9700 · 16.3
Gene control flashcards
Revision flashcards for Cambridge 9700 Gene control (syllabus 16.3). Flip, recall, then mark a real past-paper question.
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What is an operon?
A cluster of genes with related functions that are transcribed together as a single mRNA molecule. They are common in prokaryotes, with the lac operon being a classic example.
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What is the role of the lac repressor protein in the absence of lactose?
It binds to the operator region of the lac operon, physically blocking RNA polymerase from transcribing the structural genes. This is an example of negative control.
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How does lactose induce the lac operon?
Lactose (or its isomer, allolactose) acts as an inducer by binding to the lac repressor protein. This causes a conformational change in the repressor, so it can no longer bind to the operator, allowing transcription to proceed.
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What is the function of the cAMP-CAP complex?
It acts as an activator for the lac operon. When glucose is scarce, cAMP levels are high. cAMP binds to CAP, and the complex binds to the promoter, enhancing RNA polymerase binding and significantly increasing the transcription rate. This is positive control.
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What is histone acetylation and what is its effect on gene expression?
Histone acetylation is the addition of acetyl groups to histone proteins. It neutralises their positive charge, weakening their interaction with DNA. This makes the chromatin less condensed (euchromatin), increasing the accessibility of genes to transcription machinery and thus promoting gene expression.
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What is DNA methylation and what is its typical effect on gene expression?
DNA methylation is the addition of a methyl group to a cytosine base, often in promoter regions (CpG islands). It typically represses gene expression by blocking transcription factors or by recruiting proteins that compact the chromatin into heterochromatin.
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What are the two main types of transcription factors in eukaryotes?
1. **General transcription factors**: Required for the binding of RNA polymerase to the promoter of all protein-coding genes. 2. **Specific transcription factors**: Bind to enhancer or silencer sequences to activate or repress the transcription of particular genes in response to specific signals.
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How do microRNAs (miRNAs) regulate gene expression?
miRNAs are small non-coding RNA molecules that regulate gene expression post-transcriptionally. They bind to complementary sequences on target mRNA molecules, leading to either the degradation of the mRNA or the inhibition of its translation into protein.