AP Biology Study Guide

Water's high specific heat is primarily due to:

• Strong ionic bonds between neighboring molecules
• Extensive hydrogen bonding between molecules
• Its very small molecular mass and low density
• Nonpolar covalent bonds within each molecule

Hydrogen bonds must absorb significant energy to break before water's temperature rises, giving water its high specific heat.

Ice floats on liquid water because:

• Ice molecules pack more tightly than liquid water, but trapped air bubbles lift it
• Hydrogen bonds in ice form a rigid, spaced-out lattice that is less dense
• Ice has a higher proportion of oxygen atoms, which are lighter than hydrogen
• Covalent bonds in ice are longer and break apart, spacing out the molecules

The hexagonal hydrogen-bonded lattice in ice spaces molecules farther apart than in liquid water, making ice less dense.

Which best describes dehydration synthesis?

• Breaking a polymer using water
• Joining monomers and releasing water
• Denaturing a protein
• Adding water to break a peptide bond

Dehydration synthesis joins monomers into polymers, releasing one water molecule per bond formed.

A carboxyl group (-COOH) found in amino acids and fatty acids is best described as:

• Basic and able to accept a proton
• Acidic and able to donate a proton
• Nonpolar and hydrophobic
• Uncharged at all pH levels

The carboxyl group is acidic — it can ionize to -COO⁻, releasing an H⁺.

Which level of protein structure is described as the linear sequence of amino acids?

• Primary
• Secondary
• Tertiary
• Quaternary

Primary structure is simply the order of amino acids linked by peptide bonds.

A protein loses its function after being placed in boiling water. This is best explained by:

• Denaturation disrupting secondary/tertiary bonds while primary structure remains
• Hydrolysis of peptide bonds in the primary structure, breaking the chain into fragments
• Complete breakdown of the polypeptide into its individual amino acid monomers
• Conversion of the protein into a nucleic acid-like macromolecule with a new sequence

Heat denatures a protein by disrupting hydrogen bonds and other weak interactions, not the covalent peptide bonds of primary structure.

Enzymes speed up reactions by:

• Raising the free energy change (ΔG) so the products are more stable than reactants
• Converting nonspontaneous reactions into spontaneous ones by changing ΔG
• Lowering the activation energy required to reach the transition state
• Increasing the substrate concentration so the reaction reaches equilibrium faster

Enzymes lower activation energy; they do not change ΔG or reaction spontaneity.

A noncompetitive inhibitor reduces enzyme activity by:

• Binding directly at the active site, competing with the substrate for the same location
• Mimicking the substrate's shape so it blocks the active site until it is outcompeted
• Raising the enzyme's optimal temperature and pushing the enzyme out of its range
• Binding a site other than the active site and changing enzyme shape

Noncompetitive (allosteric) inhibitors bind elsewhere and change the enzyme's shape so the active site functions poorly.

Increasing substrate concentration would most effectively counteract the effect of a:

• Noncompetitive inhibitor
• Competitive inhibitor
• Denaturing agent
• Allosteric activator

Competitive inhibitors compete with substrate for the same active site, so excess substrate can outcompete the inhibitor.

Which macromolecule's monomer is a nucleotide?

• Protein
• Carbohydrate
• Lipid
• Nucleic acid

Nucleic acids (DNA, RNA) are polymers of nucleotide monomers.

Cohesion in water is responsible for:

• Water's ability to dissolve ions and polar molecules as a universal solvent
• Ice floating on liquid water as the hydrogen-bonded lattice becomes less dense
• Resistance to temperature change as heat is absorbed by hydrogen bonds
• Surface tension and movement of water up plant xylem

Cohesion (water-water attraction via H-bonds) creates surface tension and allows continuous water columns in xylem.

An enzyme functions best within a specific pH range because:

• Extreme pH changes only the charge of the substrate, leaving the enzyme's shape unchanged
• Extreme pH can disrupt the ionic and hydrogen bonds maintaining the enzyme's shape
• Every enzyme has an optimum of exactly pH 7 because water is neutral
• Extreme pH breaks the covalent peptide bonds that form the enzyme's primary structure

pH extremes disrupt the weak bonds that hold the enzyme's tertiary structure, altering the active site's shape.

Which is an example of a phospholipid's role?

• Storing long-term energy reserves as triglycerides in adipose tissue
• Forming the plasma membrane's structural bilayer
• Catalyzing biochemical reactions as a folded enzyme with an active site
• Carrying genetic information in a sequence of nucleotide bases

Phospholipids' amphipathic structure (polar head, nonpolar tails) makes them ideal for forming membrane bilayers.

The induced fit model of enzyme action states that:

• The active site is a rigid, perfectly matched shape that fits only one substrate, like a lock and key
• The substrate bends to fit the enzyme while the enzyme's shape stays completely fixed
• The enzyme changes shape permanently after one reaction and cannot be reused
• The active site changes shape slightly as the substrate binds, improving fit

Induced fit describes a slight conformational change in the enzyme's active site upon substrate binding.

Hydrolysis reactions are used by the digestive system to:

• Break down polymers into monomers by adding water
• Build polymers from monomers by removing a water molecule at each new bond
• Release oxygen gas from food molecules so cells can use it for respiration
• Denature dietary proteins by heat so enzymes can reach the peptide bonds

Digestion breaks polymers (starch, protein) into monomers via hydrolysis, adding a water molecule at each bond broken.

Which functional group would most increase a molecule's water solubility?

• Methyl group
• Hydroxyl group
• A long hydrocarbon chain
• None of these affect solubility

The polar hydroxyl group (-OH) can hydrogen bond with water, increasing solubility; methyl groups are nonpolar and decrease it.

Which bond links two amino acids together in a polypeptide?

• Peptide bond
• Glycosidic bond
• Phosphodiester bond
• Ester bond

A peptide bond forms by dehydration synthesis between the carboxyl group of one amino acid and the amino group of the next.

A fat that is solid at room temperature most likely has fatty acid tails that are:

• Unsaturated, with many cis double bonds
• Saturated, with no double bonds
• Branched with phosphate groups
• Charged and hydrophilic

Saturated fatty acids are straight and pack tightly, so the fat is solid. Cis double bonds put kinks in the chain and keep fats liquid.

Which polysaccharide do animals use for short-term energy storage?

• Starch
• Cellulose
• Glycogen
• Chitin

Animals store glucose as glycogen, mainly in liver and muscle. Plants store starch.

A solution at pH 3 has how many times more H⁺ ions than a solution at pH 6?

• 3 times
• 30 times
• 300 times
• 1000 times

The pH scale is logarithmic. Three pH units is a factor of 10 × 10 × 10 = 1000.

Which interaction mainly holds together the alpha helix and beta pleated sheet of a protein?

• Disulfide bonds between cysteine R groups
• Peptide bonds linking adjacent amino acids
• Hydrogen bonds between backbone atoms
• Ionic bonds between charged side chains

Secondary structure comes from hydrogen bonds between the backbone N-H and C=O groups, not from the R groups.

Water moving up a tall tree against gravity depends partly on adhesion, which is the attraction between:

• Water molecules and one another through hydrogen bonds
• Water molecules and the walls of xylem vessels
• Water molecules and the nonpolar, waxy cuticle of the leaf
• Oxygen atoms in neighboring water molecules sharing electrons

Adhesion to the vessel walls works together with cohesion between water molecules to pull water upward.

Which pair correctly matches a monomer with the polymer it builds?

• Glucose and protein
• Amino acid and polysaccharide
• Nucleotide and nucleic acid
• Fatty acid and starch

Nucleotides build nucleic acids, amino acids build proteins, and monosaccharides build polysaccharides. Lipids are not true polymers.

Which statement about DNA and RNA nucleotides is correct?

• Both contain deoxyribose
• RNA is double stranded and DNA is single stranded in cells
• Both use the same five carbon sugar
• DNA contains thymine and RNA contains uracil

DNA uses deoxyribose and thymine. RNA uses ribose and uracil. RNA is usually single stranded.

Why do the hydrophobic tails of phospholipids face inward in a membrane?

• They avoid contact with the surrounding water
• They are attracted to the cytoplasm
• They form covalent bonds with proteins
• They carry a positive charge

Nonpolar tails are shielded from the watery environment on both sides, so the tails cluster in the interior.

Which feature of carbon makes it the backbone of biological molecules?

• It is always charged
• It forms four covalent bonds
• It is the most abundant element in cells
• It cannot bond to hydrogen

Carbon has four valence electrons and forms four stable covalent bonds, allowing chains, rings and branches.

Which structure is found in both prokaryotic and eukaryotic cells?

• Nucleus
• Mitochondria
• Ribosomes
• Endoplasmic reticulum

Both cell types contain ribosomes (though prokaryotic 70S ribosomes are smaller than eukaryotic 80S), a plasma membrane, cytoplasm, and DNA.

The best evidence for the endosymbiotic theory is that mitochondria and chloroplasts:

• Are found only in plant cells and are much larger than the nucleus
• Contain their own circular DNA, 70S ribosomes, and double membranes
• Are surrounded by a single membrane and use 80S ribosomes like the cytoplasm
• Cannot make any ATP or proteins and depend entirely on the host cell

These features closely resemble free-living bacteria, supporting the idea that these organelles originated as engulfed prokaryotes.

As a cell increases in size, its surface-area-to-volume ratio:

• Increases, improving the efficiency of exchange with the environment
• Stays constant, since surface area and volume grow at the same rate
• Decreases, limiting efficient exchange with the environment
• Becomes irrelevant, because larger cells use organelles for all exchange

Volume increases faster (cubed) than surface area (squared) as size increases, so SA:V ratio decreases, limiting exchange.

In the fluid mosaic model, membrane fluidity increases with:

• More saturated fatty acid tails (straight chains packing tightly together)
• More unsaturated fatty acid tails (kinks preventing tight packing)
• More cholesterol at low temperatures, since it always makes membranes looser
• Longer, thicker hydrophilic phospholipid heads that prevent tight packing

Unsaturated fatty acids have kinks from double bonds that prevent tight packing, increasing membrane fluidity.

Facilitated diffusion differs from active transport because facilitated diffusion:

• Requires no ATP and moves down the concentration gradient
• Requires ATP and moves solutes against their concentration gradient
• Works only for small nonpolar molecules that pass through the bilayer
• Always depends on the sodium-potassium pump to move solutes

Facilitated diffusion is passive (no ATP) and moves solutes down their gradient via channel/carrier proteins.

A red blood cell placed in a hypertonic solution will:

• Swell and burst (lyse) as water enters the cell
• Remain the same size because water movement stays balanced
• Begin dividing by mitosis as solutes increase outside the cell
• Shrivel (crenate) as water leaves the cell

In a hypertonic solution, water moves out of the cell (toward higher solute concentration), causing the cell to shrink/crenate.

A plant cell placed in a hypotonic solution becomes:

• Turgid, due to the rigid cell wall resisting excess water entry
• Plasmolyzed, as water leaves and the membrane pulls away from the wall
• Crenated, shriveling as water moves out of the cell
• Completely lysed, bursting because the wall cannot resist the water entering

Water enters the cell, but the cell wall prevents lysis, resulting in a firm, turgid cell — this is the healthy state for plant cells.

The Golgi apparatus functions primarily to:

• Modify, sort, and package proteins and lipids from the ER
• Synthesize ATP through cellular respiration using an electron transport chain
• Store the cell's genetic material and control gene transcription
• Break down worn-out organelles and foreign particles with hydrolytic enzymes

The Golgi receives products from the ER at its cis face and modifies/sorts/packages them, shipping them from its trans face.

The sodium-potassium pump is an example of:

• Simple diffusion
• Facilitated diffusion
• Active transport
• Osmosis

It moves Na⁺ and K⁺ against their gradients using ATP, making it active transport.

Which is true of both mitochondria and chloroplasts?

• Both are found only in animal cells and have a single membrane
• Both have double membranes and their own DNA
• Both perform photosynthesis and produce sugar from sunlight
• Both lack ribosomes and import all their proteins from the cytoplasm

Both organelles have double membranes and circular DNA, consistent with endosymbiotic origin.

Integral membrane proteins differ from peripheral membrane proteins in that integral proteins:

• Are only found on the membrane's outer surface
• Span or embed within the lipid bilayer
• Cannot function as receptors
• Are made of carbohydrates

Integral proteins are embedded in or span the bilayer; peripheral proteins are attached to the membrane surface only.

Which of the following would most likely increase the rate of facilitated diffusion?

• Decreasing the number of transport proteins available in the membrane
• Increasing the concentration gradient of the solute
• Adding ATP to the system to power the transport proteins
• Cooling the membrane to near freezing so the proteins move more slowly

A steeper concentration gradient increases the rate of passive transport processes like facilitated diffusion, up to a saturation point.

Lysosomes primarily function to:

• Produce ATP through oxidative phosphorylation using enzymes in their inner membrane
• Digest macromolecules, worn-out organelles, and pathogens using hydrolytic enzymes
• Synthesize lipids and detoxify drugs using enzymes embedded in their membranes
• Store starch and water in plant cells to maintain turgor pressure

Lysosomes contain hydrolytic enzymes for intracellular digestion (autophagy, pathogen destruction).

Why can eukaryotic cells generally be larger than prokaryotic cells?

• Their cells lack a plasma membrane, so size is not limited by exchange
• They do not need ribosomes because organelles make their proteins
• Membrane-bound organelles increase internal surface area and compartmentalize functions
• They reproduce more slowly, which allows more time for growth

Internal membranes (organelles) provide additional surface area for reactions, helping overcome surface-area-to-volume limitations.

A cell surrounded by a solution with the same solute concentration as its cytoplasm is in a(n):

• Hypotonic solution
• Hypertonic solution
• Isotonic solution
• Osmotic solution

Isotonic solutions have equal solute concentration, so there is no net water movement.

Cholesterol embedded in the animal cell membrane functions to:

• Anchor the membrane to the cytoskeleton, giving the cell a fixed shape that stays rigid at all temperatures
• Form channels that let polar molecules and ions cross the bilayer freely without using any cellular energy
• Buffer membrane fluidity, preventing it from becoming too fluid at high temperatures or too rigid at low temperatures
• Act as receptor proteins on the outer surface, binding signal molecules and relaying the message into the cell

Cholesterol has a dual buffering role — it restrains excess fluidity at high temperatures and prevents excess rigidity at low temperatures.

Proteins that will be secreted from a cell are typically made by ribosomes attached to the:

• Smooth endoplasmic reticulum
• Nuclear pore
• Rough endoplasmic reticulum
• Lysosome

Ribosomes on the rough ER make proteins destined for secretion, membranes or lysosomes, then pass them on through the endomembrane system.

Which evidence supports the endosymbiotic origin of chloroplasts?

• They contain their own circular DNA and ribosomes similar to bacteria
• They lack any membranes and rely on the host cell's genetic material
• They are found only in animal cells and use lysosomal enzymes
• They have a single membrane and linear DNA bound to histones like the nucleus

Circular DNA, bacteria-like ribosomes and a double membrane all point to a free-living prokaryote ancestor.

A cell has a lower water potential than its surroundings. Water will move:

• Into the cell
• Out of the cell
• Equally in both directions with no net movement
• Only through the cell wall

Water moves from higher to lower water potential, so it enters the cell with the lower value.

The sodium-potassium pump moves ions in which way?

• 2 Na⁺ out and 3 K⁺ in, without ATP
• 3 Na⁺ out and 2 K⁺ in, using ATP
• 3 Na⁺ in and 2 K⁺ out, using ATP
• Both ions down their gradients by diffusion

The pump uses ATP to move 3 Na⁺ out and 2 K⁺ in against their gradients, creating an electrochemical gradient.

Which structure is found in plant cells but not in animal cells?

• Ribosomes
• Mitochondria
• Central vacuole
• Golgi apparatus

Plant cells have a large central vacuole, a cell wall and chloroplasts. Animal cells have none of these.

Why do cells that need to exchange materials quickly tend to be small?

• A small cell has fewer organelles, so less material must be moved
• A small cell has no membrane, so materials pass through freely
• A small cell has a larger surface area relative to its volume
• A small cell has a lower metabolic rate, so less exchange is needed

As a cell grows, volume increases faster than surface area, so exchange with the environment becomes less efficient.

A student places an animal cell in distilled water. What is the most likely result?

• The cell swells and may burst
• The cell shrinks
• The cell stays the same size
• The cell becomes turgid

Distilled water is hypotonic to the cell, so water enters. Animal cells have no cell wall to resist and can lyse.

Which organelle is the main site of lipid synthesis and detoxification in many cells?

• Nucleolus
• Smooth endoplasmic reticulum
• Peroxisome only
• Rough endoplasmic reticulum

Smooth ER lacks ribosomes and synthesizes lipids and steroids and detoxifies drugs and poisons.

Which transport process moves a large particle into a cell by wrapping it in a piece of membrane?

• Osmosis
• Facilitated diffusion
• Endocytosis
• Simple diffusion

Endocytosis brings in large materials by engulfing them in a vesicle. It requires energy.

Compartmentalization in eukaryotic cells is an advantage because it:

• Eliminates the need for enzymes, because organelles carry out reactions on their own
• Allows incompatible reactions to occur at the same time in separate locations
• Prevents any exchange between organelles so that each one works in isolation
• Makes eukaryotic cells smaller than prokaryotic cells by saving space

Membrane-bound organelles keep conditions and enzymes for different reactions apart, increasing efficiency.

Which process directly produces the most ATP per glucose molecule?

• Glycolysis, which breaks glucose into pyruvate in the cytoplasm
• Pyruvate oxidation, which converts pyruvate into acetyl CoA
• The Krebs cycle, which completes the oxidation of acetyl CoA in the matrix
• Oxidative phosphorylation (chemiosmosis via the ETC)

Oxidative phosphorylation via the electron transport chain and chemiosmosis produces roughly 26-28 of the ~30-32 total ATP per glucose.

The main purpose of fermentation is to:

• Produce large amounts of ATP directly through substrate-level phosphorylation
• Regenerate NAD⁺ so glycolysis can continue without oxygen
• Split water to release oxygen and supply electrons to the transport chain
• Fix carbon dioxide into sugar using ATP and NADPH from earlier reactions

Fermentation regenerates NAD⁺ from NADH, allowing glycolysis to continue producing its small net ATP yield in the absence of O₂.

In the electron transport chain, the final electron acceptor in aerobic respiration is:

• NAD⁺
• Pyruvate
• Oxygen
• Carbon dioxide

Oxygen accepts electrons at the end of the ETC, combining with H⁺ to form water.

The oxygen released during photosynthesis comes from:

• Water, split during the light reactions
• Carbon dioxide, which is split during the Calvin cycle
• Glucose, which is broken down in the light reactions
• The Calvin cycle, as RuBP is regenerated and releases O₂

Photolysis of water in Photosystem II releases O₂ as a byproduct; CO₂ becomes part of the sugar produced, not O₂.

Which molecules directly power the Calvin cycle?

• ATP and NADPH from the light reactions
• Glucose and oxygen from the mitochondria
• CO₂ and H₂O, which supply the energy for fixation
• FADH₂ and pyruvate from glycolysis and the Krebs cycle

The Calvin cycle uses ATP and NADPH (produced in the light reactions) to fix CO₂ into G3P/glucose.

Chemiosmosis refers to:

• ATP synthesis driven by direct transfer of phosphate from glucose breakdown
• Diffusion of chemicals across a membrane from high to low concentration
• ATP synthesis driven by electrons flowing against a gradient to oxygen
• ATP synthesis driven by H⁺ flowing through ATP synthase down its gradient

Chemiosmosis is the process where a proton gradient drives ATP synthase to produce ATP, used in both respiration and photosynthesis.

Glycolysis occurs in the:

• Mitochondrial matrix
• Inner mitochondrial membrane
• Cytoplasm
• Chloroplast stroma

Glycolysis takes place in the cytoplasm and does not require oxygen.

An athlete experiencing muscle burn during intense exercise is most likely undergoing:

• Aerobic respiration only, with plentiful oxygen at the electron transport chain
• The Calvin cycle, which fixes carbon dioxide using ATP and NADPH
• Photosynthesis in muscle cells, which store energy as glucose
• Lactic acid fermentation due to insufficient oxygen supply

When oxygen delivery can't keep up with demand, muscle cells use lactic acid fermentation to regenerate NAD⁺.

Rubisco catalyzes which step of the Calvin cycle?

• Carbon fixation — attaching CO₂ to RuBP
• Regeneration of RuBP using ATP and G3P
• Reduction of 3-PG into G3P using ATP and NADPH
• Splitting of water to release oxygen in the light reactions

Rubisco fixes atmospheric CO₂ onto the 5-carbon RuBP, the first step of the Calvin cycle.

Which best describes the relationship between mitochondria and chloroplasts regarding ATP synthesis?

• Only mitochondria use chemiosmosis, while chloroplasts make ATP by substrate-level reactions
• Both use chemiosmosis, driven by a proton gradient across an internal membrane
• Only chloroplasts make ATP, since mitochondria use only glycolysis
• Neither uses a proton gradient, since ATP is made by phosphate transfer

Both organelles use ATP synthase and a proton gradient (chemiosmosis) to generate ATP, despite different electron sources.

Alcoholic fermentation, used by yeast, converts pyruvate into:

• Lactate
• Ethanol and CO₂
• Glucose
• Oxygen and water

Yeast converts pyruvate to acetaldehyde, releasing CO₂, then to ethanol, regenerating NAD⁺.

During the Krebs cycle, most of the energy extracted from acetyl-CoA is stored in the form of:

• ATP directly
• NADH and FADH₂ (electron carriers)
• Glucose
• Free oxygen

The Krebs cycle produces mostly NADH and FADH₂, which later fuel the ETC to make most of the cell's ATP; only 2 ATP form directly.

If a poison blocked Photosystem II specifically, which process would be directly impaired first?

• The Calvin cycle only, since it needs light directly to fix carbon
• Splitting of water and the initial electron excitation for the light reactions
• Cellular respiration, since the mitochondrial electron transport chain depends on it
• DNA replication, since light energy powers the replication fork

PS II absorbs light and splits water; blocking it stops electron flow to PS I and halts water-splitting (O₂ release).

The net ATP yield of glycolysis (before considering the electron transport chain) is:

• 0 ATP
• 2 ATP
• 4 ATP
• 36 ATP

Glycolysis uses 2 ATP to invest and produces 4 ATP, for a net gain of 2 ATP via substrate-level phosphorylation.

Which process produces ATP directly by substrate-level phosphorylation rather than chemiosmosis?

• Oxidative phosphorylation in the ETC
• Glycolysis and the Krebs cycle
• Photophosphorylation in chloroplasts
• Facilitated diffusion

Glycolysis and the Krebs cycle each generate a small amount of ATP directly (substrate-level phosphorylation), unlike the ETC's chemiosmotic mechanism.

Enzymes increase the rate of a reaction by:

• Lowering the activation energy
• Increasing the free energy of the reactants
• Being used up in the reaction
• Changing the overall free energy change

Enzymes lower the activation energy but do not change the overall free energy change or get consumed.

A competitive inhibitor can be overcome by:

• Lowering the temperature
• Increasing substrate concentration
• Adding more inhibitor
• Denaturing the enzyme

A competitive inhibitor binds the active site. Enough substrate outcompetes it, restoring the maximum rate.

During cellular respiration, the proton gradient across the inner mitochondrial membrane is used to:

• Split water
• Fix carbon dioxide
• Make ATP through ATP synthase
• Make pyruvate

Protons flow back through ATP synthase, driving the phosphorylation of ADP. This is chemiosmosis.

What is the role of NADH in cellular respiration?

• It stores carbon from glucose and carries it to the Calvin cycle
• It serves as the final electron acceptor and is reduced to water
• It carries high-energy electrons to the electron transport chain
• It breaks glucose down into two molecules of pyruvate in glycolysis

NADH is an electron carrier that delivers electrons from glycolysis and the Krebs cycle to the ETC.

Which products of the light reactions are used in the Calvin cycle?

• ATP and NADPH
• ATP and glucose
• NADH and oxygen
• Carbon dioxide and water

The light reactions make ATP and NADPH, which power the reduction of carbon in the Calvin cycle.

The Calvin cycle takes place in the:

• Thylakoid membrane
• Stroma
• Inner mitochondrial membrane
• Cytosol

The stroma holds the Calvin cycle enzymes. The light reactions occur in the thylakoid membranes.

Why does fermentation allow glycolysis to continue without oxygen?

• It produces large amounts of ATP
• It makes oxygen
• It regenerates NAD⁺
• It breaks down water

Fermentation oxidizes NADH back to NAD⁺, which glycolysis needs to keep running.

In feedback inhibition, the end product of a pathway:

• Activates the first substrate
• Binds the active site of the last enzyme only
• Is converted back to the substrate
• Inhibits an enzyme early in the pathway

The end product binds an enzyme, often allosterically, early in the pathway, shutting the pathway down when enough product exists.

Which statement about ATP is correct?

• It stores energy in its adenine base, which is released when the base is broken off the molecule
• It carries genetic information from the nucleus to ribosomes, where it directs protein building
• It couples exergonic reactions to endergonic reactions by transferring a phosphate group
• It is made in chloroplasts and is consumed completely in each reaction without being regenerated

Phosphorylation by ATP transfers energy to drive endergonic reactions. Cells constantly recycle ATP and ADP.

If the pH of the environment changes dramatically, an enzyme's activity will drop because:

• Substrate molecules disappear
• The shape of the active site can change
• Activation energy increases for all reactions
• The enzyme is used up

Changes in pH alter ionic and hydrogen bonds, changing the shape of the active site.

In cell signaling, 'transduction' refers to:

• The initial binding of a ligand to its receptor on the cell
• The final change in cell behavior, such as gene expression or enzyme activity
• The diffusion of a ligand through the plasma membrane into the cytoplasm
• The conversion of a received signal into a cellular response through a relay pathway

Transduction is the relay/amplification stage that converts receptor activation into intracellular molecular changes.

Which cell cycle checkpoint verifies that all chromosomes are properly attached to spindle fibers?

• G1 checkpoint (cell size, nutrients and DNA damage)
• M checkpoint (spindle assembly checkpoint)
• G2 checkpoint (DNA replication completeness)
• S checkpoint (the cell's commitment to dividing)

The M/spindle assembly checkpoint ensures proper chromosome attachment before anaphase proceeds.

Cyclins differ from CDKs in that:

• CDK levels oscillate through the cell cycle while cyclin levels stay fairly constant
• Both stay constant through the cycle but differ in how fast they are degraded
• Both oscillate in sync, and neither is needed unless there is DNA damage
• Cyclin levels oscillate through the cell cycle while CDK levels stay fairly constant

Cyclin concentration rises and falls at specific points in the cycle, binding and activating the constantly-present CDKs at the right time.

A mutation that converts a proto-oncogene into an oncogene is best described as a:

• Gain-of-function mutation that overstimulates cell division
• Loss-of-function mutation that removes a brake on cell division
• Silent mutation that changes the DNA but has no effect on cell division
• Mutation that affects only mitochondrial DNA and the cell's energy supply

Proto-oncogenes normally promote division; a gain-of-function mutation makes them overactive, driving excess division.

The p53 gene is best classified as a:

• Proto-oncogene
• Tumor suppressor gene
• Structural gene with no regulatory role
• Mitochondrial gene

p53 halts the cell cycle for repair or triggers apoptosis when DNA is damaged, functioning as a tumor suppressor.

Apoptosis differs from necrosis in that apoptosis is:

• Uncontrolled and damages the surrounding tissue by releasing cell contents
• Seen only in cancer cells that have lost their cell cycle controls
• A controlled, programmed process that avoids harming nearby cells
• The same process as mitosis, in which the cell divides by cytokinesis

Apoptosis is a regulated form of cell death that safely removes cells without releasing harmful contents, unlike necrosis.

During which phase of the cell cycle does DNA replication occur?

• G1
• S
• G2
• M

S phase (synthesis phase) is when DNA replication occurs, producing sister chromatids.

A cell that permanently exits the cell cycle (e.g., a mature neuron) enters:

• G0
• Prophase
• Metaphase
• Telophase

G0 is a non-dividing state that some differentiated cells enter permanently.

Second messengers like cAMP function in signal transduction primarily to:

• Directly carry the original ligand across the plasma membrane into the cytoplasm
• Amplify the signal so one receptor activation triggers a large response
• Replace the receptor so that the ligand no longer has to bind a protein
• Bind directly to DNA and alter transcription without any relay proteins

Second messengers amplify a small initial signal into a much larger intracellular response.

Sister chromatids separate during which stage of mitosis?

• Prophase
• Metaphase
• Anaphase
• Telophase

Anaphase is when sister chromatids are pulled apart to opposite poles of the cell.

Loss of contact inhibition and anchorage dependence in a cell most directly suggests:

• Normal, healthy tissue undergoing regeneration after an injury
• A cancerous cell capable of uncontrolled growth and metastasis
• A cell entering G0 and exiting the active cell cycle
• Successful apoptosis with orderly removal of a damaged cell

Cancer cells typically ignore normal growth-limiting signals like contact inhibition, allowing invasive, uncontrolled growth.

Which statement about the cell cycle checkpoints is correct?

• The G1 checkpoint verifies that spindle fibers are attached to chromosomes
• The M checkpoint verifies cell size and nutrient availability for division
• Checkpoints function only in cancer cells that divide uncontrollably
• The G2 checkpoint verifies DNA replication was completed correctly before mitosis

The G2 checkpoint ensures DNA was replicated correctly and without damage before the cell proceeds into mitosis.

A cell with damaged DNA that cannot be repaired is normally directed by p53 to:

• Continue dividing regardless of damage
• Undergo apoptosis
• Immediately enter G0 permanently with no other effect
• Convert into a stem cell

When DNA damage is irreparable, functional p53 triggers apoptosis, eliminating the potentially dangerous cell.

A phosphorylation cascade in a signal transduction pathway most directly:

• Carries the ligand directly into the nucleus and attaches it to DNA
• Relays and amplifies the signal by sequentially activating a series of proteins via kinases
• Immediately destroys the receptor so the signal cannot be repeated
• Has no influence on the final cellular response, which is set by the ligand alone

A phosphorylation cascade uses a chain of kinases, each activating the next, to relay and amplify the original signal.

Chromosomes align at the metaphase plate during which stage of mitosis?

• Prophase
• Metaphase
• Anaphase
• Telophase

During metaphase, chromosomes line up at the cell's equator (metaphase plate) before separating in anaphase.

Which sequence correctly orders the stages of cell signaling?

• Transduction, reception, response
• Response, reception, transduction
• Reception, transduction, response
• Reception, response, transduction

A ligand is received by a receptor, the message is relayed and amplified, and then the cell responds.

Why is a signal transduction pathway with several steps useful to a cell?

• It amplifies the signal and gives more points of regulation
• It slows the response so the cell avoids reacting too quickly
• It removes the need for receptors by sending the ligand through the cell
• It keeps the signal from reaching the nucleus and limits gene expression

Each step can activate many molecules, so a small signal produces a large response, and each step can be regulated.

What happens at the G1 checkpoint in a healthy cell?

• Spindle attachment to every chromosome is verified
• The cell decides whether to commit to dividing
• The cell confirms that DNA replication is complete
• Cytoplasm is divided between two daughter cells

The G1 checkpoint checks cell size, nutrients and DNA damage before committing the cell to the cycle.

During which phase do chromosomes first become visible as condensed structures?

• Interphase
• Prophase
• Telophase
• G1

Chromatin condenses into visible chromosomes in prophase, and the nuclear envelope begins to break down.

Cancer cells that divide without responding to checkpoints often have mutations that:

• Remove all of the cell's DNA so division is no longer regulated
• Increase contact inhibition and make cells stop dividing sooner
• Prevent DNA replication, which halts the cycle at the S phase
• Inactivate tumor suppressors or activate oncogenes

Loss of tumor suppressor function and gain of oncogene function both push cells toward uncontrolled division.

How does cytokinesis differ in plant and animal cells?

• Animals form a cell plate and plants form a cleavage furrow
• Plants form a cell plate and animals form a cleavage furrow
• Neither cell type undergoes cytokinesis after mitosis is complete
• Both plants and animals form a cell plate across the cell center

Animal cells pinch in with a cleavage furrow. Plant cells build a cell plate that becomes a new cell wall.

A drug blocks the formation of spindle fibers. Cells treated with it would most likely stop in:

• Metaphase
• G1
• S phase
• Cytokinesis only

Without spindle fibers, chromosomes cannot align and separate, so the cell is arrested at the spindle checkpoint in metaphase.

Quorum sensing in bacteria is an example of:

• Meiosis, in which cells divide to produce haploid gametes
• Cell signaling between cells using chemical signals
• Apoptosis, in which cells undergo programmed death
• Passive transport of solutes across the plasma membrane

Bacteria release and detect signaling molecules, and respond when the population density reaches a threshold.

Which molecule is a typical second messenger?

• DNA polymerase
• Cellulose
• cAMP
• ATP synthase

cAMP relays the message inside the cell after a signal binds a receptor at the membrane.

Which statement about apoptosis is correct?

• It always causes inflammation as cell contents spill into surrounding tissue
• It is programmed cell death used in development and to remove damaged cells
• It is the same as necrosis, which is an uncontrolled form of cell death
• It happens only in cancer cells after they stop responding to checkpoints

Apoptosis is controlled and orderly. Necrosis is uncontrolled and often triggers inflammation.

Crossing over occurs during which stage of meiosis?

• Prophase I
• Metaphase I
• Anaphase II
• Telophase II

Crossing over occurs during prophase I, when homologous chromosomes synapse and exchange genetic material.

For an organism with n=4 chromosome pairs, independent assortment alone can produce how many different chromosome combinations in gametes?

• 4
• 8
• 16
• 32

2ⁿ = 2⁴ = 16 possible combinations from independent assortment of 4 chromosome pairs.

A cross between a heterozygous (Rr) and a heterozygous (Rr) pea plant for a completely dominant trait produces what phenotypic ratio?

• 1:1
• 1:2:1
• 3:1
• 9:3:3:1

Rr × Rr gives genotypes 1 RR : 2 Rr : 1 rr, which is 3 dominant : 1 recessive phenotypically.

A cross between two heterozygous individuals for two independently assorting genes (AaBb × AaBb) produces which phenotypic ratio?

• 3:1
• 1:2:1
• 9:3:3:1
• 1:1:1:1

Independent dihybrid crosses produce the classic 9:3:3:1 phenotypic ratio.

Red (RR) crossed with white (WW) snapdragons produces all pink (RW) offspring. This demonstrates:

• Codominance
• Incomplete dominance
• Complete dominance
• Sex linkage

The blended pink phenotype is the hallmark of incomplete dominance, unlike codominance where both traits appear fully and separately.

AB blood type, where both A and B antigens appear on red blood cells, is an example of:

• Incomplete dominance
• Codominance
• Polygenic inheritance
• Sex linkage

Both IA and IB alleles are expressed fully and simultaneously — this is codominance, not blending.

A colorblind son is born to a mother who is a carrier (heterozygous) for the X-linked recessive trait and a father with normal vision. This is explained because the son:

• Received the trait from his father's Y chromosome, which carries the allele
• Cannot inherit X-linked traits from his mother, so a new mutation must have arisen
• Is heterozygous like his mother, so both alleles determine his phenotype
• Is hemizygous, so his single X-linked allele from his mother determines his phenotype

Males have only one X chromosome (hemizygous), so a single recessive allele inherited from the mother is enough to express an X-linked recessive trait.

Two genes located very close together on the same chromosome will:

• Tend to be inherited together, violating independent assortment, unless separated by crossing over
• Tend to assort independently of each other, following Mendel's law, unless a mutation joins them together
• Tend to be inherited separately, because the chromosome is split into pieces at the start of meiosis
• Tend to produce more recombinant offspring than nonrecombinant offspring, since nearby genes cross over often

Linked genes tend to travel together during meiosis; only crossing over can separate them, and this happens more often the farther apart the genes are.

Nondisjunction during meiosis I most directly leads to:

• Normal haploid gametes, since the chromosomes separate correctly
• Gametes with an abnormal number of chromosomes (e.g., trisomy)
• Increased crossing over, which exchanges more genetic material between homologs
• A completely new species, as a gamete carries a changed chromosome number

Failure of homologous chromosomes to separate properly produces gametes with too many or too few chromosomes.

A test cross is used to determine:

• The exact number of chromosomes in a species by counting the offspring
• Whether an organism with a dominant phenotype is homozygous or heterozygous
• Which parent contributed the Y chromosome to the offspring
• Whether a trait shows codominance rather than complete dominance

Crossing the unknown individual with a homozygous recessive reveals its genotype based on the offspring ratio produced.

Which is a source of genetic variation unique to meiosis (not mitosis)?

• Spindle formation and DNA replication
• Crossing over and independent assortment
• Point mutations and DNA replication errors
• Cytokinesis and chromosome condensation

Crossing over (prophase I) and independent assortment (metaphase I) are meiosis-specific processes generating genetic variation.

Human skin color, controlled by multiple genes with additive effects producing a continuous range of phenotypes, is an example of:

• Codominance
• Polygenic inheritance
• Sex-linked inheritance
• Incomplete dominance

Polygenic traits are influenced by multiple genes, producing a continuous distribution of phenotypes rather than discrete categories.

Meiosis II is most similar to mitosis because in both processes:

• Homologous chromosomes separate, producing two cells with half the chromosome number
• Crossing over always occurs in prophase, exchanging alleles between chromatids
• Sister chromatids separate, producing genetically identical daughter cells from each parent cell
• The chromosome number is reduced by half, giving haploid daughter cells

Both meiosis II and mitosis separate sister chromatids; meiosis I (not II) is the reductional division that separates homologs.

An offspring's blood type is O, meaning its genotype is ii. If one parent is IAi and the other is IBi, this outcome is:

• Impossible, because each parent's alleles would always give a type A or B child
• Possible — each parent could contribute the recessive i allele
• Possible only if a new mutation changed the I alleles
• Evidence that codominance failed because both I alleles should be expressed

Each heterozygous parent carries one i allele; if both contribute i, the child is ii (type O), which is fully expected under multiple allele inheritance.

Genes located far apart on the same chromosome are more likely to be separated by crossing over than genes located close together because:

• Greater physical distance increases the probability that a crossover event occurs between them
• Distance has no effect on recombination, since crossovers occur at random locations
• Close genes always cross over first, so they separate more often
• Far apart genes are on different chromosomes, so they always assort independently

Recombination frequency increases with the physical distance between two linked genes, since there is more opportunity for a crossover to occur between them.

Which event in meiosis I results in a reduction of the chromosome number?

• Homologous chromosomes separate
• Sister chromatids separate
• DNA replicates
• Crossing over occurs in anaphase

Meiosis I separates homologous pairs, cutting the chromosome number in half. Meiosis II separates sister chromatids.

The law of segregation states that:

• All traits are inherited together as a single unit from each parent
• Alleles for a trait separate into different gametes
• Dominant alleles always win and cannot be masked in the next generation
• Genes on the same chromosome assort independently into gametes

Each gamete receives only one of the two alleles for a gene.

A dihybrid cross between two heterozygotes (AaBb × AaBb) typically gives which phenotype ratio?

• 3:1
• 1:2:1
• 9:3:3:1
• 1:1:1:1

For two independently assorting genes with complete dominance, the ratio is 9:3:3:1.

A man with an X-linked recessive disorder and a carrier woman have a child. What is the chance that a son will be affected?

• 25%
• 100%
• 0%
• 50%

The mother passes either an affected X or a normal X to a son with equal probability, so half of the sons are affected.

Pink flowers appear when red and white parents are crossed. This is best explained by:

• Incomplete dominance
• Codominance
• Epistasis
• Sex linkage

In incomplete dominance the heterozygote shows an intermediate phenotype.

A chi-square test is used to determine whether:

• Two genes are on the same chromosome and are inherited together
• Observed results differ significantly from expected results
• A trait is dominant or recessive based on the offspring ratio
• A mutation has occurred in one parent's gametes

Chi-square compares observed data with expected data under the null hypothesis, using degrees of freedom to find the probability.

A pedigree shows that two unaffected parents have an affected child. The trait is most likely:

• Dominant
• Y-linked
• Recessive
• Mitochondrial

Unaffected parents can only have an affected child if both carry a hidden recessive allele.

Genes that are linked tend to:

• Assort independently because they lie far apart on the same chromosome
• Be inherited together because they are close on the same chromosome
• Be on different chromosomes and sort independently in meiosis
• Never undergo crossing over because they are physically attached

Linked genes are close together, so crossing over rarely separates them. Recombination frequency reflects the distance between them.

Down syndrome (trisomy 21) is most often caused by:

• Nondisjunction during meiosis
• A point mutation in one gene
• A viral infection
• A deletion in mitochondrial DNA

Failure of chromosome 21 to separate in meiosis produces a gamete with an extra copy.

Which gives a gamete with a unique combination of alleles that did not exist in either parent?

• Mitosis, which copies the parent cell's chromosomes exactly
• DNA replication, which makes an identical copy of each chromosome
• Binary fission, which splits a prokaryote into two identical cells
• Crossing over combined with independent assortment

Crossing over mixes alleles on homologs, and independent assortment shuffles chromosomes, producing many new combinations.

DNA replication is described as semiconservative because:

• Both new strands are entirely newly made, and the old strands are discarded
• No original DNA is preserved, as the parent helix is degraded
• Replication copies only half the cell's DNA before the cell divides
• Each new double helix has one original strand and one newly made strand

Semiconservative replication means each daughter molecule retains one parental strand paired with one new strand.

Why is the lagging strand synthesized in short (Okazaki) fragments?

• DNA polymerase can only add nucleotides in the 3'→5' direction, opposite to the movement of the replication fork
• Helicase can unwind only short stretches of the template at a time, so the new strand must be built in pieces
• DNA polymerase can only add nucleotides in the 5'→3' direction, opposite to the replication fork's movement on that strand
• The lagging strand lacks a primer, so RNA polymerase must restart synthesis each time the fork moves forward

Since DNA polymerase synthesizes only 5'→3', the lagging strand (running away from the fork direction) must be made discontinuously.

A mutation that inserts 1 nucleotide into a gene's coding sequence would most likely:

• Swap one amino acid for another and leave the rest of the protein unchanged
• Alter only the one codon where it is inserted, leaving all later codons unaffected
• Be corrected by the ribosome during translation, giving the normal protein
• Cause a frameshift, altering every amino acid downstream of the insertion

A single nucleotide insertion (not a multiple of 3) shifts the reading frame for all codons downstream, typically producing a nonfunctional protein.

A silent mutation has no effect on the protein produced because:

• The new codon codes for a different amino acid with similar properties, so the protein folds
• The new codon still codes for the same amino acid, due to the redundancy of the genetic code
• The ribosome detects and replaces the changed codon with the original one before the amino acid is added
• The mutation changes a single base on the template but is reversed by DNA repair before transcription

The genetic code is degenerate — several codons can specify the same amino acid, so some substitutions don't change the protein.

In the lac operon, when lactose is present in the cell, it:

• Binds directly to RNA polymerase, enabling it to bind the promoter
• Binds the repressor, removing it from the operator and allowing transcription
• Binds the operator, blocking transcription of the structural genes
• Has no effect on the operon, since the repressor stays bound

Lactose acts as an inducer — it binds the repressor protein, changing its shape so it can no longer bind the operator, allowing transcription.

In gel electrophoresis, DNA fragments migrate toward the positive electrode because:

• DNA's phosphate backbone gives it a negative charge
• DNA's nitrogenous bases give it a positive charge
• Heavier DNA fragments are pulled by gravity to the bottom
• The loading dye pushes DNA toward the positive end

DNA's phosphate groups give it an overall negative charge, so it migrates toward the positive electrode in an electric field.

Which best describes the role of DNA polymerase's proofreading function?

• It detects and corrects mismatched nucleotides during replication, increasing fidelity
• It seals gaps between Okazaki fragments by joining the sugar-phosphate backbone of the new strand
• It unwinds the double helix ahead of the fork and separates the strands to prepare for replication
• It lays down short RNA primers that provide a starting point for the synthesis of new DNA strands

DNA polymerase proofreads newly added nucleotides, removing errors to keep the mutation rate very low.

During eukaryotic mRNA processing, which of the following occurs?

• Introns are added and exons are removed, producing a longer transcript
• Ribosomes bind and begin translating before transcription is complete
• A 5' cap and poly-A tail are added, and introns are spliced out
• Nothing is changed, and the mRNA is used immediately as transcribed

Eukaryotic pre-mRNA is processed by adding a 5' cap and poly-A tail and splicing out introns, leaving only exons in the mature mRNA.

DNA methylation is an example of:

• An epigenetic change that typically activates gene expression by changing the DNA base sequence itself
• A point mutation that typically silences gene expression by replacing one base with another in a gene
• A post-translational modification that typically adds sugar groups to proteins after they are made
• An epigenetic change that typically silences gene expression without altering the DNA sequence

Epigenetic modifications like DNA methylation alter gene expression heritably without changing the underlying DNA sequence.

PCR requires all of the following EXCEPT:

• Primers complementary to the target sequence
• Heat-stable DNA polymerase (Taq)
• A ribosome
• Repeated cycles of denaturation, annealing, and extension

PCR amplifies DNA using primers and heat-stable polymerase through thermal cycling — ribosomes are involved in translation, not PCR.

The trp operon is best described as:

• Inducible, normally off, and turned on when tryptophan binds the repressor
• Identical in function to the lac operon, which is inducible by its substrate
• Repressible — normally on, turned off when tryptophan acts as a co-repressor
• Found only in eukaryotes, where it controls protein synthesis

The trp operon is repressible: it's normally active, but abundant tryptophan activates the repressor to shut off transcription (negative feedback).

Heterochromatin, compared to euchromatin, is:

• Loosely packed and actively transcribed by RNA polymerase
• Found only in prokaryotes, which have no nucleus
• The site of translation, where ribosomes read mRNA
• Tightly packed and generally transcriptionally inactive

Heterochromatin is densely packed DNA that is generally inaccessible to transcription machinery, keeping those genes silent.

A restriction enzyme used in genetic engineering functions to:

• Join DNA fragments together by forming phosphodiester bonds
• Cut DNA at a specific recognition sequence, often producing sticky ends
• Synthesize new DNA strands by adding nucleotides in the 5'→3' direction
• Translate mRNA into protein by reading codons at the ribosome

Restriction enzymes recognize specific sequences and cut DNA there, generating fragments (often with complementary sticky ends) for cloning.

Which best describes the relationship between a gene's exons and introns in eukaryotic pre-mRNA processing?

• Introns are removed by splicing, and exons are joined to form mature mRNA
• Exons are removed by splicing, and introns are joined to form mature mRNA
• Both introns and exons are removed and replaced with a cap and tail
• Neither is present in pre-mRNA, which contains only regulatory sequences

Splicing removes noncoding introns, leaving only the coding exons joined together in the mature mRNA.

A missense mutation results in:

• A different amino acid being incorporated into the protein
• No change in the amino acid sequence because of the redundant code
• A premature stop codon that ends translation early
• No effect on the protein because the gene is not transcribed

A missense mutation changes the codon so that it now codes for a different amino acid than the original.

Which enzyme unwinds the DNA double helix at the replication fork?

• Ligase
• Primase
• Helicase
• RNA polymerase

Helicase breaks hydrogen bonds between the strands. Primase adds RNA primers and ligase joins fragments.

DNA polymerase can add nucleotides only to the:

• 5' end of an existing strand
• Middle of a strand
• Start of a gene without a primer
• 3' end of an existing strand

DNA polymerase synthesizes 5' to 3' by adding to a free 3' OH, so it needs a primer.

In eukaryotes, which of the following is added to pre-mRNA during processing?

• A 5' cap and a poly-A tail
• A promoter
• An operator
• A second strand

The 5' cap and poly-A tail protect the mRNA and help translation. Introns are also spliced out.

The anticodon is found on:

• mRNA
• tRNA
• DNA
• rRNA only

The anticodon on tRNA pairs with the mRNA codon in the ribosome, matching the amino acid carried.

Which mutation would most likely have the greatest effect on a protein?

• A nonsense mutation near the start of the gene
• A silent mutation that leaves the codon coding for the same amino acid
• A missense mutation at the very end of the gene, near the C-terminus
• A mutation in an intron that is removed by splicing

A nonsense mutation creates an early stop codon, producing a short, usually nonfunctional protein.

Transcription factors regulate gene expression by:

• Binding DNA and affecting whether RNA polymerase can transcribe a gene
• Cutting the DNA at specific sequences to give RNA polymerase shorter pieces
• Binding mRNA at the ribosome and controlling how fast proteins are built
• Unwinding the DNA at origins so that DNA polymerase can copy the genome

Activators and repressors bind regulatory sequences and control the rate of transcription.

In the lac operon, the repressor protein binds to the:

• Operator when lactose is absent
• Promoter when lactose is present
• Lactose permanently
• RNA polymerase only

Without lactose, the repressor sits on the operator and blocks transcription. Lactose changes the repressor so it releases.

A researcher wants to make many copies of a specific DNA segment. Which technique should be used?

• Gel electrophoresis
• PCR
• Western blot
• Karyotyping

PCR amplifies a target DNA sequence using primers and a heat-stable DNA polymerase.

Gel electrophoresis separates DNA fragments by:

• Sequence
• Color
• Size
• Gene function

Smaller fragments move farther through the gel toward the positive electrode.

Which describes the purpose of the ribosome's A, P and E sites?

• Replicating DNA
• Splicing mRNA
• Making rRNA
• Binding tRNAs during translation

The A site accepts a new tRNA, the P site holds the growing chain, and the E site releases empty tRNAs.

Homologous structures such as the forelimbs of humans, whales, and bats provide evidence for:

• Convergent evolution from unrelated ancestors facing similar pressures
• Analogous adaptation to similar environments in distant lineages
• Genetic drift as the only cause of similarity between these species
• Common ancestry, despite different current functions

Homologous structures share the same underlying anatomical origin, indicating descent from a common ancestor even though functions differ.

Bird wings and insect wings are considered analogous structures because they:

• Serve a similar function (flight) but evolved independently (convergent evolution)
• Share the same developmental origin and arose from a common ancestor's wing
• Prove that birds and insects share a recent common ancestor
• Are known only from the fossil record and show no function in living species

Analogous structures perform similar functions but arose independently, so they do NOT indicate a recent common ancestor.

If observed genotype frequencies in a population differ substantially from those predicted by Hardy-Weinberg, this suggests:

• The population is in equilibrium and no evolutionary forces are acting
• No conclusion can be drawn, since genotype frequencies are unrelated to evolution
• The population is evolving (one or more HW assumptions is being violated)
• The population size is infinite, so drift cannot alter its frequencies

Deviation from Hardy-Weinberg predictions indicates evolutionary forces (selection, drift, migration, mutation, or nonrandom mating) are acting.

A population has an allele frequency q=0.2 for a recessive allele. What is the expected frequency of homozygous recessive individuals?

• 0.2
• 0.04
• 0.4
• 0.96

q² = 0.2² = 0.04, so about 4% of the population is expected to be homozygous recessive.

A population's allele frequencies change dramatically after a natural disaster kills most of its members randomly, unrelated to genotype. This illustrates:

• Natural selection, since the survivors had better-adapted genotypes
• The bottleneck effect (a type of genetic drift)
• Gene flow, as new alleles entered from a neighboring population
• Directional selection, as one phenotype was favored over the others

The bottleneck effect is a form of genetic drift caused by a drastic, random reduction in population size.

Stabilizing selection is best illustrated by:

• Human birth weight favoring an intermediate value over extremes
• Peppered moth coloration shifting toward darker forms during industrialization
• Finch beak size splitting into two distinct extremes
• Antibiotic resistance in bacteria increasing over time

Stabilizing selection favors intermediate phenotypes and reduces variation — average birth weight has the highest survival rate.

Disruptive selection tends to:

• Increase phenotypic variation by favoring both extremes over the intermediate
• Reduce phenotypic variation by favoring the average phenotype
• Have no effect on allele frequencies, because it acts only on phenotype
• Occur only in asexual organisms, where there is no recombination

Disruptive selection favors both phenotypic extremes, which can increase variation and sometimes drive speciation.

Two populations of the same species become separated by a newly formed mountain range and evolve independently until they can no longer interbreed. This is an example of:

• Sympatric speciation
• Allopatric speciation
• Genetic drift only
• Convergent evolution

Geographic separation preventing gene flow, leading to independent divergence, defines allopatric speciation.

A mule (offspring of a horse and donkey) being sterile is an example of which reproductive isolating mechanism?

• Prezygotic — behavioral isolation
• Prezygotic — habitat isolation
• Postzygotic — hybrid sterility
• Postzygotic — hybrid inviability

Hybrid sterility is a postzygotic barrier — the hybrid survives but cannot produce viable offspring.

On a phylogenetic tree, a node represents:

• The end of a branch where a living species is placed on the tree
• The most recent common ancestor of the lineages branching from that point
• The amount of genetic change that has occurred along one lineage over time
• A complete group consisting of one ancestor and all of its descendants

Each node/branch point on a cladogram represents the most recent common ancestor shared by the descendant lineages.

Which condition is NOT required for a population to remain in Hardy-Weinberg equilibrium?

• No mutation changing the alleles
• Random mating within the population
• Natural selection actively occurring
• No gene flow into or out of the population

Hardy-Weinberg equilibrium requires the ABSENCE of natural selection (along with no mutation, no gene flow, large population, and random mating).

The high similarity of cytochrome c protein sequences between humans and chimpanzees compared to humans and yeast provides evidence that:

• Cytochrome c has no evolutionary significance because it is found in all organisms
• Humans and chimps share a more recent common ancestor than humans and yeast
• All organisms are genetically identical at the cytochrome c gene
• Molecular data cannot be used to study relationships among organisms

Greater molecular similarity generally indicates a more recent shared common ancestor.

The human appendix, a reduced structure with little current function, is best classified as a:

• Homologous structure
• Analogous structure
• Vestigial structure
• Convergent structure

Vestigial structures are reduced remnants of structures that were functional in an ancestral species.

A new plant species arises instantly from a doubling of chromosome number (polyploidy) in a single generation, without any geographic separation. This is an example of:

• Allopatric speciation
• Sympatric speciation
• Genetic drift only
• Gene flow

Sympatric speciation occurs without geographic isolation; polyploidy is a common mechanism, especially in plants.

Sexual selection, a special case of natural selection based on mating success, can explain the evolution of traits like a peacock's tail because:

• Such traits always improve survival against predators and food shortage
• Such traits arise only through genetic drift and have no effect on mating
• Such traits can increase mating success even if they carry a survival cost
• Such traits prevent gene flow and keep the population from changing

Sexual selection favors traits that increase mating success, which can evolve even when they carry some survival cost (e.g., increased predation risk).

Which statement best describes natural selection?

• Individuals change their traits during life to fit the environment, then pass them on
• Organisms always evolve toward perfect adaptation to their environment
• Individuals with heritable traits that improve survival and reproduction leave more offspring
• All traits are caused by mutations that arise within an individual's lifetime

Natural selection acts on existing heritable variation. Fitness is measured by reproductive success.

In a Hardy-Weinberg population, q² = 0.04. What is the frequency of heterozygotes?

• 0.04
• 0.32
• 0.16
• 0.64

q = 0.2 and p = 0.8, so 2pq = 2 × 0.8 × 0.2 = 0.32.

A small group of individuals starts a new population on an island. This is an example of:

• Gene flow
• Natural selection only
• The founder effect
• Directional selection

The founder effect is a type of genetic drift where the new population has a limited, unrepresentative gene pool.

Antibiotic resistance spreading in bacteria is best explained by:

• Natural selection acting on existing variation
• Bacteria deliberately choosing to adapt when exposed to the drug
• Antibiotics causing the resistance mutations in the bacteria
• Genetic drift acting as the only cause of the change

Resistant bacteria survive treatment and reproduce, so the resistance allele increases in frequency.

Which is an example of postzygotic reproductive isolation?

• Hybrid offspring that are sterile
• Mating at different times of year
• Different courtship songs
• Different habitats

Postzygotic barriers act after fertilization, such as hybrid inviability or sterility. The other choices are prezygotic.

Convergent evolution produces:

• Homologous structures in close relatives sharing a common ancestor
• Identical genomes in species that live in similar environments
• Analogous structures in unrelated species facing similar pressures
• Vestigial organs that have lost their original function

Similar selection pressures lead to similar adaptations in different lineages, such as the wings of bats and insects.

Which would increase gene flow between two populations?

• Building a barrier between the two populations
• Migration of individuals who breed in the new population
• A volcanic eruption that separates the populations
• Stronger sexual selection within each population

Gene flow is the movement of alleles between populations through migration and reproduction.

A bottleneck event usually leads to:

• Increased genetic diversity
• No change in allele frequencies
• Immediate speciation
• Reduced genetic diversity

A sharp reduction in population size removes many alleles by chance, lowering diversity.

Evidence for common ancestry includes all of the following EXCEPT:

• Shared DNA sequences among distantly related species
• Homologous structures such as limbs of different vertebrates
• Organisms always evolving toward complexity
• Transitional forms in the fossil record

Evolution has no direction toward complexity. DNA, anatomy and fossils support common descent.

On a cladogram, species that share a more recent common ancestor are:

• Always more similar in appearance
• Closer together on the tree
• Located on the same branch tip
• Older than other species

Nearness of branching points shows relatedness, not appearance.

Exponential population growth is best modeled by a curve that:

• Rises in a J-shape with no resource limitation
• Levels off at carrying capacity because resources become limited
• Declines steadily over time as resources are used up
• Oscillates around a fixed point as births and deaths balance

Exponential growth produces a J-shaped curve because growth rate is proportional to population size, without resource limits.

Logistic growth differs from exponential growth because logistic growth:

• Never slows down, because births always exceed deaths
• Slows as the population approaches carrying capacity due to limiting factors
• Applies only to bacteria growing in laboratory cultures
• Always produces a J-shaped curve, as growth rate is proportional to size

Logistic growth incorporates carrying capacity (K), producing an S-shaped curve as density-dependent factors slow growth.

Which is a density-dependent limiting factor?

• A hurricane destroying habitat regardless of population size
• A volcanic eruption destroying habitat in the area
• Competition for food intensifying as population density increases
• A seasonal temperature drop unrelated to population density

Density-dependent factors like competition, predation, and disease intensify as population density increases.

In a predator-prey relationship, this interaction is best classified as:

• +/+ (mutualism, in which both species benefit)
• −/− (both species are harmed by the interaction)
• +/0 (commensalism, in which one species is unaffected)
• +/− (predator benefits, prey harmed)

Predation benefits the predator (+) while harming the prey (−).

Only about 10% of energy is transferred between trophic levels mainly because:

• Producers absorb all remaining energy before it reaches the consumers
• Most energy is lost as heat through cellular respiration at each level
• Energy is destroyed at each level, which violates conservation of energy
• Consumers leave most available food uneaten, so little energy is transferred

Metabolic processes (respiration) release most energy as heat rather than passing it on to the next trophic level, following the 10% rule.

Nitrogen-fixing bacteria such as Rhizobium are important because they:

• Break down organic nitrogen compounds into N₂ gas, returning it to the atmosphere
• Live only in the ocean, where they supply nitrogen to algae
• Convert atmospheric N₂ into a usable form (ammonia/ammonium) for plants
• Compete with plants for the available nitrogen in the soil

Nitrogen-fixing bacteria convert unusable atmospheric N₂ into ammonia, which plants can use to build proteins and nucleic acids.

Which biogeochemical cycle has essentially no significant atmospheric gas phase?

• Carbon cycle
• Nitrogen cycle
• Water cycle
• Phosphorus cycle

Phosphorus cycles mainly through rock weathering, soil, and water — it lacks a major atmospheric gas component, unlike C, N, and water cycles.

Two species of birds that feed on different parts of the same tree (one on insects in the bark, one on seeds in the canopy) illustrate:

• Competitive exclusion, in which one species is eliminated from the tree
• Predation, in which one bird species feeds on the other
• Resource partitioning, reducing direct competition
• Parasitism, in which one species harms its host

Resource partitioning (niche differentiation) allows similar species to coexist by using different parts of a shared resource.

An r-selected species is most likely to:

• Produce few offspring, provide extensive parental care, and mature slowly
• Live near carrying capacity, with stable populations and long life spans
• Produce many offspring, provide little parental care, and mature quickly
• Invest heavily in each offspring, grow large, and delay reproduction

r-selected species prioritize rapid reproduction (many offspring, minimal care) suited to unstable or resource-rich environments.

Ocean acidification, caused by increased atmospheric CO₂ dissolving into seawater, primarily threatens:

• Nitrogen-fixing bacteria in the soil, which cannot tolerate acid
• Calcifying organisms such as coral and shellfish
• Freshwater fish only, since lakes are less buffered than oceans
• Terrestrial plants that rely on atmospheric CO₂ for photosynthesis

Increased CO₂ forms carbonic acid in seawater, lowering pH and interfering with calcium carbonate shell/skeleton formation.

Habitat fragmentation reduces biodiversity primarily by:

• Increasing gene flow between populations as patches are connected
• Increasing the carrying capacity of each remaining patch
• Eliminating density-dependent factors so populations grow without limit
• Isolating populations, reducing gene flow, and shrinking available resources

Fragmentation splits habitats into smaller, isolated patches, reducing gene flow and resource availability, threatening population viability.

Net primary productivity (NPP) is calculated as:

• Gross primary productivity minus the energy producers use for their own respiration
• Gross primary productivity plus the energy producers use in respiration
• Total energy stored by organisms at the top trophic level
• The rate of decomposition of dead organic matter in the ecosystem

NPP = GPP − respiration by producers; it represents the energy actually available to consumers.

Which best describes commensalism?

• One species benefits, the other is unaffected
• Both species benefit from the relationship (mutualism)
• One species benefits and the other is harmed (parasitism or predation)
• Both species are harmed by competing for the same resources

Commensalism is a (+/0) relationship — one organism benefits while the other experiences neither benefit nor harm.

A greater number of species and stronger species interactions in an ecosystem generally leads to:

• Decreased ecosystem stability and lower resilience to disturbance
• No change in ecosystem function, because species are interchangeable
• Increased ecosystem stability and resilience to disturbance
• Guaranteed extinction of keystone species due to increased competition

Higher biodiversity is generally associated with greater ecosystem stability and resilience to environmental disturbances.

The competitive exclusion principle states that:

• Two species with identical niches can coexist indefinitely in the same habitat as long as resources are abundant
• Two species that share a habitat will compete until one goes extinct, even if their niches clearly differ from each other
• Two species cannot indefinitely occupy the exact same niche in the same habitat — one will be excluded or niches will diverge
• Predators limit the number of competing species so that no single prey species can dominate the whole community over time

When two species compete for the identical niche and resources, one typically outcompetes the other, or resource partitioning allows coexistence.

A population grows exponentially until it reaches carrying capacity. After that, growth most likely:

• Continues at the same rate
• Becomes negative forever
• Levels off
• Doubles each generation

Limiting factors restrict growth near K, giving the S-shaped logistic curve.

Why are there usually fewer top predators than herbivores in an ecosystem?

• Predators are always larger, so they need more habitat
• Only about 10% of energy passes to the next trophic level
• Herbivores eat predators and so reduce their numbers
• Energy is created at each level and then lost as heat

Most energy is lost as heat and waste, so each level supports less biomass.

A keystone species is one that:

• Is the most abundant species in the community and makes up most of its biomass
• Is the top predator in a food chain and has no natural predators of its own
• Is introduced from another region and spreads rapidly, outcompeting the native species
• Has a disproportionately large effect on its ecosystem relative to its abundance

Removing a keystone species, such as a sea star in a tide pool, can reorganize the community.

Which process returns nitrogen to the atmosphere as N₂?

• Nitrogen fixation
• Denitrification
• Nitrification
• Ammonification

Denitrifying bacteria convert nitrate back to N₂ gas. Nitrogen fixation does the opposite.

Secondary succession differs from primary succession because it:

• Starts on bare rock with no soil present
• Begins with soil already present after a disturbance
• Always results in desert as the end community
• Takes longer than primary succession in the same area

After a fire or farming, soil remains, so succession proceeds faster than on new rock.

A parasite and its host represent which type of interaction?

• +/- where one benefits and the other is harmed
• +/+ where both species benefit (mutualism)
• 0/+ where one benefits and the other is unaffected (commensalism)
• -/- where both are harmed (competition)

Parasites benefit at the expense of the host.

Biomagnification occurs when:

• Energy increases at each higher level of the food chain
• Nutrients are recycled between organisms and the soil
• Producers absorb sunlight to make organic matter
• A toxin becomes more concentrated at higher trophic levels

Persistent toxins like DDT accumulate in tissues, so top predators carry the highest concentrations.

An invasive species often succeeds in a new ecosystem because it:

• Lacks natural predators or competitors there
• Always has a higher mutation rate than native species
• Is native to the area and well adapted to local conditions
• Cannot reproduce, so it does not need to compete

Without the controls of its home range, an invasive species may outcompete native species.

Which would most likely decrease biodiversity in an ecosystem?

• Protection of habitat corridors that connect populations
• Reintroduction of a keystone predator to the ecosystem
• Introduction of a dominant invasive species
• Reducing pollution entering the habitat

Invasive species can displace many natives, reducing species richness and evenness.

Which property of water most directly explains capillary action that helps move water up plant xylem?

• High specific heat
• Low density of ice
• Cohesion and adhesion
• Nonpolarity

Water molecules stick to each other and to xylem walls through hydrogen bonding.

The secondary structure of a protein is stabilized mainly by:

• Disulfide bridges
• Hydrogen bonds between backbone atoms
• Peptide bonds
• Hydrophobic interactions among R groups

Alpha helices and beta sheets form through backbone hydrogen bonding.

Why is a high surface-area-to-volume ratio an advantage for a cell?

• It increases the cell's total volume and so allows it to hold more
• It reduces the need for a membrane around the cell
• It allows more efficient exchange of materials relative to cell volume
• It slows diffusion so that cell contents are not lost

Small cells exchange nutrients and wastes efficiently.

In facilitated diffusion, a solute moves:

• Against its concentration gradient using energy from ATP
• Packaged inside vesicles that fuse with the membrane
• Only across the nuclear envelope through nuclear pores
• Down its concentration gradient through a channel or carrier without ATP

No energy input is required because movement follows the gradient.

During the light reactions, electrons that replace those lost by photosystem II come from:

• Water
• Carbon dioxide
• Glucose
• NADPH

Splitting water releases electrons, protons and oxygen.

Which change would most directly decrease ATP production by oxidative phosphorylation?

• A mutation that increases the activity of ATP synthase in the inner mitochondrial membrane
• Providing the cell with more oxygen so the electron transport chain can pass electrons faster
• A higher concentration of NADH delivered to the electron transport chain from the citric acid cycle
• An uncoupler that dissipates the proton gradient across the inner mitochondrial membrane

ATP synthase relies on the proton gradient.

Cyclin-dependent kinases regulate the cell cycle by:

• Copying DNA during S phase as part of replication
• Phosphorylating target proteins when bound to a cyclin
• Forming the spindle from microtubules during mitosis
• Digesting proteins in the nucleus after they are used

Their activity rises and falls with cyclin levels.

A mutation that inactivates the p53 protein would most likely:

• Cause cells with damaged DNA to stop dividing permanently
• Allow cells with damaged DNA to keep dividing
• Trigger apoptosis in cells that have healthy, intact DNA
• Speed up DNA repair so damage is fixed before division

p53 normally halts the cycle or triggers apoptosis.

A heterozygous (Aa) individual is crossed with a homozygous recessive (aa). Expected phenotypic ratio of offspring:

• 3:1
• 9:3:3:1
• 1:1
• All dominant

Half the offspring are Aa and half are aa.

Two genes on the same chromosome show fewer recombinant offspring than expected under independent assortment because:

• They are located close together and tend to be inherited together
• They are located far apart and are separated by frequent crossovers
• They are on different chromosomes and assort independently of each other
• They code for proteins that are never expressed in the same cell

The closer genes are, the less often crossing over separates them.

A nonsense mutation most likely results in:

• A single amino acid change in the protein
• A premature stop codon and a truncated protein
• A shifted reading frame for all later codons
• No change in the amino acid sequence at all

The mutation converts an amino acid codon into a stop codon.

In the lac operon, when lactose is present:

• Allolactose binds the repressor, which releases the operator, allowing transcription
• Lactose binds the operator directly, which blocks RNA polymerase from transcribing
• Allolactose activates the repressor, which tightens on the operator, stopping transcription
• The repressor is degraded by lactose, which prevents RNA polymerase from binding the promoter

The operon is inducible by lactose.

In a population at Hardy-Weinberg equilibrium, q squared = 0.16. What is the frequency of heterozygotes?

• 0.48
• 0.24
• 0.16
• 0.84

q = 0.4, p = 0.6, and 2pq = 0.48.

Which provides evidence of common ancestry?

• Homologous structures such as the limb bones of vertebrates
• Analogous structures such as the wings of bats and the wings of insects
• Similar body shapes in unrelated species living in the same habitat
• Vestigial traits that are completely absent from every related species

Similar structures with different functions reflect a shared ancestor.

Primary succession differs from secondary succession because primary succession:

• Begins after a forest fire
• Begins with climax species
• Occurs only in water
• Begins in an area with no soil

Pioneer species like lichens create soil first.

About how much energy is typically transferred between trophic levels?

• About 90 percent
• About 50 percent
• About 10 percent
• About 1 percent

Most energy is lost as heat and metabolic processes.