Lecture 18 - Cell Division Cycle and Mitosis

Question 1

Basic goals of cell division?

Answer 1

1. Approx doubling in wolume
2. Exact duplication of genome
3. Exact segregation of duplicated genomes
4. Division into cells w/ one complete genome copy

Question 2

How are the phases of the cell cycle defined?

Answer 2

G1 - gap before DNA synth (2C DNA/cell)

S - DNA synthesis (gradually increasing DNA/cell)

G2 - gap after DNA synth (4C DNA/cell)

M - mitosis (4C DNA/cell until cytokensis)

Question 3

Describe events of G1 phase

Answer 3

“Resting” phase

General biosynth

External signals and internal state (size, DNA integrity) combine to make division to stay in G1 (G0) or proceed to S

Question 4

Describe the events during S phase

Answer 4

Histone synth

Synth of enzumes required for DNA replicaiton

DNA replication

Duplication of centrioles

Question 5

Describe the events during G2 phase

Answer 5

Continued synth of proteins required for mitosis and cytokinesis

Addition of cohesins (links sister chromatids)

Addition of condensins and Histone H1 phosphorylation (condense chromosomes at start of M)

Question 6

Mitosis - Prophase

Answer 6

Chromosomes condense

Spindle forms (duplicated centrosomes separate)

Kinetochores assemble onto centromeric DNA

Question 7

Prometaphase

Answer 7

Phosphorylation of lamain int filaments leads to nuclear envelope breakdown

Spindle invades nucleus

Kinetochores capture microtubules

Question 8

Metaphase

Answer 8

Biopolar attachment of chromosomes to spindle restuls in equal tension

Chromosomes aligned at the metaphase plate

Question 9

Metaphase/Anaphase checkpoint

Answer 9

Tension on kinetochores activates Anaphase Promoting Complex (APC) - triggers destruction of cohesin (anaphase)

Question 10

Types of movement that lead to chromosomal separation in Anaphase?

Answer 10

Chromosome-to-pole

Pole-pole separation

Question 11

Telophase

Answer 11

Phosphatase activity reverses effects of mitotic kinase on chromosome condensation

Spindle begins to disassemble, nucleus reassembles

Prep for cytokinesis

Question 12

Cytokinesis

Answer 12

Contractile ring of antiparallel microfilaments associated w/ cell membrane and non-muscle myosin constrict cell

Orientation of mitotic apparatus determines its position

Question 13

What are the major checkpoints of the cell cycle?

Answer 13

G1/S border - “start”

G2/M border (enter M)

Metaphase/Anaphase transition (exit M)

Question 14

What regulates the cell cycle?

Answer 14

Cyclin dependent kinase (Cdks)

Catalytic subunit (Cdk) + regulatory subunit (cyclin) = complete CDK

Question 15

How are CDKs regulated?

Answer 15

1. Cyclin subunit must be synthesized and must bind to kinase
2. Catalytic subunit is phosphorylated - one phosphate is needed for Cdk activity, the other inhibits Cdk activity
3. A phosphatase removes inhibitory phosphate, activating Cdk

((((Two protein kinases add 2 phosphates to Cdk. One adds the activating phosphate, the other adds the inhibitory phosphte

The Cdk inhibitory kinase is called Wee1, the activating kinase is Cak ))))

Question 16

How are Cdks turned off?

Answer 16

Cyclin subunit is ubiquitylated by an E3 ubiquitin ligase

Cyclin destroyed by proteolysis

Activating phosphate is removed by phosphatase

Question 17

Metaphase/Anaphase checkpoint

Answer 17

Securin binds and inhibits Separase

Tension on all kinetochores activates APC

APC degrades Securin, Separase is activated

Separase cleaves the cohesins between chromatids - Anaphase occurs

Question 18

Describe how cells deal with DNA damage (give p53 example)

Answer 18

DNA damage leads to activation of p53 (prolonged activation leads to apoptosis)

Active p53 > active p21 gene

Leads to translation of p21 protein which binds to and inactivates cyclins

Blocks entry into S phase