Cell Cycle Control

Question 1

Name and describe the stages in mitosis:

Answer 1

Prophase- the DNA condenses and chromosome structure appears (nucleolus disappears)
Prometaphase- Microtubules from centrosomes attach to kinetochore of chromatid
Metaphase- the sister chromosomes align in the centre so each chromatid facing opposite poles
Anaphase- the chromosomes separate due to shortening of MTs
Telophase- nucleomembranes reappear around the 2 chromosomes
Cytokinesis- splitting of the membrane by a contractile ring

Question 2

Name and describe the stages in interphase:

Answer 2

Most of the cell sits in interphase
G1= Gap1, 1st growth stage, biochemical function, double in number of organelles
S= synthesis, DNA in nucleus is replicated
G2= Gap 2, cell finishes growing, allows mitosis to begin

Question 3

How does the cancer cell cycle control the activity of their proteins?

Answer 3

1. Phosphorylation/ dephosphorylation
2. Make more of protein, inhibit and degrade

Question 4

How does the cancer cell cycle involve phosphorylation/ dephosphorylation to control proteins?

Answer 4

Proteins can be switched on and off, by phosphorylation and dephosphorylation
Both can either activate or deactivate, it depends

Question 5

What is phosphorylation?

Answer 5

The transfer of a phosphate group from ATP to the OH group on a protein
This is done by a protein kinase

Question 6

What is dephosphorylation?

Answer 6

Removal of the phosphate group
This is done by a phosphatase

Question 7

How does the cancer cell cycle involve making more proteins/ inhibit and degrade to control proteins?

Answer 7

Proteins can be switched on, de novo synthesis from mRNA (make more) by transcription/ translation
Proteins can be switched off , binding to an inhibitory protein that keeps it inactive, or, by targeting for degradation, attatchment of one or more ubiquitin (Ub- small regulatory protein) by a ubiquitin ligase
This signals that the protein has to go to the proteasome for degradation and recycling of components

Question 8

What are the three components involved in cell cycle control?

Answer 8

Cyclins
Cyclin dependent kinases (CDKs)
Cyclin kinase inhibitors (CKIs)

Question 9

Describe cyclins as a component in cell cycle control:

Answer 9

Presence in cell is determined by which stage if cell cycle someone is at
During interphase the conc of cyclins increase, at mitosis is the highest conc of cyclin, at the end of mitosis, cyclin is degraded
Many types: A, B, C, D1, D2, D3, E

Question 10

Do cyclins have enzymatic activity?

Answer 10

Have no enzymatic actvity
When bound to CDKs, the kinase can become enzymatically active

Question 11

Describe CDKs as a component in cell cycle control:

Answer 11

Can’t function unless bound to cyclin
CDK 1, 2, 4, 6
CDK’s conc is stable during cell cycle
CDKs activity is controlled by cyclins and CKIs

Question 12

Describe CKIs as a component in cell cycle control:

Answer 12

Bind to CDKs
Different families:
-INK CDK inhibitors; p15, p16, p18, p19- all based on size
-KIP/CIP inhibitors; p21, p27, p57

Question 13

What needs to occur in order for a Cyclin-CDK complex to become activated?

Answer 13

The CDK has to be phosphorylated at 3 sites and dephosphorylated at 2 sites

Question 14

What is the cyclin complex at the G1 phase?

Answer 14

Cyclin D and CDK 4 and 6

Question 15

What is the cyclin complex at the G1/S phase?

Answer 15

Cyclin E and CDK 2

Question 16

What is the cyclin complex at the S phase?

Answer 16

Cyclin A and CDK 2

Question 17

What is the cyclin complex at the Mitosis phase?

Answer 17

Cyclin B and CDK 1

Question 18

Describe the replication of DNA in the S phase:

Answer 18

Controlled by S-CDK complexes (Cyclin A/CDK2)
DNA replication must occur first and only once per cell cycle
It begins at the origin of replication when the pre-replicative complex is assembled

Question 19

What does S-CDK do?

Answer 19

DNA replication can only occur when the complex is activated
Triggers DNA replication recruiting DNA polymerase
Blocks re-replication by causing dissociation and degradation of CDC6 and exports other proteins from the pre-replicative complex from the nucleus

Question 20

What controls the chromosome segregation in the mitotic phase?

Answer 20

Controlled by m-CDK complexes (Cyclin B/CDK1) and APC (ubiquitin ligase)

Question 21

What needs to happen for chromosome segregation in the mitotic phase?

Answer 21

Synthesis and accumulation of cyclin B of M phase after S phase
Formation of complexes, M-CDK still inactive
At the end of G2, phosphatase CDC25 remove phosphorylation causes activation of M-CDK complexes

Question 22

What does active M-CDK do?

Answer 22

Inhibits entry into M phase if DNA replication is incomplete
Trigger chromosome condensation (by phosphorylation of condensins)
Trigger cytoskeleton rearrangements (by phosphorylation of MAP proteins that stabilise MT cause an increase in dynamic stability of MT)
Trigger nuclear envelope breakdown (by phosphorylation of Laminin and nuclear pore proteins)

Question 23

What occurs in metaphase for the chromosomes to separate, going into anaphase?

Answer 23

Key protein called Anaphase Promoting Complex (APC)
Its a ubiquitin ligase that promotes the destruction of several regulatory proteins
Active APC catalyses the degradation of securin (inhibitory protein to separase), allowing to separate the cohesion complex that keeps the sister chromatids together
If Mts not attached to the kinetochore then APC activation is blocked

Question 24

Describe the process of returning to the G1phase from the M phase:

Answer 24

Requires the inactivation of the M Phase cyclin- CDK by APC mediated ubiquitination that causes proteolysis of M cyclins so that;
-chromosomes decondense
-Laminin gets dephosphorylated and the nuclear envelope reassembled
-Mitotic spindle de-assembles and cytoskeleton rearranges

Question 25

What are the outcomes of M-CDK formation?

Answer 25

Triggers rearrangement of MTs
Causes nuclear envelope to break down
Triggers DNA to condense into chromosomes

Question 26

Describe the continue of proliferation from the G1 to S phase:

Answer 26

Controlled by G1-CDK complexes (Cyclin D/CDK 4 and 6) and Rb

Question 27

What triggers the progression from G1 to S?

Answer 27

The accumulation of G1 phase cyclins and their formation of G1-CDK complexes
This is stimulated by extracellular signals that promote proliferation-> mitogens, hormones, GFs
The degradation of CDK inhibitors

Question 28

What is Rb?

Answer 28

Retinoblastoma protein
Resting cell not receiving any GF signals, in nucleus there is an active Rb, prevents transcription and translation

Question 29

How does self proliferation occur from G1 to S phase?

Answer 29

When GF signals, an intracellular signalling pathway which causes an active G1-CDK and G1/S-CDK complex which phosphorylates active Rb protein (so inactive) so a transcription factor is now active so transcription and translation of genes allowing self proliferation

Question 30

Describe the resting cell which remains in the G1 phase so it can’t enter the S phase:

Answer 30

The transcription factor E2F
CKI (p16) inactivates G1-CDK complexes
Rb becomes dephosphorylated (active)
It then binds to E2F
Rb-E2F complexes prevent E2F mediated transcription of genes required for S phase

Question 31

Describe the active cell which allows the G1 phase to enter the S phase:

Answer 31

Active G1-CDK phosphorylates and inactivates Rb
Phosphorylated Rb releases E2F
E2F activates transcription genes that encode proteins required for S-phase like ‘G1/S cyclins’ and ‘S cyclins’
So the cell can enter S phase

Question 32

What are the factors that control DNA damage from G1 to S phase and why?

Answer 32

Cell must repair damage before chromosomes are replicated
The G1 checkpoint is controlled by p53 (the guardian of the genome) and CDKI

Question 33

How is p53 involved in controlling DNA damage when there is NO DNA damage?

Answer 33

In absence of DNA damage, p53 is targeted for degradation in proteasome with MDM2 (a ubiquitin ligase) so cell cycle progression

Question 34

How is p53 involved in controlling DNA damage when there is DNA damage?

Answer 34

With DNA damage, there is an activation of PK which phosphorylates p53
Once p53 is phosphorylated it becomes stable, so it can bind to specific regions on DNA to allow transcription to take place
p53 binds to p21 gene, which leads transcription of p21 mRNA and translation to protein
p21 is a CKI, making the cyclin-CDK complex inactivated, so cell cycle is stoped and repair genes produced/regulated
If DNA damage is too severe to repair, p53 triggers cell death

Question 35

Describe microtubules:

Answer 35

They are motor protein so use energy
Kinesisns walk towards +ve end
Dyneins walk towards -ve end

Question 36

What does it mean that microtubules are in a dynamic state?

Answer 36

Occurs during mitosis
MTs switch between periods of growth to shrinkage (catastrophe) and shrinkage to growth

Question 37

How long does mitosis last for in the cell cycle?

Answer 37

1-2 hours

Question 38

How long does Interphase last for in the cell cycle?

Answer 38

20-22 hours

Question 39

Name the 3 classes of microtubules:

Answer 39

Astral
Kinetochore
Overlap MTs

Question 40

Describe the Astral MTs:

Answer 40

Star like
Radiating from centrosome and thought to contribute to pole separation
Keep the two opposite poles, hold the cell membrane

Question 41

Describe the Kinetochore MTs:

Answer 41

Attach to the kinetochore on duplicated chromosome

Question 42

Describe the Overlap MTs:

Answer 42

Interpolar
Overlap at the equator, responsible for bipolar shape of the spindle
From one pole to another, they overlap in the middle