L20: Cell Cycle Checkpoints & Mitosis

Question 1

Name the stages of mitosis with their functions

Answer 1

• prophase: chromosomes replicate
• prometaphase: nucleus dissolves & mitotic spindles attach to centrosomes
• metaphase: chromosomes align at centre of cell
• anaphase: mitotic spindles pull sister chromatids to opposite poles
• telophase: mitotic spindles disappears, nuclear envelope begins to reform & cell devision begins
• cytokinesis: cells dividde, 2 daighter cells formed

Question 2

What are the mitotic checkpoint mechanisms used?

Answer 2

DNA damage, just after G2 & spindle defects, just before M (between metaphase and anaphase)

Question 3

cdk and cyclin levels remain constant in the cell cycle. True or false?

Answer 3

False, cdk levels are relatively constant but cyclin partners vary
-cdk activity remains low (due to Wee1 activity) then as trigger kicks in it rapidly rises until metaphase then lowers

Question 4

How do cyclin partners vary in the cell cycle?

Answer 4

• cyclin A rises in S phase and disappears in M

- cyclin B starts to accumulate in G2 and peaks before its destruction in metaphase

Question 5

Wee1 and Myt1 activity has what effect?

Answer 5

Causes inhibitory phosphorylation on cyclin

• small amount of cdc25B (phosphotase that reverses inhibitory phosphorylation) starts activating
• this triggers cdc25A & C and rapidly rises into active form & rapid activation of cyclin & shuts don inhibitory processs

Question 6

Where do centrosomes duplicate?

Answer 6

In G1

Question 7

What are the funcions of polo like kinasees (plk)?

Answer 7

• entry into M phase
• centrioles and centrosomers biogenesis
• mitotic chromosomes
• cytokinesis

Question 8

What is the function of aurora kinases?

Answer 8

They regulate spindle and sister chromatid function. There are 2 types

• A: functions at centrosomes to maintain spindle integriy
• B: acts as part of chromosomal passenger complex (CPC), along with NCENP, survivin and boreatin & is on chromosome arms early in mitosis but then moves to centromere and kinetochores later in mitosis to aid in chromosome condensation or promote correct attachment of microtubules to kinetochores

Question 9

What are the correct forms of spindle attachment?

Answer 9

• Amphitelic: both kinetochores attached to pole at same point, forms tension, can pull apart
• Merotelic: one kinetochore attached to both spindle cores, forms pulling force, can pull
  Both of these forms have an inactive spindle checkpoint

Question 10

What causes active spindle checkpoints?

Answer 10

Monotelic and syntelic forms of spindle attachemnt, which are incorrect

• montelic: only one kinetochore attached to core
• syntelic: kinetochore attached to both core while other reamins unattached

Question 11

How are mitotic cyclins recognised?

Answer 11

By APC-Cdc20 via N-terminal deestruction motifs or boxes
- we get waves of destruction, measured by decline in fluorescence in live G2 cells injectde w fluro tagged targets of APC- cyclin A,B and securin

Question 12

When does cyclin A destruction start?

Answer 12

Almost immediately after nuclear envelope breakdown (just before prometaphase)
- whilse SAC is active, by virtue of a longer N-terminal D box which can outcompete the SAC proteins for cdc 20

Question 13

When does cyclin B and securin destruction occur?

Answer 13

Almost immediately after alignment of last pair of sister chromatids on metaphase plate
- later substrates are destroyed by APC Cdh1

Question 14

What is the function of Emil?

Answer 14

Prevents cdc20 from activating the APC in prophase
- when destroyed in early prometaphase, spindle checkpoint instead stops activation of APC thus preserving cyclin B to maintain cdk1 activity
Stops destruction of cyclin B by APC
- cdh1 which was occuring in G1, allowing cyclin B to start accumulating prior to later mitosis