AP Biology · Summary

Unit 4 summary

1. A signaling pathway has three stages: reception (a ligand binds a receptor that fits it), transduction (relay molecules such as G proteins, second messengers and kinases pass the signal on) and response.
2. Polar or large signals bind membrane receptors; small nonpolar signals such as steroid hormones cross the membrane and bind receptors inside the cell.
3. A blocked link stops every step after it; a link stuck in its active form runs every step after it even with no signal.
4. Negative feedback reverses a change and holds a variable steady; positive feedback adds to a change and pushes a process to an end point.
5. S phase doubles the DNA but not the chromosome count; mitosis gives each daughter cell the same chromosomes as the parent had in G1.
6. Checkpoints in G1, G2 and metaphase stop the cycle until conditions are right; cyclins rise and fall and switch their CDKs on and off.
7. Cancer comes from mutations that make a proto-oncogene overactive (an oncogene) or disable a tumor suppressor gene such as p53, so division no longer stops.