AP Biology Unit 6: Gene Expression and Regulation

DNA is double-stranded and stores genetic instructions. RNA is usually single-stranded and helps cells use those instructions during gene expression. Nucleotides include a sugar, phosphate group, and nitrogenous base. Complementary base pairing supports accurate replication and transcription. DNA uses thymine; RNA uses uracil instead.

For nucleotide parts, base-pairing rules, and DNA vs RNA differences, use the DNA and RNA structure study guide.

Replication happens before cell division. It is semiconservative: each new DNA molecule has one original strand and one newly built strand. Helicase separates strands, DNA polymerase builds complementary strands, and ligase joins fragments on the lagging strand.

For semiconservative replication, enzymes, leading and lagging strands, and practice questions, open the DNA replication AP Biology guide.

RNA polymerase reads the DNA template strand and builds a complementary RNA strand. mRNA carries the message to ribosomes. In eukaryotes, RNA processing adds a 5′ cap, a poly-A tail, and removes introns while keeping exons.

For a full comparison of where transcription and translation happen, what each process makes, and how students confuse them, use the transcription vs translation guide.

Ribosomes read mRNA codons. tRNA brings amino acids, and anticodons pair with codons. Amino acids link into a polypeptide that folds into a protein whose shape and function can affect phenotype.

Cells do not express every gene all the time. Gene regulation saves energy and allows cell specialization. Prokaryotes often use operons; eukaryotes regulate expression at many points, including transcription factors that increase or decrease transcription.

For activators, repressors, prokaryotic vs eukaryotic control, and practice questions, use the gene regulation AP Biology guide.

Operons are groups of genes controlled together through a promoter, operator, repressor, and regulatory gene. The lac operon responds to lactose availability; the trp operon responds to tryptophan availability.

For promoter, operator, repressor logic, lac and trp overviews, and practice questions, use the operons AP Biology guide.

Most cells in an organism share the same genome but express different genes. Different expression patterns produce different proteins and therefore different cell structures and functions. A neuron, muscle cell, and pancreatic cell can share the same genome but express different genes.

For the full reasoning ladder, worked examples, and practice questions, use the gene expression and cell specialization AP Biology guide.

Mutations are changes in DNA sequence. Some have no effect; some alter proteins; some affect regulation. Mutations create genetic variation that natural selection can act on when variation is inherited.

Important types include point mutations, insertions, deletions, frameshift mutations, and silent, missense, and nonsense mutations.

For mutation types, the reasoning ladder, practice questions, and FRQ tracing strategy, use the mutations AP Biology guide.

When discussing variation and selection, connect to Unit 7 Natural Selection and Hardy-Weinberg equilibrium.

Biotechnology uses tools to analyze or manipulate DNA. PCR amplifies DNA, gel electrophoresis separates fragments by size, plasmids can move genes into bacteria, recombinant DNA combines sequences from different sources, and DNA sequencing identifies base order.

Viruses and bacteria matter in Unit 6 because they show how genetic information can be copied, expressed, mutated, transferred, and studied. Bacteria are especially useful in biotechnology because plasmids can carry genes. Viruses matter because viral genomes and replication cycles help explain genetic variation and infection.

Use the viruses and bacteria hub, the AP Biology viruses guide, and the AP Biology virus review for deeper practice.

FRQ scoring checklist

1. Identify the molecule or process involved
2. Trace DNA → RNA → protein when relevant
3. Explain how a mutation or regulatory change affects the product
4. Use the data table, graph, sequence, or gel pattern
5. Connect molecular change to phenotype or cell function
6. Avoid unsupported claims like “the organism evolves” unless population-level data is given

Mini FRQ scenarios with strong answers

For 10 full AP-style FRQ prompts with scoring checklists, model answer bullets, and common mistakes, use the AP Biology Unit 6 FRQ practice page.

For 45 full MCQs with topic filters, score tracking, and answer explanations, use the AP Biology Unit 6 practice questions page.