l10

Question 1

describe the eukaryotic cell cycle

Answer 1

1. Parent cellular contents are duplicated
2. Cellular contents are divided into two child cells

Question 2

at which phase does DNA replication happen?

Answer 2

S phase (WAY BEFORE MITOSIS)

Question 3

is M phase only mitosis?

Answer 3

no; includes mitosis and cytokinesis

Question 4

purpose of cell cycle checkpoints

Answer 4

• checkpoints ensure conditions are suitable for division
• allow cells to control different stages

Question 5

which checkpoint is known as the start checkpoint?

Answer 5

before DNA synthesis

Question 6

what is the start transition checkpoint?

Answer 6

• is the environment favourable?
• if yes, will proceed to S
• if no, stops

Question 7

what is the G2/M transition checkpoint ?

Answer 7

• is all DNA replicated?
• is environment favourable?
• if yes, proceeds to mitosis

Question 8

what is the metaphase-to-anaphase transition checkpoint?

Answer 8

• are all chromosomes attached to the spindle?
• if yes, proceeds to anaphase and cytokinesis

Question 9

three checkpoints

Answer 9

start transition
G2/M
metaphase-to-anaphase

Question 10

what are checkpoints mediated by?

Answer 10

the controlled activity of cyclin-dependent kinases (Cdks)

Question 11

what is the function/role of Cdks?

Answer 11

phosphorylate target proteins for the cell cycle to progress

Question 12

what do Cdks require?

Answer 12

cyclins

Question 13

describe the changes in function of cyclin-Cdks throughout the cell cycle stages

Answer 13

different cyclin-Cdks are active at different cell cycle stages:
- Cdks are present throughout all cell cycle stages
- Cdks require cyclins to be active
- Cyclins are made and broken down at different cell cycle stages
- Cyclin-Cdk combination controls which target proteins are phosphorylated

Question 14

what would cyclin-Cdk complexes phosphorylate at the start transition checkpoint?

Answer 14

Dna helicases for DNA replication

Question 15

what would cyclin-Cdk complexes phosphorylate at the G2/M checkpoint?

Answer 15

• lamin (nuclear envelope breakdown)
• microtubule-associated proteins (spindle assembly)

Question 16

describe the specificity of Cdks

Answer 16

one Cdk may work with multiple cyclins, but cyclins only work at one stage

Question 17

in yeast, what Cdk partner is always present no matter the stage of the cell cycle?

Answer 17

Cdk1; it is only the cyclin that changes

Question 18

in vertebrates, how do Cdk partners and cyclins change depending on the stage of the cycle?

Answer 18

must be different cyclins; Cdk partner can be the same or different

Question 19

what are two ways in which Cdk activity is regulated?

Answer 19

synthesis and degradation of cyclins

Question 20

degradation of cyclins

Answer 20

• cyclins can be ubiquitinated
• polyubiquitination marks cyclins for degradation by the proteasome
• if Cyclins are broken down, Cdks will no longer be active

Question 21

give an example of cyclin ubiquitination

Answer 21

• APC/C & Cdc20 act as an E3 ubiquitin ligase
• Ubiquitination of M-cyclin causes its degradation by the proteasome

Question 22

draw a flow chart of how cyclin-Cdk regulation occurs

Question 23

describe how Cdks are activated by cyclins and Cdk-Activating Kinase (CAK)

Answer 23

• Cyclin is absolutely required for Cdk activity by exposing the T-loop
• however, this only leads to partial activation (ie the cdk is still inactive)
• Once T-loop is exposed, CAK can phosphorylate Cdk to further activate it

Question 24

describe how protein phosphatases reverse the effects of cyclin-Cdks

Answer 24

• Protein phosphatases (PP2As) remove phosphorylation from proteins
• PP2As usually have the same targets as Cyclin-Cdks
• PP2A and M-Cdk participate in a complex feedback loop

Question 25

describe how M-Cdk and PP2A compete for the same targets

Answer 25

Before M-phase - PP2A is active - PP2A dephosphorylates M-Cdk targets to stop progression into mitosis During M-phase - M-cyclin is produced - M-Cdk is active - M-Cdk indirectly inhibits PP2A activity - M-cyclin phosphorylates its targets to promote mitosis progression

Question 26

describe how Cyclin-Cdks can be inactivated by CKI binding

Answer 26

- Cdk inhibitor proteins (CKIs) bind Cyclin-Cdks - CKIs disrupt the Cyclin-Cdk interaction, which inactivates the Cdk - eg p27, p21

Question 27

describe how Cyclin-CDKs can be inactivated by phosphorylation

Answer 27

- The inhibitory phosphorylation is at a different site from the activating phosphate added by Cak - Wee1 kinase can add an inhibitory phosphorylation

Question 28

describe how Cdc25 phosphatase can activate Cyclin-Cdks

Answer 28

- Cdc25 phosphatase can remove the inhibitory phosphorylation on M-Cdk - Cdc25 must be phosphorylated to be active

Question 29

how do Positive feedback loops ensure robust Cyclin-Cdk activation?

Answer 29

- Active M-Cdk activates its activators -> Cdc25 phosphorylation - Active M-Cdk inhibits its inhibitors -> PP2A phosphatase inactivation, Wee1 kinase phosphorylation (making it inactive) Creates an “all or nothing” choice to proceed

Question 30

slide 24

Question 31

'green' lights for cyclin-Cdk activity

Answer 31

favourable extracellular environment

Question 32

'red' lights for cyclin-Cdk activity

Answer 32

DNA damage, unreplicated DNA, DNA damage, chromosome unattached to spindle

Question 33

give an example of cell retention in G1

Answer 33

cells can be retained in G1 by Rb inhibition of E2F: - E2F is a transcription factor required for S-phase gene expression - Rb binds and inhibits E2F - Rb can be inhibited by phosphorylation

Question 34

what happens if there is an Rb mutation?

Answer 34

you always have E2F around, meaning you are constantly undergoing DNA synthesis, leading to cell division out of control

Question 35

what do mitogens do?

Answer 35

they trigger production of the cell cycle control protein, Myc

Question 36

what are mitogens?

Answer 36

extracellular signaling molecules

Question 37

describe the action of mitogens

Answer 37

- They activate a signaling pathway including RTKs, Ras, and a MAP kinase cascade - These upregulate expression of Myc - Myc is a key transcription factor involved in cell cycle control

Question 38

describe Myc function

Answer 38

1. Myc activates G1-cyclin gene expression 2. G1-Cdk phosphorylates Rb 3. Phosphorylated Rb releases E2F 4. E2F activates expression of S-phase genes Positive feedback through: - E2F autoactivation - G1/S-Cdk & S-Cdk phosphorylation of Rb

Question 39

how does DNA damage blocks cell cycle progression via CKI synthesis?

Answer 39

1. DNA damage induces p53 transcription factor activation 2. p53 activates p21 expression 3. p21 is a CKI that inhibits G1/S-Cdk and S-Cdk

Question 40

what is the practical importance of p53?

Answer 40

half of all human cancers have a mutation with p53