Week 9: Cell Cycle Control

Question 1

Describe the cell cycle phases.

Answer 1

G1 -> S (DNA replication -> G2 -> M phase (mitosis and cytokinesis)

Cells becomes smaller and rounder during mitosis.

Question 2

What is a model organism for studying cell cycle?

Answer 2

Xenopus egg (African clawed frog). Factors present in cytoplasm from M-phase cell triggers entry into M phase (ie. maturation promoting factor, MPF).

Inject cytoplasm from M-phase cell - drives oocyte into M phase. If inject cytoplasm from interphase cell - oocyte does not enter M phase

Question 3

What are the 3 main cell cycle control points?

Answer 3

At end of G1 -> is environment favourable?
At end of G2 -> Is DNA replicated and is all DNA damage repaired?
Mid mitosis -> Are chromosomes properly attached to mitotic spindle?

Question 4

What are Cdks?

Answer 4

Control system depends on cyclically activated protein kinases - Cyclin-Dependent Kinases (Cdks) - highly conserved throughout evolution

Different Cyclin - Cdk complexes trigger diff steps in cell cycle

Question 5

How are cyclins regulated?

Answer 5

Cyclin concentrations are regulated by transcription and proteolysis.
ie. cyclin in an active cyclin-Cdk complex may be targetted with ubiquitin chain flagging it for proteasome destruction and inactivating the Cdk.

Question 6

What does the activity of the cyclin-Cdk complexes depend on?

Answer 6

Activity of cyclin-Cdk complexes depends on phosphorylation and dephosphorylation. When M cyclin-Cdk complex is formed, it is phosphorylated by Wee1 and the phosphate is then removed by Cdc25.

Question 7

How can Cdk activity be blocked?

Answer 7

Cdk Activity can be blocked by Cdk inhibitor proteins

Question 8

How can control system pause cycle (3 ways)?

Answer 8

Cdks are stably inactivated in G1. End of G1 -> Cdk inhibitors block entry to S. Transition of G2-M -> inhibition of activating phosphatase (Cdc25) blocks entry to M. End of M -> inhibition of APC activation delays exit from mitosis.

Question 9

How do mitogens work?

Answer 9

Mitogens stimulate cell division by promoting entry into S phase. Growth factors stimulate cells to grow. Some extracellular signal proteins inhibit cell survival, division or growth.

Mitogens promote production of cyclins that stimulate cell division.
Mitogens bind to mitogen receptor causing it to begin intracellular signaling pathway in which an active Rb (retinoblastoma) protein is blocking a transcription regulator. The mitogen path causes activation of G1-Cdk and G1/S Cdks which phosphorylates Rb causing it to release transcription factor leading to cell proliferation.

Question 10

What does DNA damage do in cell cycle?

Answer 10

can temporarily halt progression through G1. Cells can delay division for prolonged periods by entering non-dividing states.
In absence of DNA damage, p53 is degraded by proteasomes. DNA damage activates protein kinases that phosphorylate p53 activating it. It binds to regulatory region of the p21 gene causing it to be transcribed and translated becoming a p21 (Cdk inhibitor protein) which then inactivates the G1/S and S-Cdk complexes.

Question 11

What does S-Cdk do?

Answer 11

initiates DNA replication and blocks Re-replication

Cdc6 is bound to ORC (origin recognition complex). DNA helicase binds causing Cdc6 to dissociate (G1), then this prereplicative complex (pre-RC) goes on to have helicase activated by S-Cdk in S phase and replication machine is recruited leading to completion of DNA replication.
Incomplete replication can arrest cell cycle in G2

Question 12

What does M-Cdk do?

Answer 12

M-Cdk drives entry into M phase and mitosis

Inactive Cdc25 phophatase is positively regulated by active M-Cdk to form active Cdc25 phosphatase which hydrolyses the inhibitory phosphates on inactive M-Cdk to form active M-Cdk that positively regulates Cdc25.

Question 13

What are cohesins and condensins?

Answer 13

Cohesins and condensins help configure duplicated chromosomes for separation. Cohesins tie together sister chromatids in each duplicated chromosome. Condensins coil each sister chromatid to compact it and make it easier to separate during mitosis.

Question 14

What do microtubules and actin/myosin do in mitosis and cytokinesis?

Answer 14

Microtubules form mitotic spindle. Actin/myosin form contractile ring between cells.

Question 15

What are the stages of mitosis?

Answer 15

Prophase, prometaphase, metaphase, anaphase and telophase + cytokinesis.

Question 16

What occurs in interphase?

Answer 16

cell increases in size, DNA replication, centrosome is duplicated.

Question 17

What occurs in prophase?

Answer 17

Duplicated chromosomes condense, mitotic spindle assembles, centrosomes begin to move apart.

Question 18

What occurs in prometaphase?

Answer 18

nuclear envelope breaks down and chromosomes attach to spindle microtubules using kinetochores

Question 19

What occurs in metaphase?

Answer 19

chromosomes align at spindle equator.

Question 20

What occurs in anaphase? Describe this.

Answer 20

sister chromatids pulled apart

Proteolysis triggers sister-chromatid separation at anaphase: APC = anaphase-promoting complex - catalyses the ubiquitination and destruction of securin - inhibits activity of separase freeing it to destroy cohesins.

Question 21

What is anaphase A vs B?

Answer 21

A - chromosomes are pulled poleward. B - poles are pushed and pulled apart.

Question 22

What occurs in telophase?

Answer 22

chromosomes arrive at each pole, new nuclear envelope reassembles around each set of chromosomes.

Question 23

How is nuclear envelope reformed?

Answer 23

Phosphorylation of nuclear pore proteins and lamins occurs in prometaphase. Dephosphorylation of nuclear pore proteins and lamins occurs causing them to reassemble in telophase. Continued fusion of the nuclear envelop vesicles lead to the interphase nucleus.

Question 24

What occurs in cytokinesis?

Answer 24

Cytoplasm is divided between two daughter cells by contractile ring of actin and myosin filaments

Question 25

What is apoptosis?

Answer 25

Apoptosis - cell suicide - programmed cell death. Occurs in development in adult tissues where death balances division.

Question 26

How is apoptosis mediated?

Answer 26

intracellular proteolytic cascade.
Two procaspase molecules have their prodomains cleaved and then join to form an active caspase molecule. Caspases undergo an amplifying cascade causing them to specialize.

Question 27

How is apoptosis regulated?

Answer 27

Bcl2 family of intracellular proteins.
An apoptotic stimulates Bax or Bak molecules in mitochondria to release cytochrome c molecules . These then bind to adaptor protein which assembles. Procaspase-9 joins to create an apoptosome which leads to apoptosis.

Question 28

How can extracellular signals cause apoptosis?

Answer 28

DISC - death-inducing signalling complex - includes initiator procaspase (* or 10) as well as adaptor proteins.

A killer lymphocyte has a Fas ligand which binds to a Fas death receptor. This causes the assembly of adaptor proteins and procaspase-8 (together called DISC). Activation of the caspase-8’s then causes the activation of executioner caspases which cause apoptosis.