4.5.2Additional lessons
Regulating Transcription and Translation — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 48 of 101
Part 1 of 5Gene Expression
Cells save energy and gain specialised roles by switching genes on and off.
Definition
- Gene expression is the process by which the information in a gene is used to make a product that affects the phenotype.
Main stages
- Expression usually involves transcription of DNA into RNA and, for protein-coding genes, translation of mRNA into a polypeptide.
Selective use
- Every body cell holds the same genes, but each cell type expresses only those that match its function.
Regulation
- Control can occur at transcription, mRNA processing, translation and protein activity, but transcription is the main control point.
Part 2 of 5Regulating Transcription
Different combinations of transcription factors in different cells give each cell type its own pattern of expression.
Promoter
- This DNA sequence next to a gene is the site where RNA polymerase binds to begin transcription.
Transcription factors
- Proteins that bind to specific DNA sequences and help or block the binding of RNA polymerase.
Enhancers
- DNA sequences, sometimes far from the gene, that bind activator proteins and raise the rate of transcription.
Repressors
- Some regulatory proteins bind and prevent RNA polymerase from starting, which lowers the rate of transcription.
| Regulatory element | Bound by | Effect |
|---|---|---|
| Promoter | RNA polymerase and general factors | Starts transcription |
| Enhancer | Activator proteins | Increases transcription rate |
| Silencing sequence | Repressor proteins | Reduces or stops transcription |
Part 3 of 5Regulating Translation
mRNA lifespan
- The longer an mRNA molecule survives in the cytoplasm, the more polypeptide copies can be made from it.
Nucleases
- Enzymes that break down nucleic acids destroy mRNA, so the message is only temporary and expression can be turned down.
Rate of translation
- Factors affecting ribosome binding and initiation change how quickly each mRNA is translated.
Speed of response
- Controlling mRNA breakdown lets a cell change protein levels quickly when conditions change.
Part 4 of 5Switching on a Gene
1
A signal such as a hormone or a nutrient reaches the cell and activates a transcription factor.
2
The transcription factor binds to the enhancer or promoter of the target gene.
3
RNA polymerase attaches to the promoter more readily, so transcription speeds up and mRNA is made.
4
The mRNA is translated until nucleases degrade it, and the amount of protein then falls again.
Part 5 of 5Exam Checklist & Tips
4.5.2 – Key Points for the Exam
- Gene expression The use of the information in a gene to produce an RNA or protein product.
- Transcription factor A protein that regulates transcription by binding to DNA.
- Promoter A sequence where RNA polymerase attaches to begin transcription.
- Nuclease An enzyme that digests nucleic acids.
Common Mistakes
- Saying that cells of one organism contain different genes;
- they contain the same genes but express different ones.
- Describing transcription factors as enzymes that copy DNA.
- Forgetting that mRNA is broken down and does not last indefinitely.
- Using the term expression to mean replication of DNA.
Last updated:
\n