Chapter 19 – The Eukaryotic Cell Cycle

Flashcards of vocabulary terms and definitions from the Eukaryotic Cell Cycle lecture.

Cell cycle

Four phases: G1, S, G2, and M.

G1 START/Restriction point

Where cells commit to division.

Cyclin-CDK complexes

Drive cell cycle progression.

Checkpoint pathway surveillance mechanisms

Guarantee each cell cycle step is completed correctly before the next is initiated.

Prophase

Chromosomes condense. Nuclear envelope breaks down. Spindle poles duplicate. Microtubules form the mitotic spindle apparatus.

Metaphase

Spindle microtubules from each pole attach to chromosome kinetochores and center sister chromatid pairs in the spindle.

Anaphase

Spindle microtubule shortening and motor proteins pull each sister chromatid toward an opposite spindle pole.

Telophase

Chromosomes decondense, and each presumptive daughter cell reassembles a nuclear membrane around its chromosomes.

Cytokinesis

Cell divides into two daughter cells.

Cyclin

Different cyclins present only in the cell cycle stage they promote activate CDKs at different cell cycle stages.

The ubiquitin-proteasome system

Limits presence of a cyclin to the appropriate cell cycle stage.

CDK inhibitors (CKIs)

Inhibit CDK activity by binding directly to the cyclin-CDK complex.

CDKs

Initiate every aspect of each cell cycle stage by phosphorylating many different target proteins.

Cyclin-Dependent Kinases

Small serine/threonine kinases that require a regulatory cyclin subunit for activity.

CDK activity

Activity and substrate specificity of any given CDK is defined by the particular cyclin to which it is bound; each CDK activity is cell-cycle-stage-specific.

CAK kinase

Activates CDKs

Wee1 kinase

Inhibits CDKS

Cdc25 phosphatase

Activates CDKs

Sic1

Binds and inhibits S phase CDKs

CKIS p27KIP1, p57KIP2, and p21CIP

Binds and inhibits CDKs

INK4

Binds and inhibits G₁ CDKs

Rb

Binds E2Fs, preventing transcription of multiple cell cycle genes

SCF

Degradation of phosphorylated Sic1 or p27 KIP1 to activate S phase CDKs

APC/CCdc20

Degradation of securin, initiating anaphase. Induces degradation of B-type cyclins

Extracellular signals

Regulate cell cycle entry.

G1/S CDKs

Trigger chromosome duplication at DNA origin of replication sites.

Cohesins

Link replicated DNA molecules to ensure accurate segregation during mitosis.

Mitotic CDKs

Induce entry into mitosis in all eukaryotes by inducing chromosome condensation, nuclear envelope breakdown, and spindle formation.

Mitotic CDKs

Inactivated by inhibitory phosphorylation of the CDK subunit until completion of DNA replication; promote their own activation through positive feedback loops that inactivate Wee1 kinase and activate Cdc25 phosphatase.

Chromosome attachment to the mitotic spindle

Sister chromatids must be stably bi-oriented on the mitotic spindle to be accurately segregated during mitosis.

Cohesin cleavage

Cohesin cleavage by separase initiates chromosome segregation during anaphase.

Exit from mitosis

Exit from mitosis is triggered by mitotic cyclin degradation and requires protein phosphatase reversal of mitotic CDK phosphorylation of many different proteins, permitting mitotic spindle disassembly, decondensation of chromosomes, and reassembly of the nuclear envelope.

Meiosis

Meiosis involves one cycle of chromosome replication followed by two cycles of cell division to produce haploid germ cells.