D.2.2 · 基因表达

What would explain this observation?

• Two cells with the same genome make different proteins. Regulation changes which information is used and when.
• Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.

Build the model

• Gene expression can be regulated at transcription and later stages. A transcription factor 转录因子 can affect transcription through interactions with DNA and other proteins. Epigenetic modifications can influence expression without changing the underlying DNA sequence.
• transcription factor: A protein that influences transcription; epigenetic modification 表观遗传修饰: A change affecting expression without changing the DNA sequence.

Choose evidence that can test it

• Distinguish a measured mRNA change from a protein or functional change. Translation, degradation and protein modification can all intervene. Regulation is specific to a cell context and environmental conditions.
• Compare treated and control expression data after checking normalization and replicates. Use evidence from a supplied perturbation to discuss causal models, rather than equating every association with a direct mechanism.

Work from known quantities

• State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
• Known: normalized transcript abundance is 12 in treatment and 3 in control. Fold change = 12/3 = 4. Equal normalized abundance does not require equal absolute cell counts in the two raw samples.

Example:

Transcript abundance is 15 in treatment and 5 in control. Find fold change. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.

Check the conclusion and its limits

• An epigenetic change is not automatically a DNA-sequence mutation. More mRNA does not guarantee proportionally more active protein.
• Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.

Warn:

Every change in gene expression is a mutation. This claim is false: An epigenetic change is not automatically a DNA-sequence mutation. More mRNA does not guarantee proportionally more active protein.

Key:

Gene expression: regulate products, not just genes: Distinguish a measured mRNA change from a protein or functional change. Translation, degradation and protein modification can all intervene. Regulation is specific to a cell context and environmental conditions.