OCR · A-Level · Biology
Course version: H420 · Version from Sept 2025
(b) the regulatory mechanisms that control gene expression at the…
Full specification wording
Cellular control: (b) the regulatory mechanisms that control gene expression at the transcriptional level, post-transcriptional level and post-translational level
Gene expression can be regulated at three levels.
Checked against the OCR H420 specification (opens in a new tab) on .
- Point 2 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Gene expression can be regulated at three levels. At the transcriptional level, control determines whether mRNA is made: transcription factors and chromatin modification, such as methylation or acetylation of histones, affect access of RNA polymerase to genes, and operon-style control in prokaryotes is a classic example. At the post-transcriptional level, control acts on the mRNA transcript: RNA processing such as splicing, and the rate of mRNA degradation, determine how much functional mRNA reaches the ribosome. At the post-translational level, control acts on the finished protein: activation by phosphorylation, or degradation, modifies how much active protein is present. Learners should place each mechanism at the correct level rather than mixing them.
What earns the marks
- Transcriptional control determines whether a gene is transcribed into mRNA, for example through transcription factors or chromatin modification affecting RNA polymerase access.
- Post-transcriptional control acts on the mRNA after transcription, for example through splicing or the rate of mRNA degradation.
- Post-translational control acts on the synthesised protein, for example through activation by phosphorylation or through protein degradation.
- Each mechanism must be assigned to the correct level, since the level describes the stage at which regulation occurs.
- Regulation at any level changes the amount or activity of the final functional protein, so gene expression is controlled without changing the DNA sequence.
Common mistakes and how to put them right
Placing mRNA degradation at the transcriptional level.
Put it right by recognising it acts on the transcript, so it is post-transcriptional.
Describing protein phosphorylation as transcriptional control.
Put it right by identifying it as post-translational, acting on the finished protein.
Assuming regulation always changes the DNA base sequence.
Put it right by explaining that these mechanisms alter expression without altering the sequence.
Exam tips
- For each example, name the level first, then the mechanism, so the examiner can see the match.
- Use the prefixes trans- and post- to check the stage: transcription of DNA, then the transcript, then the protein.
- Avoid vague answers such as 'controls the gene'; specify the molecule or process being regulated.
Try the lesson
The start of the Cellular control: (b) the regulatory mechanisms that control gene expression at the transcriptional level, post-transcriptional level and post-translational level lesson, made from checked teaching notes for this specification point.
Have a go first
In a bacterial cell, a repressor protein binds to the operator region of an operon. What effect does this binding have on transcription of the structural genes?
Listen
Gene expression can be regulated at three levels. At the transcriptional level, control determines whether mRNA is made: transcription factors and chromatin modification, such as methylation or acetylation of histones, affect access of RNA polymerase to genes, and operon-style control in prokaryotes is a classic example. At the post-transcriptional level, control acts on the mRNA transcript: RNA processing such as splicing, and the rate of mRNA degradation, determine how much functional mRNA reaches the ribosome. At the post-translational level, control acts on the finished protein: activation by phosphorylation, or degradation, modifies how much active protein is present. Learners should place each mechanism at the correct level rather than mixing them. A strong answer earns marks for these points. Transcriptional control determines whether a gene is transcribed into mRNA, for example through transcription factors or chromatin modification affecting RNA polymerase access. Post-transcriptional control acts on the mRNA after transcription, for example through splicing or the rate of mRNA degradation. Post-translational control acts on the synthesised protein, for example through activation by phosphorylation or through protein degradation. Each mechanism must be assigned to the correct level, since the level describes the stage at which regulation occurs. A common slip: Placing mRNA degradation at the transcriptional level. Instead, recognising it acts on the transcript, so it is post-transcriptional. Exam tip: For each example, name the level first, then the mechanism, so the examiner can see the match.
Key words
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Transcriptional control of gene expression
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Post-transcriptional control of mRNA
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Post-translational control of proteins
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Regulation without changing DNA sequence
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See every definition
- Transcriptional control of gene expression
- Control that determines whether a gene is transcribed into mRNA, for example through transcription factors or chromatin modification affecting RNA polymerase access.
- Post-transcriptional control of mRNA
- Control that acts on the mRNA after transcription, for example through splicing or the rate of mRNA degradation.
- Post-translational control of proteins
- Control that acts on the synthesised protein, for example through activation by phosphorylation or through protein degradation.
- Regulation without changing DNA sequence
- Control at any level changes the amount or activity of the final functional protein, so gene expression is controlled without changing the DNA sequence.
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