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BL2301 Cell Biology > The cell cycle > Flashcards

The cell cycle Flashcards

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1
Q

Define the cell cycle

A

The process by which a cell duplicates its contents in order to produce two daughter cells

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2
Q

What 2 major processes does the cell cycle involve?

A
  • DNA replication

- Accurate chromosome segragation

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3
Q

What occurs during the S phase

A

Replication of chromosomes

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4
Q

What occurs during the M phase

A

Segregation of chromosomes (mitosis) before the cell divides (cytokinesis)

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5
Q

Name the 5 stages of mitosis in the order that they occur

A
  1. Prophase
  2. Prometaphase
  3. Metaphase
  4. Anaphase
  5. Telophase
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6
Q

What key change occurs at the metaphase to anaphase transition?

A

Sister chromatid cohesion (formed during S phase) is lost and the sister chromatids are pulled appart

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7
Q

What are the 4 main stages of the cell cycle in order?

A

G1 phase
S phase
G2 phase
M phase

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8
Q

What is the function of the gap (G) stages?

A
  • allow cell to grow
  • prepare for S and M phases
  • monitoring of internal and external cell environment
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9
Q

What occurs when conditions are not favourable at the G1-S transition?

A

Cells enter specialised resting state referred to as Gzero

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10
Q

How is cell cycle progression controlled?

A
  • complex network of regulatory interactions
  • include ordered biochemical switches and checkpoints that ensure the next stage has not started before all earlier things are completed
  • monitoring of outside signals in response to developmental/environmental cues
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11
Q

Why is budding yeast a useful model for the study of the cell cycle?

A
  • Generation time of under 2 hours
  • Simple to grow in the lab
  • Very amenable to genetic manipulation
  • Can be maintained in haploid/diploid form
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12
Q

What is the advantage of using haploid organisms for studies?

A
  • Genes can be mutated to investigate function without interference from a second copy
  • However can only use conditional lethal mutants, as inactivation of essential genes would cause organism death
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13
Q

Why is xenopus laevis a good model for cell cycle investigations?

A
  • Egg extracts and sperm can be mixed to recapitulate early cell cycle events
  • Egg extracts can be analysed biochemically by depletion of individual factors
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14
Q

What are the 3 main regulatory points in the cell cycle?

A

G1-S
G2-M
Metaphase-anaphase

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15
Q

How is the progression through the cell cycle regulated?

A

CDK-cyclin complexes

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16
Q

What are CDK-cyclin complexes composed of?

A
  • catalytic subunit containing protein kinases (CDKs) which are present at a constant level
  • regulatory subunits - cyclins which undergo synthesis and destruction
17
Q

What are the 3 essential classes of cyclins and their roles?

A
  • G1/S cyclins which activate in late G1, committing to S-phase entry (start)
  • S phase cyclins interact with CDKs after the ‘start’ to initiate DNA replication
  • M phase cyclins activate CDKs to drive entry into mitosis
18
Q

Appart from activating CDK enzymes, what is the other role of cyclins?

A

Confer substrate specificity

19
Q

Describe an additional method of cell cycle regulation

A

Some substrates are only available at some points during the cell cycle

20
Q

How are cyclin dependant kinases activated?

A
  • Cyclin binds to T-loop on ATP molecules in kinase, causing partial activation
  • kinase CAK then phosphorylates serine/threonine in the T-loop in order to further enhance activity
21
Q

How is M-CDK activity regulated?

A
  • Wee1 protein kinase phosphorylates Cdk1 on two neighbouring amino acids to the active site causing inhibition
  • Cdc25 phosphoprotein phosphatase removes phosphates in order to relieve inhibition
22
Q

How is M-CDK activity amplified during the G2-M transition?

A

Through positive feedback loops

23
Q

How do CDK inhibitor proteins work?

A

e.g p27

distorts active site and partially blocks the ATP-binding site

24
Q

How is the metaphase-anaphase transition triggered?

A

Via regulated proteolysis

  • Protein destruction triggered by catalysed by a ubiquitin ligase known as the anaphase promoting complex or cyclosome (APC/C)
  • S- and M-cyclins are a major target of APC/C
25
Q

How does APC/C carry out regulated proteolysis?

A
  • Becomes covalently attached to a lysine side chain of the target protein
  • Polyubiquitin chains are then formed by linking more ubiquitin chains via Lys48
26
Q

How does APC/C interact with the proteasome?

A
  • Has 3 domains: ring, central cylinder and cap
  • Unfoldase at the ring unfolds the target protein, ubiquitins are then cleaved off and then cyclins are broken down at the central cylinder by protease
27
Q

How is APC/C activated?

A

mid-mitosis - Cdc20

late mitosis/ early G1 - Cdh1

28
Q

What needs to happen after S/M-CDK activity has been abolished in order to complete mitosis & cytokinesis?

A

Proteins previously phosphorylated during the S phase to early mitosis need to be dephosphorylated before mitosis/cytokinesis can be completed

29
Q

Why is the protein securin targetted by APC/C action?

A
  • Securin inhibits the activity of the protease separase
  • When degraded at the end of metaphase, separase can then cleave off subunits of protein cohesin resulting in sister chromatid separation at anaphase
30
Q

What is SCF?

A

A ubiquitin ligase that functions during S phase

31
Q

What is the function of SCF?

A

targets various CKIs for destruction in late G1 and is responsible for G1/S cyclin destruction in early S-phase

32
Q

How is SCF regulated differently to APC/C?

A

SCF is always presents but requires substrates in order to be phosphorylated

33
Q

What is required for the initiation of chromosome replication?

A
  • triggered by an increase in S-Cdk activity

- Assembly of pre-RC complexes occurs from late mitosis-early G1 when CDK activity is low

34
Q

How is DNA replication regulated so that it only occurs once per cell cycle?

A
  • S-CDK phosphorylates 2 proteins (Sld2 and Sld3) in order to facilitate the assembly of the CMG complex (replicative helicase in eukaryotes)
  • CMG is then fully activated using protein kinase DDK
  • Mechanisms such as protein phosphorylation, degredation, nuclear exclusion and inhibitor binding then prevent pre-RC complexes from reassembling until the next cell cycle
35
Q

Overall, which stages in the cell cycle are caused by M-CDK activity and which stages are caused by APC/C destruction of securin and cyclins?

A

MCDK - prophase, prometaphase, metaphase

APC/C - anaphase, telophase, cytokinesis

36
Q

Which events are triggered by M-CDK activity?

A
  • chromosome condensation (phosphorylates condensin subunits)
  • spindle assembly
  • nuclear envelope breakdown
  • rearrangement of the actin cytoskeleton
  • rearrangement of the Golgi apparatus
37
Q

How can APC/C activation be inhibited?

A
  • If chromosome fails to attach to spindle it will bind to the Mad2 protein which then inhibits Cdc20 which goes on to inhibit APC/C

BL2301 Cell Biology (17 decks)

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