Unit 6: Gene Expression and Regulation
AP Biology: 94 practice questions with detailed explanations.
Unit Study Guide
Executive Summary
Unit 6 follows information flow: DNA → RNA → protein, and the controls that decide which genes a cell expresses. Regulation creates cell specialization.
DNA and RNA
DNA: double-stranded, deoxyribose, A–T/G–C, stores information. RNA: single-stranded, ribose, A–U/G–C, transmits and processes it. Both polymerize 5′ → 3′.
Replication
Semiconservative: each daughter double helix keeps one original strand. Helicase unwinds; primase lays RNA primers; DNA polymerase III extends new strands; leading strand is continuous, lagging strand builds Okazaki fragments joined by ligase. Proofreading keeps errors rare.
Transcription and processing
RNA polymerase reads the template strand 3′ → 5′ and builds mRNA 5′ → 3′ at the promoter. In eukaryotes, pre-mRNA gets a 5′ cap, poly-A tail, and splicing — introns removed, exons joined (alternative splicing multiplies protein variety).
Translation
Ribosomes read codons (three bases); tRNA anticodons carry matching amino acids. Initiation at AUG; elongation moves through the ribosome's A, P, E sites; stop codons end the chain. The genetic code is universal and redundant (multiple codons per amino acid).
Regulation
Prokaryotes use operons: the lac operon is inducible (repressor off when allolactose binds), the trp operon repressible (corepressor tryptophan activates the repressor). Eukaryotes layer transcription factors, enhancers, chromatin modification, and RNA processing. Cell specialization = differential gene expression.
Mutations and biotechnology
Point mutations: silent (no change), missense (one amino acid), nonsense (early stop). Frameshifts (insertion/deletion) scramble everything downstream. Biotechnology: PCR amplifies DNA; gel electrophoresis separates fragments by size; bacterial transformation and recombinant DNA produce proteins.