Cell Communication and Cell Cycle
What you must understand
Cells communicate through signal transduction: a signal (ligand) binds a receptor, triggering a transduction cascade that produces a cellular response. Receptors include G protein-coupled receptors, receptor tyrosine kinases, and intracellular receptors (for lipid-soluble signals like steroids). Amplification in cascades means a small signal produces a large response.
The cell cycle has interphase (G1, S, G2) and the mitotic (M) phase. DNA replicates in S phase. Checkpoints (G1, G2, M) monitor conditions; failure leads to uncontrolled division and cancer. Mitosis produces genetically identical diploid cells; meiosis produces haploid gametes with genetic variation via crossing over and independent assortment.
Traps
- Mitosis maintains chromosome number; meiosis halves it.
- Crossing over occurs in prophase I of meiosis, not mitosis.
- A steroid hormone typically binds an intracellular receptor and alters gene expression, unlike a membrane receptor.
- Signal transduction does not require the signal to enter the cell; the receptor transmits it.
Rule of thumb
For any signaling question, identify the receptor location: membrane (polar/peptide signals) or intracellular (lipid-soluble signals).
Worked example
A mutation that keeps a G protein permanently active would cause continuous signaling even without ligand, potentially driving uncontrolled cell division.