How do cells control which genes are expressed and when?
Topic 6.5 Regulation of Gene Expression: explain how gene expression is regulated in prokaryotes and eukaryotes, including operons and regulatory sequences.
A focused answer to AP Biology Topic 6.5, covering the lac and trp operons, promoters, regulatory sequences, transcription factors and epigenetic control, and how regulation lets cells respond to the environment, with a worked operon example.
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What this topic is asking
The College Board (Topic 6.5) wants you to explain how gene expression is regulated in prokaryotes (operons such as lac and trp) and eukaryotes (promoters, regulatory sequences, transcription factors, and epigenetic control), and why regulation lets cells respond to their environment efficiently.
Why regulate gene expression
Prokaryotic operons
Eukaryotic regulation
Try this
Q1. State the difference between an inducible and a repressible operon. [2 points]
- Cue. An inducible operon is normally off and is switched on by its substrate (lac); a repressible operon is normally on and is switched off by its product (trp).
Q2. Explain how DNA methylation regulates gene expression. [2 points]
- Cue. Methylation of DNA usually silences a gene by preventing transcription, without changing the base sequence, and the pattern can be inherited by daughter cells.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2021 (style)4 marksSection II (long FRQ excerpt). In E. coli, the lac operon controls the genes for lactose metabolism. (a) Explain why the lac operon genes are not transcribed when lactose is absent. (b) Predict and explain what happens to transcription of the operon when lactose is added.Show worked answer →
A 4-point explain-and-predict FRQ on the lac operon.
(a) Explain (2 points): (1 point) when lactose is absent, a repressor protein binds the operator, blocking RNA polymerase; (1 point) so the genes are not transcribed, which saves resources because the enzymes are not needed.
(b) Predict and explain (2 points): (1 point) when lactose is present, it (as allolactose) binds the repressor and changes its shape so it can no longer bind the operator; (1 point) RNA polymerase can now transcribe the genes, so the lactose-metabolizing enzymes are produced. This is an inducible system.
Markers reward the role of the repressor and operator and explaining how lactose induces transcription.
AP 2018 (style)1 marksSection I (multiple choice). In eukaryotes, proteins that bind to regulatory DNA sequences and control the rate of transcription are called: (A) ribosomes. (B) transcription factors. (C) tRNAs. (D) restriction enzymes.Show worked answer →
The correct answer is (B).
Transcription factors are proteins that bind to regulatory sequences (such as promoters and enhancers) and increase or decrease the rate at which a gene is transcribed. Ribosomes (A) translate mRNA; tRNAs (C) carry amino acids; restriction enzymes (D) cut DNA in biotechnology.
Related dot points
- Topic 6.6 Gene Expression and Cell Specialization: explain how differential gene expression produces specialized cell types from one genome.
A focused answer to AP Biology Topic 6.6, covering differential gene expression, cell differentiation, the role of signalling and transcription factors, stem cells, and how one genome builds many cell types, with a worked example.
- Topic 6.3 Transcription and RNA Processing: explain how RNA polymerase transcribes a gene into mRNA and how the primary transcript is processed in eukaryotes.
A focused answer to AP Biology Topic 6.3, covering RNA polymerase, the template strand, the differences between transcription and replication, and eukaryotic RNA processing (cap, tail, splicing), with a worked transcription example.
- Topic 6.7 Mutations: explain the types of mutations and how they affect gene products, phenotype and the variation available to a population.
A focused answer to AP Biology Topic 6.7, covering point mutations (silent, missense, nonsense), frameshift mutations, chromosomal mutations, their effects on proteins and phenotype, and their role as the source of new variation, with a worked example.
- Topic 5.5 Environmental Effects on Phenotype: explain how environmental factors can affect the phenotype produced by a given genotype.
A focused answer to AP Biology Topic 5.5, covering how temperature, nutrients, pH and other environmental factors influence phenotype, the genotype-by-environment interaction, and norms of reaction, with a worked example.
- Topic 4.4 Feedback: explain how negative feedback maintains homeostasis and how positive feedback amplifies a response, using examples from cellular and organismal systems.
A focused answer to AP Biology Topic 4.4, covering negative feedback and homeostasis, positive feedback and amplification, set points, and how feedback data are analyzed, with a worked chi-square example.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)