AP Biology — Unit 6: Gene Expression and Regulation

What AP Biology Unit 6 covers

The College Board Course and Exam Description breaks Unit 6 into 8 topics:

1. 6.1 DNA and RNA Structure
2. 6.2 DNA Replication
3. 6.3 Transcription and RNA Processing
4. 6.4 Translation
5. 6.5 Regulation of Gene Expression
6. 6.6 Gene Expression and Cell Specialization
7. 6.7 Mutations
8. 6.8 Biotechnology

AP Biology Unit 6 practice questions

• DNA and RNA both carry genetic information using nucleotide bases — so why can't RNA just replace DNA as the permanent storage molecule?

  RNA is single-stranded and less stable than DNA. Its uracil and single strand can't self-correct like DNA's double helix with thymine.

• A drug intercalates between DNA base pairs and disrupts hydrogen bonding. What happens to the double helix?

  The double helix unwinds and loses integrity. Hydrogen bonds between complementary bases (A-T, G-C) held the two strands together.

• Your friend says mRNA, tRNA, and rRNA all do the same job because they're all just 'RNA.' What are they missing?

  Each RNA type has a distinct role, not the same job. mRNA carries the message, tRNA brings amino acids, and rRNA builds the ribosome.

• A drug blocks helicase. DNA replication begins but then stalls. Trace exactly where the process breaks down.

  Helicase unwinds and separates the helix at the fork. Without it, the strands can't part, so polymerase has no single-stranded template.

• DNA polymerase can only add nucleotides in the 5' to 3' direction — so how does the lagging strand get copied at all?

  The lagging strand is built discontinuously as short Okazaki fragments. Each runs 5' to 3' away from the fork, then ligase joins them.

• DNA polymerase needs an RNA primer before it can start synthesizing. Your friend says that's a design flaw. What's the real reason?

  DNA polymerase can only extend a strand, not start one. Primase lays a short RNA primer with a free 3'-OH for it to begin.

• Semiconservative replication sounds like DNA only keeps half of itself. What actually gets conserved?

  Each daughter DNA has one parental strand and one new strand — half old, half new. Both molecules are conserved, just split.

• A mutation destroys the promoter region of a eukaryotic gene. RNA polymerase is still present and functional. What happens?

  Transcription can't initiate without a functional promoter. RNA polymerase has nowhere to bind, so no mRNA is made even with the enzyme present.

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Key terms in Unit 6

These 12 terms show up in the Unit 6 cards. Each one links to its definition in the AP Biology key-term reference.

• Bacterial transformation
• Differential gene expression
• DNA polymerase
• DNA replication
• Frameshift mutation
• Gel electrophoresis
• Gene expression
• Lagging strand
• Nonsense mutation
• Semiconservative replication
• Silent mutation
• Transcription factor