AP Biology Unit 6 review: Gene Expression and Regulation
Unit 6 follows information from DNA through RNA to proteins and explains how cells regulate that flow. It is 12–16% of multiple choice. Expect to connect a sequence or regulatory change to measured RNA, protein or phenotype, rather than assume every mutation has the same effect.
Unit 6 on the May 2027 exam
| 2027 exam | |
|---|---|
| Weight | 12–16% of the multiple-choice section |
| Exam date | Monday, May 3, 2027, afternoon session |
| Multiple choice | 60 questions in 90 minutes, worth 50% of the exam score |
| Free response | 6 questions in 90 minutes, worth 50%: two long questions (9 points each) and four short questions (4 points each). |
| Calculator | Both sections allow a four-function calculator with square root or a nongraphing scientific calculator. Bluebook includes Desmos scientific. Handheld graphing calculators are not allowed in 2027. |
| Class time | About 18–20 class periods in College Board's pacing guide |
| 2026 results | 15% of students scored a 5 on the whole exam, and about 71% scored 3 or higher |
Sources: College Board's course and exam description, exam page, 2027 exam dates and score distributions.
What Unit 6 covers
College Board's course description splits the unit into 8 topics. The second column is what you should be able to do for each one.
| Topic | What you should be able to do |
|---|---|
| 6.1 DNA and RNA Structure | Relate complementary pairing and antiparallel strands to information storage and copying. |
| 6.2 DNA Replication | Explain semiconservative copying, synthesis direction and the roles of replication enzymes. |
| 6.3 Transcription and RNA Processing | Read a DNA template and explain transcription, splicing and other eukaryotic RNA processing. |
| 6.4 Translation | Use an mRNA codon sequence to predict a polypeptide and the effects of a changed reading frame. |
| 6.5 Regulation of Gene Expression | Predict how promoters, transcription factors and regulatory sequences alter gene expression. |
| 6.6 Gene Expression and Cell Specialization | Explain how cells with the same genome produce different proteins and specialized functions. |
| 6.7 Mutations | Distinguish substitutions, insertions, deletions and chromosome changes and evaluate their possible effects. |
| 6.8 Biotechnology | Interpret PCR, gel electrophoresis, sequencing and genetic-engineering evidence. |
Locate a change in the DNA-to-protein pathway
Use measurements at successive stages to narrow the explanation. Equal DNA copy number does not guarantee equal expression.
| Change | First likely effect | What to measure |
|---|---|---|
| Promoter mutation | Altered transcription initiation | Amount of primary RNA transcript. |
| Splice-site mutation | Different retained or removed RNA segments | Mature mRNA length and sequence. |
| Synonymous coding substitution | Same encoded amino acid | Check the codon before predicting a protein sequence change. |
| Missense substitution | One encoded amino acid changes | Protein structure or activity; effects depend on location. |
| Early stop codon | Shorter translated product | Protein size and activity. |
| One-base coding deletion | Reading frame shifts downstream | Regroup downstream bases into triplets. |
| Translation inhibitor | Less new protein despite existing mRNA | Compare RNA abundance and new protein production. |
Common mistakes
Each of these loses points on questions you otherwise understand.
- Treating gene and allele as synonyms. An allele is a version of a gene. The CED identifies this confusion on printed p. 109.
- Calling every base substitution a frameshift. A substitution changes a base without shifting triplet boundaries. CED p. 109 and topic 6.7.
- Saying a mutation denatures a protein. Explain whether sequence, amount or activity changes; denaturation describes loss of folded structure. CED p. 109.
- Assuming all mutations are harmful. A change can be neutral, harmful or beneficial in its environment. Predict from the sequence and evidence. CED p. 109.
6 practice questions with answers
Written by Ashore for this page, in the style of the multiple-choice section. Try each one before you open the answer.
Question 1 · 6.3
A DNA template segment reads 3′-TAC GGA CTT-5′. Which RNA sequence is transcribed?
- A5′-AUG CCU GAA-3′
- B5′-UAC GGA CUU-3′
- C5′-ATG CCT GAA-3′
- D5′-GAA CCU AUG-3′
Show the answer
A. RNA is complementary and antiparallel to the template. Reading the template 3′ to 5′ produces RNA 5′-AUG CCU GAA-3′. RNA uses U instead of T.
Question 2 · 6.4
An mRNA coding region has 246 nucleotides, including one terminal stop codon and no untranslated bases. How many amino acids does it encode?
- A80
- B82
- C81
- D246
Show the answer
C. 246/3 = 82 codons. The terminal stop codon recruits release factors rather than an amino acid, leaving 81 amino acids. The start codon does encode an amino acid.
Question 3 · 6.7
A codon changes from GAA to GAG; both encode glutamate. What change is directly predicted?
- AA shift in every following reading frame
- BNo change in the amino acid at this position
- CDeletion of the next amino acid in the chain
- DTermination at this position in the chain
Show the answer
B. Both codons specify the same amino acid. This is a synonymous substitution. It does not by itself establish that all aspects of RNA processing or expression remain unchanged, but the encoded residue is unchanged.
Question 4 · 6.5
A promoter mutation is followed by 70% less mRNA and less protein, while DNA copy number is unchanged. Which explanation fits best?
- AIncreased DNA replication at the gene
- BIncreased export of the protein from the cell
- CReduced breakdown of existing mRNA
- DReduced transcription of the affected gene
Show the answer
D. A promoter controls transcription initiation. Reduced transcription can explain less mRNA and then less protein without changing gene copy number. Protein export alone does not explain reduced mRNA.
Question 5 · 6.8
Starting with 5 target DNA molecules, ideal PCR doubles the target each cycle. How many molecules are present after 6 cycles?
- A30
- B160
- C320
- D640
Show the answer
C. Ideal amplification gives 5 × 2⁶ = 320 molecules. Six cycles means six doublings; multiplying by six would assume a fixed linear increase.
Question 6 · 6.8
Two linear DNA fragments of 250 and 900 base pairs run on the same agarose gel. Which result is expected?
- AThe 250-base-pair fragment moves farther toward the positive electrode
- BThe 900-base-pair fragment moves farther toward the positive electrode
- CBoth fragments move toward the negative electrode at the same speed
- DBoth fragments remain at the wells because DNA has no net charge
Show the answer
A. DNA has a negatively charged phosphate backbone and moves toward the positive electrode. Under the same gel conditions, smaller linear fragments pass through the matrix more readily and travel farther.
How to study Unit 6
Practice writing a complementary strand with both ends labeled. Transcribe a short template, divide the RNA into codons and translate using a supplied codon table. Repeat after a substitution and a one-base deletion to separate a changed residue from a shifted reading frame.
For a regulation experiment, build three columns: DNA amount, RNA amount and protein amount or activity. State which observations support transcriptional, post-transcriptional or translational control. For a gel, label the wells, electrode directions and fragment sizes before interpreting any lane.
Ashore's Biology course has a short lesson for every topic in this unit, each ending with a few questions on what it taught.
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