Eukaryotic Cell Cycle Overview - Ch. 16 AI

Flashcards covering key concepts of the eukaryotic cell cycle, including phases, regulation, checkpoints, and mechanisms.

What are the four major phases of the eukaryotic cell cycle?

G1, S, G2, and M phase (mitosis + cytokinesis).

What occurs during G1 phase?

Protein synthesis, growth, and decision whether to divide.

What occurs during S phase?

DNA replication and doubling of nuclear DNA content.

What occurs during G2 phase?

Protein synthesis and preparation for mitosis.

What occurs during M phase?

Mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Why do early embryo cell cycles proceed rapidly?

They lack G phases and do not express embryonic genes.

What sustains early embryonic divisions before the mid-blastula transition?

Maternal nutrients stored in the yolk.

What begins at the mid-blastula transition?

G phases begin, cell division slows, embryo begins growth.

What drives transitions at specific points of the cell cycle?

The cell cycle control system.

What proteins are involved in regulating each control step?

Cdc proteins active only during specific phases.

What are cyclins?

Regulatory subunits of cyclin-Cdk complexes.

What are Cdks?

Cyclin-dependent kinases; catalytic subunits always present but not always active.

What do different cyclin-Cdk complexes control?

Different phases of the cell cycle.

What is MPF?

Maturation-promoting factor, later identified as M-Cdk.

How was MPF discovered?

Injection of M-phase cytoplasm into frog oocytes induced M-phase entry.

What processes are triggered by active M-Cdk?

Chromatin condensation, nuclear envelope breakdown, Golgi fragmentation, mitotic spindle formation.

When is M-cyclin at its highest level?

During M-phase.

What must happen to M-cyclin for mitotic exit to occur?

It must be degraded.

What adds ubiquitin to M-cyclin?

The anaphase promoting complex (APC).

What activates M-Cdk?

Synthesis of M-cyclin, activation phosphorylation, and removal of inhibitory phosphate by Cdc25.

What kinase adds the inhibitory phosphate on M-Cdk?

An inhibitory kinase (Wee1 in yeast).

What phosphatase activates M-Cdk?

Cdc25.

What is the role of positive feedback in M-Cdk activation?

Active M-Cdk activates more Cdc25, increasing M-Cdk activation.

What two cyclin-Cdk complexes are required for S phase entry?

G1/S-Cdk and S-Cdk.

What triggers the G1 checkpoint?

DNA damage.

What kinase detects double-strand DNA breaks?

ATM.

What does ATM do when activated?

Phosphorylates and stabilizes p53.

What does p53 activate?

Transcription of p21.

What does p21 do?

Inactivates G1/S-Cdk and S-Cdk, causing G1 arrest.

How does Chk2 participate in DNA damage response?

It inhibits Cdc25 and stabilizes p53.

Which tumor suppressors help with DNA repair after arrest?

BRCA1 and BRCA2.

What happens when BRCA1 or BRCA2 are mutated?

Increased breast cancer risk.

What regulates the animal cell cycle?

Extracellular growth factors, not nutrient availability.

What is G0?

A quiescent phase outside the cell cycle.

Name cell types mostly in G0.

Neurons, fibroblasts (until wound), some stem cells.

When is the restriction point reached?

Late G1, committing the cell to S phase.

What pathway do mitogens activate?

Ras-MAP kinase pathway.

What does the Ras-MAP pathway stimulate?

Synthesis of G1/S and S cyclins.

What is the function of mitogens?

Stimulate division by relieving cell-cycle braking mechanisms.

What does Rb protein do when active?

Inhibits E2F and prevents transcription of proliferation genes.

How do cyclin/Cdk complexes inactivate Rb?

They phosphorylate it.

What happens when Rb is phosphorylated?

E2F is released and activates genes for S phase.

What is START in budding yeast?

Commitment step in G1 to enter S phase.

What regulates START?

Nutrient availability, mating factors, cell size.

What is the primary control point in fission yeast?

G2 to M transition.

What does Wee1 do?

Adds inhibitory phosphate to M-Cdk when cell size is too small.

What does Cdc25 do?

Removes inhibitory phosphate from M-Cdk to activate it.

What happens in a wee1- mutant?

Cells divide too early and are small.

What happens in a cdc25- mutant?

Cells cannot enter M-phase and become very large.

What holds sister chromatids together?

Cohesins.

What cleaves cohesins?

Separase.

What activates separase?

APC-mediated degradation of securin.

What must occur before anaphase can begin?

All chromosomes must be attached to the spindle (spindle assembly checkpoint).

What are the two major APC targets?

Securin and M-cyclin.

What does degradation of M-cyclin accomplish?

M-Cdk inactivation and exit from M phase.

What do meiosis I and II produce?

Gametes (haploid cells).

What is diplotene arrest?

Primary oocytes pause in prophase I, sometimes for decades.

What happens during diplotene arrest?

Chromosomes de-condense, gene expression and oocyte growth occur.

When does meiosis resume?

Upon hormonal stimulation before ovulation.

When does metaphase II arrest occur?

After meiosis I, awaiting fertilization.

What triggers exit from diplotene arrest?

Activation of M-Cdk.

Why is there no nuclear envelope reassembly between meiosis I and II?

M-Cdk activity stays partially high.

When does anaphase II occur?

Only after fertilization triggers release from metaphase II arrest.