Free AP® Biology Practice Test (2026)

Question 1

A single-base substitution changes one mRNA codon in the middle of a gene's coding sequence from UGG (tryptophan) to UGA, one of the three stop codons. What is the most likely effect on the protein?

Accepted Answer

The polypeptide ends early at that codon and is shorter than normal

Answer

The protein is unchanged, because the genetic code is redundant

Answer

One amino acid is replaced by a different one, and the rest of the chain is normal

Answer

Every amino acid after the mutation is different, because the reading frame shifts

Question 2

In a population in Hardy-Weinberg equilibrium for one gene with two alleles, 16% of individuals show the recessive phenotype. What fraction of the population is heterozygous?

Accepted Answer

0.48

Answer

0.16

Answer

0.36

Answer

0.24

Question 3

A cube-shaped cell grows so that the length of each side doubles. What happens to its surface-area-to-volume ratio?

Accepted Answer

It is halved, so each unit of cytoplasm has less membrane for exchange

Answer

It doubles, so exchange with the surroundings becomes more efficient

Answer

It stays the same, because surface area and volume both increase

Answer

It falls to one-eighth of its original value

Question 4

Producers in a grassland capture 20,000 kcal/m² of energy per year. If about 10% of the energy at each trophic level passes to the next, about how much reaches the tertiary consumers?

Accepted Answer

20 kcal/m² per year

Answer

2,000 kcal/m² per year

Answer

200 kcal/m² per year

Answer

2 kcal/m² per year

Question 5

A chemical makes the inner mitochondrial membrane leaky to H⁺ ions, so protons flow back into the matrix without passing through ATP synthase. Which result is expected in cells exposed to it?

Accepted Answer

Oxygen use continues but ATP output falls, and more energy is lost as heat

Answer

Electron transport stops, because oxygen can no longer accept electrons from the chain

Answer

Glycolysis stops at once, because it depends on the mitochondrial proton gradient

Answer

ATP output rises, because protons now return to the matrix faster than before

Question 6

In winter, a pond freezes at the surface while liquid water stays below the ice. Which property of water best explains why the ice forms on top?

Accepted Answer

Hydrogen bonds hold the molecules of ice in an open lattice, so ice is less dense than liquid water

Answer

Water's high specific heat keeps the bottom of the pond from ever cooling

Answer

Covalent bonds inside water molecules break during freezing, so ice weighs less

Answer

Cohesion keeps the surface layer from mixing with the water underneath

Question 7

A hormone makes liver cells break down glycogen within seconds. The response still happens when the hormone is attached to beads too large to enter the cells, and also when a drug blocks transcription. Which conclusion is best supported?

Accepted Answer

The receptor is on the cell surface, and the response changes the activity of proteins already present

Answer

The hormone must diffuse across the membrane to reach a receptor in the cytoplasm

Answer

The response depends on new mRNA made in the nucleus

Answer

The hormone is a steroid that acts directly as a transcription factor

Question 8

A fruit fly heterozygous for two genes (AaBb, having received A and B from the same parent) is crossed with an aabb fly. The offspring phenotypes are 42% AB, 42% ab, 8% Ab and 8% aB. What is the best conclusion?

Accepted Answer

The genes are linked and about 16 map units apart

Answer

The genes assort independently, because all four phenotypes appear

Answer

The genes are linked and about 8 map units apart

Answer

The genes are on different chromosomes, and some genotypes do not survive

Question 9

In E. coli, the lac operon genes are normally transcribed only when lactose is present. Which mutation would most likely cause them to be transcribed even when lactose is absent?

Accepted Answer

An operator mutation that stops the repressor from binding

Answer

A promoter mutation that stops RNA polymerase from binding

Answer

A repressor mutation that keeps it bound to the operator even when lactose is present

Answer

A mutation that inactivates the gene for the lactose-digesting enzyme

Question 10

Which observation is the best evidence that two organisms inherited a trait from a common ancestor, rather than evolving similar traits independently?

Accepted Answer

Whale flippers and human arms have the same bones in the same order but different uses

Answer

Bats and butterflies both have wings that they use to fly

Answer

Sharks and dolphins both have streamlined bodies and fins

Answer

Cacti in the Americas and some spurges in Africa both store water in thick, spiny stems

Question 11

A plant cell has a solute potential of −0.7 MPa and a pressure potential of +0.3 MPa. It is placed in a solution whose water potential is −0.6 MPa. Which describes the net movement of water?

Accepted Answer

Out of the cell, because the cell's water potential (−0.4 MPa) is higher than the solution's

Answer

Into the cell, because the cell's solute potential is lower than −0.6 MPa

Answer

Into the cell, because the cell's water potential is −1.0 MPa

Answer

No net movement, because the cell wall prevents water from leaving

Question 12

An inhibitor slows an enzyme-catalyzed reaction, but adding a large excess of substrate brings the rate back to nearly its normal maximum. How does the inhibitor most likely work?

Accepted Answer

It competes with the substrate for the active site

Answer

It binds an allosteric site and changes the active site no matter how much substrate is present

Answer

It denatures the enzyme by breaking its hydrogen bonds

Answer

It binds permanently to the active site with covalent bonds

Question 13

Along a rocky coast, sea otters eat sea urchins, and sea urchins graze on kelp. If the otters are removed from the area, which change is most likely?

Accepted Answer

Urchin numbers rise and the kelp forest shrinks

Answer

Urchin numbers fall, because urchins lose a source of food

Answer

Kelp increases, because otters no longer disturb it

Answer

Urchins and kelp stay the same, because otters are at the top of the food chain

Question 14

Dividing cells are treated with a drug that prevents microtubules from assembling. Which outcome is most likely?

Accepted Answer

Cells stall in mitosis with condensed chromosomes, because sister chromatids cannot be pulled apart

Answer

Cells stop in G₁, because DNA cannot be replicated without microtubules

Answer

Cells skip mitosis and divide their cytoplasm, giving daughter cells half the DNA

Answer

Cells copy their DNA twice during the same S phase

Question 15

In a globular protein, a mutation replaces an amino acid with a nonpolar R group, buried in the protein's interior, with one that has a charged R group. Why might the protein stop working?

Accepted Answer

The charged R group disrupts interactions that hold the folded shape, changing the tertiary structure

Answer

The new amino acid breaks the peptide bonds along the backbone

Answer

Only the primary structure changes, so the shape cannot be affected

Answer

The change alters hydrogen bonding between the base pairs of the protein

Question 16

One gene in a mammal produces one protein in liver cells and a related but different protein in muscle cells. Which process most directly explains this?

Accepted Answer

Different combinations of exons are joined when the pre-mRNA is spliced

Answer

Liver and muscle cells use different versions of the genetic code

Answer

Muscle cells lose part of the gene during development

Answer

DNA polymerase copies the gene differently in each tissue

Question 17

Isolated chloroplasts are kept in bright light but given no CO₂. Which change is expected first?

Accepted Answer

ATP and NADPH build up, because the Calvin cycle stops using them

Answer

Oxygen release speeds up, because water no longer competes with CO₂ for electrons

Answer

Sugar production continues, using CO₂ stored inside the thylakoids

Answer

The light reactions stop at once, because CO₂ is their final electron acceptor

Question 18

A storm kills 90% of a beetle population at random, regardless of the beetles' traits. In the survivors, an allele that had a frequency of 0.05 now has a frequency of 0.20. Which process best explains the change?

Accepted Answer

Genetic drift through a bottleneck

Answer

Directional selection in favor of the allele

Answer

Gene flow from a neighboring population

Answer

Mutation creating new copies of the allele

Question 19

Red-green color blindness is an X-linked recessive trait. A woman with normal vision whose father was color-blind has children with a man with normal vision. What is the probability that any one of their sons is color-blind?

Accepted Answer

1/2

Answer

1/4

Answer

0

Answer

1

Question 20

Cells that make a secreted enzyme are given a brief pulse of radioactive amino acids. In what order does the label most likely appear?

Accepted Answer

Rough ER → Golgi complex → secretory vesicles → outside the cell

Answer

Golgi complex → rough ER → secretory vesicles → outside the cell

Answer

Free ribosomes in the cytosol → nucleus → Golgi complex → outside the cell

Answer

Smooth ER → lysosomes → Golgi complex → outside the cell

Question 21

Which of the following is an example of a positive feedback loop?

Accepted Answer

Pressure on the cervix triggers oxytocin release, which strengthens contractions that press harder on the cervix

Answer

Rising blood glucose triggers insulin release, which lowers blood glucose

Answer

A rise in body temperature triggers sweating, which cools the body

Answer

High thyroid hormone levels reduce the pituitary signal that stimulates the thyroid

Question 22

A population grows logistically toward a carrying capacity of 800. At about what population size is it adding new individuals fastest?

Accepted Answer

400

Answer

200

Answer

800

Answer

600

Question 23

An enzyme from a hot-spring bacterium works fastest at 75 °C. Its reaction rate is lower at 40 °C and near zero at 95 °C. What best explains the lower rate at 40 °C?

Accepted Answer

Molecules move more slowly, so enzyme and substrate collide less often

Answer

The enzyme is denatured at 40 °C

Answer

The active site changes shape permanently below 75 °C

Answer

Less substrate can dissolve at lower temperatures

Question 24

During DNA replication, why is one new strand (the lagging strand) built in short fragments?

Accepted Answer

DNA polymerase can add nucleotides only to a free 3′ end

Answer

Helicase can unwind only one parental strand at a time

Answer

The lagging strand is copied by RNA polymerase, which works in short bursts

Answer

Ligase must first remove RNA from the lagging strand

Question 25

On an island, seeds are either very small or very large. Finches with medium-sized beaks handle neither kind well. If beak size is heritable, what is the most likely change over many generations?

Accepted Answer

Small and large beaks both become more common, and medium beaks become rarer

Answer

Medium beaks become most common, because they average the two extremes

Answer

Beak sizes stay the same, because individual finches cannot change their beaks

Answer

All birds come to have large beaks, because large seeds hold more energy

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