AP Biology, AP exams, 12–16%

Unit 6: Gene Expression and Regulation

Unit 6 follows information from DNA through RNA to protein and then asks how that flow is controlled. Replication, transcription and RNA processing, translation, and the regulation of gene expression are all fair game, along with mutations and the biotechnology techniques used to study them. Interpreting gel electrophoresis and sequencing data is an expected skill.

Position: 6 of 8 units
Published weighting: 12–16% in the official framework
Topics covered: 6 topics in this guide

What is covered

Topics inside this unit.

These topics describe the scope students most often review for this part of AP Biology. Treat them as a study checklist rather than an exhaustive list of what a single exam form can sample.

DNA and RNA structure: Nucleotide composition, antiparallel strands, and the structural differences between DNA and the RNA types.
Replication: Semiconservative replication, the enzymes involved, and why leading and lagging strands are synthesized differently.
Transcription and RNA processing: Promoters, RNA polymerase, and the capping, splicing, and tailing that convert pre-mRNA into mature mRNA.
Translation: Ribosome structure, transfer RNA, the codon table, and the initiation, elongation, and termination stages.
Regulation of gene expression: Operons in prokaryotes, transcription factors and epigenetic modification in eukaryotes, and cell specialization.
Mutations and biotechnology: Point, frameshift, and chromosomal mutations, plus polymerase chain reaction, gel electrophoresis, and transformation.

Study tips

How to practice Gene Expression.

Section-specific guidance that follows from how this material is usually tested.

Practice reading the codon chart quickly in both directions; it is provided, so speed rather than memorization is the skill.
Trace one mutation from DNA change to amino acid change to protein function to phenotype in a single written chain.
Rehearse interpreting a gel: what the lanes represent, what the ladder does, and what fragment size implies.

Where this fits

Unit 6: Gene Expression and Regulation is 6 of 8 units in the AP Biology guide, and it carries a published weighting of 12–16% in the official framework. Use the full guide to balance it against everything else you have to review.

Back to the AP Biology guide Official exam information

Sample Gene Expression questions

Practice questions, not yet expert-reviewed

Original practice questions written to the published framework for this section. No expert has reviewed them, and they are not taken from any official exam. Try each one before you open the answer.

Question 1Foundational
What is the name of the process that uses one strand of DNA as a template to build a complementary RNA molecule?
1. ATranslation
2. BTranscription
3. CReplication
4. DReverse transcription
Show answer
Answer: B. Transcription
RNA polymerase reads the template strand and assembles a complementary RNA molecule, a process called transcription. Translation is the tempting distractor, but it happens later at the ribosome and converts the RNA message into a chain of amino acids. Replication copies DNA into DNA rather than into RNA.
Key insight: Transcription changes the language from DNA to RNA; translation changes it from RNA to protein.
Question 2Foundational
How many consecutive messenger RNA nucleotides specify a single amino acid during translation?
1. AOne
2. BTwo
3. CFour
4. DThree
Show answer
Answer: D. Three
A codon is a set of three nucleotides, and each codon specifies one amino acid or a stop signal. Two nucleotides would give only 16 combinations, too few for the 20 amino acids, while three gives 64 combinations. The extra combinations make the code redundant, so several codons can specify the same amino acid.
Key insight: Sixty-four codons for twenty amino acids is why the genetic code is redundant but not ambiguous.
Question 3Intermediate
Why is one strand at a DNA replication fork synthesized discontinuously as a series of short fragments?
1. ADNA polymerase can add nucleotides only to a free 3-prime end
2. BThat template strand is shorter than the other template strand
3. CHelicase unwinds that side of the helix more slowly
4. DPrimase is unable to make a primer on the other strand
Show answer
Answer: A. DNA polymerase can add nucleotides only to a free 3-prime end
Because synthesis runs only in the 5-prime to 3-prime direction, the strand whose template runs the wrong way relative to the moving fork must be built in short pieces that point away from the fork. Those pieces are Okazaki fragments, later sealed by ligase. The two template strands are the same length, so a length difference cannot be the cause.
Key insight: Every replication oddity, including Okazaki fragments and primers, follows from strict 5-prime to 3-prime synthesis.
Question 4Intermediate
In a prokaryotic operon, what is the name of the DNA sequence that a repressor protein binds in order to block transcription?
1. AThe promoter
2. BThe operator
3. CThe terminator
4. DThe regulatory gene
Show answer
Answer: B. The operator
The repressor binds the operator, which sits between the promoter and the structural genes and physically obstructs RNA polymerase. The promoter is the tempting distractor, but that is where RNA polymerase itself binds. The regulatory gene is a separate gene that codes for the repressor protein rather than being the site the repressor binds.
Key insight: Promoter is where polymerase binds; operator is where the repressor binds.
Question 5Advanced
A single eukaryotic gene produces several different polypeptides in different tissues even though the DNA sequence is identical in every cell. Which RNA processing mechanism best explains this?
1. ASemiconservative replication
2. BMethylation of the promoter
3. CAlternative splicing
4. DA nonsense mutation in the gene
Show answer
Answer: C. Alternative splicing
During processing, different combinations of exons can be retained in the mature messenger RNA, so one gene yields several related proteins. Promoter methylation is the tempting distractor, but it silences a gene rather than diversifying its products. A nonsense mutation would change the DNA sequence, which the question rules out.
Key insight: One gene can encode many proteins because exons are assembled in more than one combination.

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Unit 6: Gene Expression and Regulation