AP Biology practice questions and exam guide

Which checkpoint primarily monitors DNA damage before the cell commits to division?

1. G1 checkpoint
2. G2 checkpoint
3. M-phase (spindle) checkpoint
4. S-phase initiation

Answer: A — G1 checkpoint

The G1 checkpoint is the major decision point: if DNA damage is detected (often via p53), the cell pauses or enters apoptosis before replicating. G2 (B) verifies completed replication; M checkpoint (C) ensures spindle attachment.

Photosynthesis in plants:

1. Releases CO₂
2. Consumes O₂
3. Occurs in mitochondria
4. Converts light energy into chemical energy stored in glucose — through light-dependent reactions in thylakoids and the Calvin cycle in stroma, releasing O₂

Answer: D — Converts light energy into chemical energy stored in glucose — through light-dependent reactions in thylakoids and the Calvin cycle in stroma, releasing O₂

D) Defining process. A/B/C) Each contradicts.

A keystone species:

1. Is the most numerous species in an ecosystem
2. Is always at the top of the food chain
3. Exerts an outsized effect on community structure relative to its abundance — its removal triggers disproportionate changes (e.g., sea otters and kelp forests)
4. Has no measurable impact on community composition

Answer: C — Exerts an outsized effect on community structure relative to its abundance — its removal triggers disproportionate changes (e.g., sea otters and kelp forests)

C) Defining ecological role. A/B/D) Each is incorrect.

Convergent evolution explains the similarity between unrelated organisms (e.g., dolphin and shark body shapes) because:

1. They are closely related descendants of a common ancestor
2. The similarity is purely random
3. Similar selective pressures (e.g., aquatic locomotion) produce analogous adaptations in lineages that do not share a recent common ancestor — distinct from homology
4. Sexual selection alone drove the resemblance

Answer: C — Similar selective pressures (e.g., aquatic locomotion) produce analogous adaptations in lineages that do not share a recent common ancestor — distinct from homology

C) Defining mechanism. A/B/D) Each is incorrect.

In photosynthesis, which structure contains chlorophyll and is the site of light reactions?

1. Stroma
2. Thylakoid membrane (within the chloroplast)
3. Cell wall
4. Mitochondrion

Answer: B — Thylakoid membrane (within the chloroplast)

Light reactions occur in the thylakoid membrane (chlorophyll, PSII, PSI, ATP synthase). Calvin cycle (CO₂ fixation) occurs in the stroma.

Which best describes a phospholipid in aqueous solution?

1. Fully hydrophobic and excluded from water
2. Fully hydrophilic with no membrane-forming tendency
3. Amphipathic — hydrophilic phosphate head and hydrophobic fatty acid tails, spontaneously forming bilayers
4. Identical to a triglyceride in every respect

Answer: C — Amphipathic — hydrophilic phosphate head and hydrophobic fatty acid tails, spontaneously forming bilayers

C) Amphipathic structure drives bilayer self-assembly. A/B/D) Each contradicts the structure.

Which event during meiosis is the primary source of genetic variation among offspring?

1. DNA replication during S-phase
2. Independent assortment of homologous chromosomes during metaphase I AND crossing over during prophase I
3. Cytokinesis
4. Mitotic division of germ cells

Answer: B — Independent assortment of homologous chromosomes during metaphase I AND crossing over during prophase I

Meiosis generates variation through two main mechanisms: independent assortment (each homologous pair independently aligns at metaphase I, giving 2^23 combinations in humans) and crossing over (homologs exchange segments at prophase I). Both happen in meiosis, not mitosis (D).

In a monohybrid cross between two heterozygotes (Aa × Aa), what is the predicted phenotypic ratio?

1. 1:1
2. 3:1 dominant:recessive
3. 9:3:3:1
4. 1:2:1

Answer: B — 3:1 dominant:recessive

Aa × Aa Punnett square yields genotypes 1 AA : 2 Aa : 1 aa, which gives phenotypes 3 dominant : 1 recessive. (C) is the dihybrid ratio; (D) is the genotypic ratio.

A common misconception about evolution is that it:

1. Has a goal or 'tries' to produce more complex or 'better' organisms — evolution has no foresight; it acts on existing variation through differential reproduction in current environments
2. Acts on populations over generations
3. Depends on heritable variation
4. Can be observed in real time (e.g., antibiotic resistance)

Answer: A — Has a goal or 'tries' to produce more complex or 'better' organisms — evolution has no foresight; it acts on existing variation through differential reproduction in current environments

A) Trick: corrects the teleology misconception. B/C/D) Each is correct about evolution.

In the light-dependent reactions, water is split at:

1. Photosystem II, releasing O₂, electrons (which replace those excited from chlorophyll), and H⁺ contributing to the thylakoid proton gradient
2. Photosystem I
3. The Calvin cycle
4. Mitochondrial complex IV

Answer: A — Photosystem II, releasing O₂, electrons (which replace those excited from chlorophyll), and H⁺ contributing to the thylakoid proton gradient

A) Oxygen-evolving complex of PSII. B) PSI passes electrons to NADP+. C/D) Wrong locations.

A "dominant allele":

1. Is the rarer allele
2. Cannot be inherited
3. Always determines lethality
4. Masks the expression of a recessive allele at the same locus when heterozygous (e.g., Aa expresses the A phenotype)

Answer: D — Masks the expression of a recessive allele at the same locus when heterozygous (e.g., Aa expresses the A phenotype)

D) Defining concept. A/B/C) Each is incorrect.

A peptide bond forms between which two functional groups?

1. The carboxyl group of one amino acid and the amino group of another, with loss of water
2. Two carboxyl groups via a phosphate bridge
3. Two amino groups joined by hydrogen bonds
4. An R-group and a backbone hydroxyl

Answer: A — The carboxyl group of one amino acid and the amino group of another, with loss of water

A) Dehydration synthesis joins −COOH and −NH₂. B/C/D) Misstate the chemistry.

What is incomplete dominance?

Heterozygote shows a blended phenotype (e.g., red × white snapdragons → pink).

What does the Krebs cycle produce per glucose (2 cycles)?

6 NADH, 2 FADH₂, 2 ATP, 4 CO₂.

Why do cells need a high surface-area-to-volume ratio?

Larger surface relative to volume = faster diffusion of nutrients and wastes. Limits cell size and drives folding/microvilli.

How does mitosis differ from meiosis?

Mitosis: 1 division → 2 identical diploid cells (growth/repair). Meiosis: 2 divisions → 4 unique haploid gametes (sexual reproduction).

What polymer is made of glucose for energy storage in animals?

Glycogen (highly branched, stored in liver and muscles).

What is tRNA's role?

Carries specific amino acids; anticodon pairs with mRNA codon at the ribosome.