AP Biology Unit 6 Practice Test: Gene Expression MCQs (Free PDF)

Here's a free AP Biology Unit 6 practice test: 20 AP-style MCQs on gene expression and regulation, seven data-based stimulus sets across topics 6.1 through 6.8, and a full rationale for every answer, all aligned to the 2025 CED.

Below, three mistakes I see students make on Unit 6 and how to avoid them, plus one worked example so you can see the reasoning.

Unit 6 is one of the three heaviest units on AP Biology at 12-16% of the exam (see the full 8-unit breakdown for weightings on the rest), and it's also one of the most stimulus-dense: DNA-to-mRNA-to-protein sequence tables, operon activity tables, restriction digest fragment sizes, tissue-specific expression grids.

AP Biology's Section I is stimulus-heavy (there's a full MCQ strategy walkthrough if you want the general playbook), and Unit 6 stimuli tend to be some of the most technical on the paper. You'll see:

• Short DNA sequences that you have to transcribe and translate
• lac and trp operon activity tables under different sugar or amino-acid conditions
• Restriction digest fragment tables where you infer cut positions from fragment sizes
• Tissue-specific expression grids that split transcription from translation across cell types
• Mutation classification tables covering silent, missense, nonsense and frameshift changes

The practice pack mirrors that spread: 15 of the 20 questions sit on a data-table stimulus. There's a Meselson-Stahl replication table, a full DNA-mRNA-peptide table where you check strand direction and reading frame, a tissue-specific expression grid across four cell types, a mutation-effect table, an EcoRI restriction digest, and both lac and trp operon activity data.

If you can read those stimulus types confidently, most of Unit 6 becomes pattern recognition.

Every year I see students walk into Unit 6 MCQs making the same three moves. All three show up in recent Chief Reader Reports, so spot them before the exam does.

The 2023 Chief Reader Report Q6 flagged this as a top Unit 6 misconception. Students see "different cell types express different proteins" and answer "different mutations". That's wrong.

Every somatic cell in your body has essentially identical DNA; a liver cell and a neuron have the same genome. What differs is which genes are expressed, and that's controlled by different sets of transcription factors turning on cell-type-specific genes. The same report also flagged the related error that promoters and enhancers differ between cell types (they don't; the transcription factors that bind them do).

The correct causal chain to keep in your head: same DNA → different transcription factors present → different genes expressed → different proteins made → different cell function.

The 2025 Chief Reader Report Q6 called this out explicitly: "A change in genotype/mutation results in a nonfunctional protein/phenotype" was the flagged misconception. In practice mutations run on a spectrum:

• Silent mutations change a codon to a synonym (same amino acid, no protein change).
• Missense mutations swap one amino acid, which may have no effect, reduced function, or complete loss depending on where in the protein it lands.
• Nonsense mutations introduce a premature stop codon and are usually severe.
• Frameshift mutations (insertions or deletions not in multiples of 3) usually destroy the protein.

On MCQs about heterozygotes (wild-type paired with a mutant allele), remember that reduced-function proteins can still support a close-to-normal phenotype, which was exactly what the 2025 Q6 was testing.

The 2023 Chief Reader Report flagged this bluntly: "Operons are found in eukaryotes. In fact, operons are a prokaryotic gene regulatory strategy." Students memorise the lac and trp operons for Unit 6 and then apply the same model to human cells on MCQs.

Eukaryotic gene regulation uses a different toolkit:

• Enhancers and silencers (which can be far from the gene)
• Chromatin remodelling (histone acetylation opens DNA, methylation typically closes it)
• Alternative splicing
• Post-transcriptional controls like miRNA

If an MCQ describes a human, animal or plant cell regulatory question and any answer choice mentions "operon" or "repressor binding to operator", that answer is wrong.

Here's a question straight from the Unit 6 pack. The stimulus: the lac operon in E. coli codes for enzymes that break down lactose. β-galactosidase activity was measured under four conditions.

The stem: 'Which claim about the lac operon is best supported by comparing conditions 2 and 4 in Table 2?'

• (A) Lactose alone induces the operon; glucose is required for full activity.
• (B) Glucose fully activates the operon on its own without any lactose.
• (C) Lactose has no effect on operon expression at all.
• (D) Glucose lowers operon expression even when lactose is present in the medium.

Answer: D. Condition 4 (lactose only) gives 100% activity, but condition 2 (lactose and glucose) gives only 6%. Adding glucose sharply reduces expression even though the inducer lactose is still there, matching catabolite repression: high glucose lowers cAMP, so CAP cannot activate the operon. A reverses the effect of glucose. B is contradicted by condition 1 (glucose alone, activity is 1%). C is contradicted by the change from condition 3 (1%) to condition 4 (100%), which shows lactose does affect expression.

On any translation MCQ that gives you a DNA sequence, remember the two-step rule: the template strand is read 3' to 5' by RNA polymerase, and the mRNA is built 5' to 3'. Then ribosomes read the mRNA 5' to 3' in codons. Students constantly reverse this and use the coding (nontemplate) strand as if it were the template.

A reliable move:

1. Find the promoter or the 3' end of the template.
2. Transcribe to mRNA in the 5' to 3' direction.
3. Find the AUG start codon.
4. Translate three bases at a time until you hit a stop codon (UAA, UAG, UGA).

Related translation-rate errors show up in the 2024 Chief Reader Report Q6, where students misread ms/codon histograms and confused tRNA abundance with translation rate.

When an MCQ shows a mutation, immediately ask whether it shifts the reading frame.

• Insertions or deletions of 1 or 2 nucleotides are frameshifts and are usually catastrophic downstream of the mutation site.
• Insertions or deletions of 3 nucleotides (or multiples of 3) don't shift the frame and just add or remove one amino acid.
• Point mutations are single-base swaps and end up silent, missense or nonsense.

If the stem asks "which mutation has the biggest effect on protein function", frameshift and nonsense (early stop) are your top candidates. Silent is almost always the least severe.

Unit 6.8 biotech questions typically describe a technique and ask what it does, or describe a goal and ask which tool to use. Match by purpose:

• PCR amplifies a specific DNA region using primers and Taq polymerase (needs heat cycles).
• Gel electrophoresis separates DNA fragments by size, and smaller fragments run faster and further from the well.
• Restriction enzymes cut DNA at specific recognition sequences.
• CRISPR edits a specific sequence using a guide RNA.
• DNA sequencing determines base order.

Recognising the tool from the purpose lets you skip past four descriptions and go straight to the matching answer.

A 3-pass method works well for a unit-length pack like this one.

• First pass, untimed: give yourself about 90 seconds per question, work through all 20, and use the rationales to understand every question you got wrong or guessed on. This is a learning pass, not a scoring pass.
• Second pass, timed: 30 minutes for 20 questions matches the AP pace of 1.5 minutes per MCQ. Circle any answer you're not confident on and move on. This is where you build exam pacing.
• Third pass, a week later: retry the questions you missed on the first two passes without looking at the answers, and see whether the reasoning has stuck.

Once you're comfortable with the MCQ format, our FRQ guide covers the mechanism-language patterns the graders reward on the essay section.