AP Biology / Gene Expression and Regulation
Lesson
Copying the message and reading it
Replication preserves information. Transcription and translation express it.
Learning goals
- Describe complementary base pairing in replication and transcription.
- Use a codon table idea to explain translation.
Explanation
DNA replication uses each strand as a template. New strands form with complementary bases, which is why the process is called semiconservative: each daughter duplex keeps one old strand. Enzymes proofread, but they are not perfect.
Transcription writes an RNA copy of a gene. Translation reads RNA in three-base codons at a ribosome and builds a polypeptide. Several codons can name the same amino acid, so some DNA changes are silent. A frameshift, by contrast, can scramble every codon after the insertion or deletion.
Key terms
- Transcription. Building an RNA copy from a DNA template.
- Translation. Building a polypeptide from an mRNA sequence at a ribosome.
Common mistakes
- Pairing U with T in DNA, or leaving U out of RNA.
- Saying a mutation always changes the protein.
Practice
Original Marlow Works items. Check the answer explanation after you try.
A cell’s surface-area-to-volume ratio is important because it affects:
Original Marlow Works item — not a College Board question.
Take your time—this is practice, not a test.
Answer explanation
Why can a substitution be silent while a one-base insertion rarely is?
Answer. A substitution may still encode the same amino acid; an insertion shifts the reading frame for the rest of the message.
The code is read in triplets. Frame matters more than a single swapped synonym.
Related resources
External links with reuse status. Marlow Works is independent and does not copy restricted exam or textbook material.
Official / link only
AP Biology course page
Official eight-unit framework, science practices, and lab resources.
College Board · All rights reserved · accessed 2026-10-01
Open sourceReference only — do not copy
OpenStax Biology 2e
Biology reference with commercial-use restrictions on the current publisher page.
OpenStax · CC BY-NC-SA 4.0 with additional restrictions · accessed 2026-10-01
Link only unless permission is obtained.
Open source