6.5Gene Expression and Regulation

Regulation of Gene Expression — AP BIOLOGY Revision Notes & Topical Questions

5 min read 11 parts Lesson 6 of 9 2 quizzes

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6.5 Regulation of Gene Expression — Notes — AP Biology notes: Describe the types of interaction that regulate gene expression in prokaryotes and eukaryotes.Importance — the genes a cell expresses determine its structure and function, and regulation controls when, where and how mu…

Transcription factor
Epigenetic change
RNA interference
Proto-oncogene
Energy saving — a bacterium does not need lactose-digesting enzymes when there is no lactose
Main control point — transcription, which depends on regulatory DNA sequences and regulatory proteins

Section 1 of 11Why and how gene expression is regulated

Importance — the genes a cell expresses determine its structure and function, and regulation controls when, where and how much of a product is made.

Energy saving — a bacterium does not need lactose-digesting enzymes when there is no lactose.

Main control point — transcription, which depends on regulatory DNA sequences and regulatory proteins.

Regulatory proteins — bind specific DNA base sequences to switch genes on or off, e.g. transcription factors.

Repressors bind DNA and block transcription (negative control).
Activators bind DNA and increase transcription (positive control).

Regulatory sequences — DNA sequences that regulatory proteins bind, e.g. promoters (next to the gene, bind RNA polymerase), operators (bind repressors) and enhancers (act at a distance).

Allosteric proteins — binding of a small molecule (a sugar or hormone) changes the shape and DNA-binding ability of a regulatory protein, linking expression to conditions.

Other levels — epigenetic changes, small RNAs, alternative splicing, mRNA lifespan, translation and post-translational modification.

Section 2 of 11Prokaryotic operons

Prokaryotic operons

Operon = a cluster of prokaryotic genes with related functions under one promoter and operator, transcribed as a single mRNA.

Coordinate regulation — groups of genes are switched on or off together in response to one signal.

Components — structural genes (enzymes), a regulatory gene (repressor), a promoter (RNA polymerase binding) and an operator (regulatory protein binding).

Section 3 of 11The lac operon (inducible)

Function — controls the enzymes for lactose uptake and digestion, including beta-galactosidase (lacZ), and is off unless induced by lactose.

Lactose absent — the regulatory gene makes a repressor that binds the operator, blocks RNA polymerase and no enzymes are made.

Lactose present — allolactose (the inducer) binds the repressor, changes its shape and releases the operator, so RNA polymerase transcribes the genes.

Positive control — when glucose is low, cAMP rises and CAP–cAMP binds near the promoter, increasing RNA polymerase binding,

so lactose is used only when glucose is unavailable.

Section 4 of 11The trp operon (repressible)

The trp operon (repressible)

Function — controls the enzymes that make tryptophan and is on unless the product is abundant.

Tryptophan absent or scarce — the repressor cannot bind the operator, so transcription proceeds.

Tryptophan present — it acts as a corepressor, binds and activates the repressor,

which binds the operator and switches transcription off,
so energy is not wasted.
- (feedback control of a biosynthetic pathway)

Section 5 of 11Eukaryotic regulation

Gene organisation — generally not in operons; each gene has its own promoter and there are more levels of control.

Chromatin structure — tightly wound heterochromatin is hard to transcribe, loose euchromatin is accessible.

Histone acetylation loosens chromatin and increases transcription.
DNA methylation, typically at cytosine, usually silences genes and is an epigenetic mark passed on when cells divide.

Transcription factors — general factors bind the promoter and specific factors bind enhancers,

which can be far upstream, downstream or within introns;
- DNA bending brings enhancer activators to the promoter complex.
Activators aid RNA polymerase binding and repressors prevent it.

Tissue-specific control — combinations of factors give precise control.

Coordinate regulation — genes sharing regulatory sequences or a shared transcription factor can be regulated together even when on different chromosomes (e.g. heat-shock protein genes), unlike prokaryotic operons.

Section 6 of 11Post-transcriptional and later control

Post-transcriptional and later control

Small RNAs — microRNAs and siRNAs bind complementary mRNA after transcription, causing breakdown or blocking translation (RNA interference),

so less protein is made even though the gene was transcribed.

Alternative splicing and mRNA stability (cap and poly-A tail) change which proteins are made and for how long.

Translational and post-translational control — translation initiation can be blocked,

and proteins may be activated by cleavage or phosphorylation or degraded after tagging with ubiquitin.

Section 7 of 11Epigenetics and misregulation

Epigenetic changes — reversible modifications to DNA or histones; the marks are copied when cells divide,

giving stable but reversible control, and diet, stress or toxins can influence them.

X chromosome inactivation — one X in each female mammal cell becomes condensed and silent early in development; the choice is random,

so females are mosaics.
- (orange and black patches of tortoiseshell cats)

Genomic imprinting — certain genes are expressed only from the allele inherited from one parent.

Inheritance — some marks pass through a few generations but most are erased in gametes.

Proto-oncogenes — stimulate cell division and become oncogenes through mutation or overexpression.

Tumour suppressors — genes such as p53 halt the cell cycle or trigger apoptosis, and losing both alleles removes the brake.

Regulatory change and evolution — altering when and where a gene is expressed can change body plans (e.g. limb length or beak shape) while leaving protein structure intact,

and such mutations can be selected.

Section 8 of 11Operons and prokaryotes compared

Feature	lac operon	trp operon
Genes code for	Lactose digestion enzymes	Tryptophan synthesis enzymes
Default	Off	On
Repressor	Active alone	Inactive alone
Effect of signal	Lactose inactivates repressor, so on	Tryptophan activates repressor, so off
Type	Inducible (catabolic)	Repressible (anabolic)

Prokaryotes and eukaryotes compared:

Feature	Prokaryotes	Eukaryotes
Gene organisation	Operons, one mRNA for several genes	Individual genes with own promoters
Main control	Repressors and activators at operator or promoter	Chromatin state, transcription factors, enhancers
mRNA processing	None	Capping, tailing, splicing
Transcription and translation	Coupled in the same space	Separated by the nuclear envelope

Section 9 of 11More from the specification

Section 10 of 11Key terms

Term	Meaning
Operon	group of genes under one promoter and regulated together
Operator	DNA site where a repressor binds
Repressor	protein that binds DNA and reduces transcription
Inducer	molecule that inactivates a repressor, turning an operon on
Transcription factor	protein that aids or blocks RNA polymerase binding
Enhancer	distal regulatory sequence bound by transcription factors
Epigenetic change	reversible modification of DNA or histones
RNA interference	mRNA breakdown or translation block by small RNAs
Proto-oncogene	gene that stimulates cell division, e.g. before mutation to an oncogene

Section 11 of 11Exam technique

Saying lactose binds the operator — the inducer binds the repressor.

Saying acetylation silences genes — histone acetylation increases transcription.

Thinking eukaryotes use operons — eukaryotic genes are individually regulated.

Mixing up operator and promoter — the promoter binds RNA polymerase and the operator binds the repressor.

Confusing inducible and repressible — lac is inducible (off unless lactose) and trp is repressible (on unless tryptophan).

Last updated: 15 October 2026