How is a cell able to switch genes on and off so that the right polypeptides are made at the right time?
The control of transcription by specific transcription factors which move from the cytoplasm to the nucleus. In eukaryotes, transcription of target genes can be stimulated or inhibited when specific transcription factors bind to DNA. The effect of oestrogen on gene transcription. The control of translation of mRNA by RNA interference using small interfering RNA (siRNA), which can lead to the breakdown of mRNA or block its translation.
An exam-focused answer to the AQA A-Level Biology 3.8 dot point on regulating gene expression. Explains how specific transcription factors control transcription, how oestrogen acts as a transcription factor complex, and how siRNA in RNA interference breaks down or blocks mRNA to control translation.
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What this dot point is asking
AQA wants you to explain that gene expression is controlled at two stages: transcription (by transcription factors, with oestrogen as the worked example) and translation (by RNA interference using siRNA). You must be able to describe the route oestrogen takes into the cell and how siRNA silences a gene.
Controlling transcription with transcription factors
Every body cell contains the same genes, but cells differ because they transcribe different genes. The main control point is transcription, regulated by proteins called transcription factors.
- An activator transcription factor binds to the DNA at the promoter region and stimulates RNA polymerase, so transcription begins and mRNA is made.
- A repressor (inhibitory) transcription factor binds and prevents RNA polymerase from transcribing the gene, so no mRNA is made.
If a gene is not transcribed, no mRNA is produced, so the polypeptide that the gene codes for is not synthesised. This is how a cell controls which proteins it makes.
The effect of oestrogen
Oestrogen is a steroid hormone that switches on specific target genes by acting as part of a transcription factor.
- Oestrogen is lipid-soluble, so it diffuses directly through the phospholipid cell-surface membrane into the cytoplasm.
- Inside the cell it binds to a receptor site on a transcription factor (the oestrogen receptor). The receptor is normally an inactive shape.
- Binding changes the shape of the receptor, making it complementary to a specific DNA base sequence. The oestrogen-receptor complex is now an activated transcription factor.
- The complex enters the nucleus and binds to a specific region of DNA near the target gene.
- This stimulates RNA polymerase to transcribe the gene, so more mRNA and more of the corresponding protein are produced.
Controlling translation: RNA interference
Even after mRNA is made, expression can be controlled at the level of translation. One important mechanism is RNA interference (RNAi), which uses small interfering RNA (siRNA).
How siRNA silences a gene:
- A double-stranded RNA molecule is cut by an enzyme into short fragments of siRNA (about 20 to 25 base pairs).
- One strand of an siRNA fragment combines with proteins to form a silencing complex.
- The siRNA strand has a base sequence complementary to part of the target mRNA, so it binds to that mRNA by base pairing.
- The associated proteins then cut (hydrolyse) the mRNA into fragments, so it can no longer be translated. In some cases the siRNA simply blocks the mRNA, preventing ribosomes from translating it.
Either way, the polypeptide coded for by that mRNA is not produced, so the gene is effectively switched off after transcription.
Common mistakes
Try this
Q1. Explain why a transcription factor must enter the nucleus to control a gene. [2 marks]
- Cue. The gene (DNA) is in the nucleus; the transcription factor must bind to a specific DNA base sequence there to stimulate or inhibit RNA polymerase and so control transcription.
Q2. Suggest why a drug that mimics siRNA against a faulty mRNA could treat a genetic disease. [2 marks]
- Cue. The siRNA-like molecule would be complementary to the faulty mRNA and bind to it, leading to its breakdown so the faulty protein is not made, reducing harmful effects.
Q3. Explain why oestrogen affects only certain cells in the body. [2 marks]
- Cue. Only cells with the specific oestrogen receptor (transcription factor) can form the activated complex, so only those target cells respond by transcribing the oestrogen-responsive genes.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2019 AQA Paper 24 marksDescribe how oestrogen stimulates the transcription of a target gene.Show worked answer →
A 4-mark answer should track oestrogen from outside the cell to the start of transcription.
- Oestrogen is lipid-soluble so it diffuses through the phospholipid cell-surface membrane into the cytoplasm.
- It binds to a receptor on a transcription factor (the oestrogen receptor), forming an oestrogen-receptor complex and changing the receptor's shape (it becomes complementary to the DNA).
- The complex moves into the nucleus and binds to a specific base sequence on the DNA (a promoter region).
- This stimulates RNA polymerase to begin transcription of the target gene, increasing mRNA and hence protein production.
Markers reward the lipid-soluble entry, the shape change of the receptor, and binding to DNA to stimulate RNA polymerase.
2021 AQA Paper 23 marksExplain how siRNA can prevent the translation of an mRNA molecule.Show worked answer →
A 3-mark answer should describe complementary binding and the consequence.
- siRNA is a short, double-stranded RNA that is cut into small single strands and combined with proteins.
- One strand has a base sequence complementary to part of the target mRNA, so it binds to the mRNA by base pairing.
- This guides proteins that cut (hydrolyse) the mRNA into fragments, so it cannot be translated into a polypeptide (the gene is effectively silenced).
Markers reward complementary base pairing and the breakdown/blocking of mRNA preventing translation.
Related dot points
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