H5 The cell cycle
5.1 cyclins and cyclin-dependent kinases (cdks)
Cell cycle basics
Average cell cycle length ≈ 16 hours (15 h interphase + 1 h mitosis), but
varies by cell
chromosomes become visible only during mitosis.
Many adult cells reside in G0 (quiescent state) and are not dividing.
Mitogens/growth factors can stimulate G0 cells to re-enter the cycle
Before G1, cell cycle progression depends on mitogens; after G1, the cell is
irreversibly committed to division.
Cyclins and cdks (cyclin-dependent kinases)
Cyclins: regulatory proteins whose concentrations rise and fall cyclically
o controlled by gene transcription and protein degradation.
Cdks: catalytic subunits, concentrations do not fluctuate during the cell
cycle.
o kinases
Cyclins bind specific cdks conformational changes activate cdk active
site.
Different cyclin–cdk complexes regulate different phase transitions
o Cyclin D–cdk4/6: drives progression through G1; links growth factor
signaling (eg. EGF pathway) to cell-cycle entry; helps regulate cyclin
E expression.
o Cyclin E–cdk2: drives G1 → S transition.
o Cyclin A–cdk2: required for S phase progression.
5.1 cyclins and cyclin-dependent kinases (cdks)
Cell cycle basics
Average cell cycle length ≈ 16 hours (15 h interphase + 1 h mitosis), but
varies by cell
chromosomes become visible only during mitosis.
Many adult cells reside in G0 (quiescent state) and are not dividing.
Mitogens/growth factors can stimulate G0 cells to re-enter the cycle
Before G1, cell cycle progression depends on mitogens; after G1, the cell is
irreversibly committed to division.
Cyclins and cdks (cyclin-dependent kinases)
Cyclins: regulatory proteins whose concentrations rise and fall cyclically
o controlled by gene transcription and protein degradation.
Cdks: catalytic subunits, concentrations do not fluctuate during the cell
cycle.
o kinases
Cyclins bind specific cdks conformational changes activate cdk active
site.
Different cyclin–cdk complexes regulate different phase transitions
o Cyclin D–cdk4/6: drives progression through G1; links growth factor
signaling (eg. EGF pathway) to cell-cycle entry; helps regulate cyclin
E expression.
o Cyclin E–cdk2: drives G1 → S transition.
o Cyclin A–cdk2: required for S phase progression.
