Explain gibberellin activation through DELLA proteins
Trace how gibberellin removes DELLA repression and allows transcription-promoting factors to activate genes.
A Level; Cambridge International Biology 9700, 2028–2030. Written answers are self-checked; this is not practical-skills certification.
Before you begin
- Genes are DNA sequences that can be inherited; meiosis and fertilisation link parental genomes.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the effect of DELLA protein breakdown?
What you will learn
- Explain that DELLA proteins repress factors that promote transcription
- Relate gibberellin-triggered DELLA breakdown to gene activation
Original teaching model. Counts and records are stipulated; drawings are schematic.
| Feature | Meaning |
|---|---|
| Shared DNA identities | E, C and R in both samples |
| Sample A messages | E 6; C 2; R 0 |
| Sample B messages | E 0; C 4; R 5 |
| C comparison | B has 2 more and twice A’s count |
Build the genetic explanation
Without gibberellin signalling, DELLA proteins act as repressors by inhibiting factors that would otherwise promote transcription. Gibberellin signalling leads to DELLA protein breakdown. The transcription-promoting factors are released and can increase transcription of target genes, including genes involved in growth responses. This is a regulatory chain from hormone signal to protein degradation to altered gene expression.
Check the evidence and limits
Gibberellin does not need to bind directly to DNA in this model. DELLA proteins are repressors; their breakdown removes inhibition from transcription-promoting factors.
Pause and explain
A student says gibberellin activates a gene by acting as a transcription factor. Correct the explanation.
Worked example
What changes when gibberellin causes DELLA breakdown?
Show the worked solution
- DELLA repression is removed.
- Transcription-promoting factors can act.
Answer Transcription-promoting factors are released from DELLA inhibition and can increase transcription of target genes.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Trace gibberellin signalling to increased transcription.
Give me a hint
Set out the alleles or steps before deciding the outcome.
Compare my reasoning
- Gibberellin signalling promotes DELLA breakdown.
- Released factors promote transcription.
Gibberellin signalling causes DELLA protein repressors to break down. Factors that promote transcription are no longer inhibited and can increase transcription of target genes.
Look for these in your work
- Genetic terms used correctly
- Steps linked
A mutation makes DELLA proteins resistant to breakdown. Predict one effect on gibberellin-responsive genes.
Give me a hint
Use the given evidence and state your assumptions.
Compare my reasoning
- DELLA repression persists.
- Transcription-promoting factors remain inhibited.
The factors may remain inhibited, so gibberellin-responsive genes may show reduced or delayed transcription despite the hormone signal.
Look for these in your work
- Prediction or interpretation supported
- Limits stated
A student says gibberellin activates a gene by acting as a transcription factor. Correct the explanation.
Give me a hint
Apply the inheritance or gene-control model to the new context.
Compare my reasoning
- Gibberellin is a hormone signal, not a DNA-binding protein in this pathway.
- It promotes breakdown of DELLA repressors.
Gibberellin is the signal. It promotes breakdown of DELLA protein repressors, releasing factors that promote transcription; those factors regulate DNA transcription.
Look for these in your work
- Model applied accurately
- Unsupported certainty avoided
Common mistakes
- Saying DELLA proteins are transcription factors that activate genes
- Saying gibberellin directly removes DNA from a promoter
What can you explain now?
What is the effect of DELLA protein breakdown?
Compare with the explanation
Promoting factors are no longer inhibited
They can increase transcription of target genes.
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Make it stick
How does gibberellin remove DELLA repression of transcription?
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