Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- A pathway links a signal to a response. Fast responses switch existing enzymes, channels or transporters on or off (often by phosphorylation). Slower responses activate a transcription factor, changing gene expression and the proteins the cell makes.
- New proteins can change the phenotype of a cell or organism (fruit ripening with ethylene; bacteria glowing by quorum sensing; HOX genes setting the body plan).
- Some pathways trigger apoptosis: ordered, programmed cell death, as when tissue between developing digits is removed.
- Quorum sensing: every bacterium releases an autoinducer; only at high population density does its concentration pass the threshold that switches on new genes.
- A mutation in any domain of the receptor or any later component can change the response. So can chemicals that bind any component.
Key relationships
| Change | Where | Effect on response |
|---|---|---|
| ligand cannot bind (mutation or blocking drug) | ligand-binding domain | none, even with plenty of signal |
| intracellular domain cannot change shape | receptor | none, even though the ligand binds |
| component stuck “on” (mutation or toxin) | receptor, relay protein or kinase | response with no signal |
| inhibitor of a kinase | partway down | blocked from that step on, even if earlier steps are on |
| chemical mimics the ligand | receptor | response without the natural signal |
| Response | Needs new proteins? | Speed |
|---|---|---|
| changed enzyme or channel activity | no | seconds to minutes |
| changed gene expression, phenotype | yes | minutes to days |
| apoptosis | uses enzymes activated by the signal | hours |
Assumptions behind the reasoning
- Each step only works if the step before it has switched it on (a linear chain). Real pathways branch, so a block in one branch may leave another branch working.
- “Downstream” means later in the chain, nearer the response.
- A fictional data set with one culture per condition supports a claim; it does not prove it.
Mistakes to avoid
- Saying every response needs new gene expression.
- Treating apoptosis as cells bursting from damage.
- Saying extra ligand overcomes a receptor that cannot bind or cannot signal.
- Forgetting that gain-of-function mutations switch a pathway on.
- Placing an inhibitor’s effect upstream of where it acts.
Quick self-check
- A drug blocks the ligand-binding site of a receptor. What happens to the response? (It falls, because the ligand cannot bind and nothing downstream is switched on.)
- A relay protein is stuck on, and a kinase after it is inhibited. Is there a response? (No: the block is downstream of the stuck protein, so the signal stops at the kinase.)
- Why do quorum-sensing bacteria glow only at high density? (Only then does the autoinducer concentration reach the threshold that switches on light genes.)
Next: practice questions.