D2.2.11 (HL)—External factors
External factors regulate gene expression through hormone signalling, transcription factors and bacterial operon responses to substrates or products in cells.
- Syllabus
- First assessment 2025
- Objective
- D2.2.11
- Level
- HL
Hormones and Lactose Can Switch Gene Expression
HL onlyExternal chemical signals can change gene expression in both eukaryotes and bacteria, but the regulatory mechanism depends on the cell type.
| External factor | Mechanism and expression outcome |
|---|---|
| Steroid hormone in a eukaryotic cell | The hormone binds an intracellular receptor; the activated receptor acts with transcriptional regulators at DNA and changes transcription |
| Lactose in E. coli | Without lactose, a repressor binds the lac operator and blocks transcription. Lactose binds and inactivates the repressor, allowing transcription of genes needed for lactose metabolism |
Both mechanisms connect an external condition to selective protein production, so the cell makes a response only when the signal or substrate is present.
The lac operon is an inducible bacterial system and is not the mechanism used by steroid hormones in eukaryotic cells. One biochemical example is sufficient for the syllabus scope.
HL Gene Expression Control
HL only- Transcription factors, promoters, enhancers, activators and repressors control RNA polymerase activity.
- mRNA lifetime limits translation; poly-A shortening and nucleases help remove transcripts.
- DNA methylation and histone modification alter chromatin access without changing base sequence, creating epigenetic patterns during differentiation.
- The genome is all genetic information; the transcriptome and proteome vary with cell type, time and environment.
- Some epigenetic marks persist through cell division or inheritance, although most are reset during gamete formation; genomic imprinting is an exception.
- Hormones regulate eukaryotic transcription through receptors and transcription factors; lac and trp operons illustrate bacterial control.
- Twin studies and environmental exposures help separate genetic, epigenetic and environmental effects on phenotype.
Concept essentials
- Hormones can regulate transcription through receptors and transcription factors.
- The lac operon is induced when lactose prevents repressor binding.
- The trp operon is repressed when tryptophan is abundant.
- External factors alter gene activity without necessarily changing DNA sequence.