Cell Cycle

Question 1

Give five reasons as to why different cells divide at different rates

Answer 1

Embryonic vs. Adult
Complexity of system 
Necessity of renewal 
State of differentiation (some cells e.g neurons never divide)
Tumour cells

Question 2

Why is it important to regulate cell division?

Answer 2

Abnormal mitosis results in:
cell death
mutations in oncogenes and tumour suppression genes
chromosomal instability (loose/gain whole chromosomes during cell division)
Deviation of protein levels of cell cycle regulators
Contact inhibition of growth
Attack machinery that regulates chromosome segregation

Question 3

What is the cell cycle?

Answer 3

Orderly sequence of events in which a cell duplicates its contents and divides in two
simply: duplication, division + co-ordination

Question 4

What occurs during the M-phase?

Answer 4

Nuclear division

Cytokinesis (cell division)

Question 5

What occurs during interphase?

Answer 5

DNA + organelle replication

Protein synthesis

Question 6

Why is mitosis the most vulnerable period of the cell cycle?

Answer 6

Cells are more likely to be killed (irradiation, heat shock, chemicals)
DNA damage cannot be repaired
Gene transcription silenced- no new proteins
Metabolism slows down

Question 7

What happen during G0 phase?

Answer 7

Cell cycle machinery dismantled

Functions normally

Question 8

What happens during G1 phase?

Answer 8

Decision point- is it able to replicate?

Question 9

What happens during S phase?

Answer 9

Synthesis of DNA/Protein

Replication of all the organelles

Question 10

What happens during G2 phase?

Answer 10

Another decision point

Question 11

What do the centrosomes do?

Answer 11

Consists of two centrioles, barrels of nine triplet microtubules
Microtubules organising centre
Duplicate before entering S phase

Question 12

What are the 6 phases of Mitosis?

Answer 12

Prophase
Prometaphase
Metaphase
Anaphase
Telophase
Cytokinesis

Question 13

What happens during prophase I?

Answer 13

Condensation of chromatin

Strong and packed so the chromosomes can move without too much damage

Question 14

What is a condensed chromosome made up of?

Answer 14

2 sister chromatids each with a kinetochore

Attached by centromere

Question 15

What happens during prophase II?

Answer 15

Duplicated centrosomes migrate to opposite sides of the nucleus and organise the assembly of spindle microtubules
Mitotic spindle from outside nucleus between the 2 centrosomes

Question 16

What is the spindle?

Answer 16

Radial microtubule arrays from around each centrosome
They grow until they meet in the centre
Polar microtubules form

Question 17

What happens during prometaphase I?

Answer 17

Chromosome aligned at equator of the spindle Breakdown of nuclear envelope
Spindle formation largely complete
Attachment of chromosomes to spindle via kinetochores

Question 18

What happens during prometaphase II?

Answer 18

Microtubule from opposite pole is captured by sister kinetochore
Chromosomes attached to each pole conger to the middle
Chromosomes slides rapidly towards centre along microtubules

Question 19

What happens during Anaphase A?

Answer 19

Cohesin, which holds sister chromatids together, breaks down
Microtubules shorten
Daughter chromosomes pulled towards opposite poles

Question 20

What happens during Anaphase B?

Answer 20

Daughter chromosomes migrate towards poles

Spindle poles migrate apart

Question 21

What happens during telophase?

Answer 21

Daughter chromosome arrive at spindle
Nuclear envelope reassembles
Assembly of contractile ring that separates the two cells

Question 22

What happens during cytokinesis?

Answer 22

Ring contracts
Midbody (remnants from microtubules that coordinated migration of chromosomes) begins to form
Cells divide

Question 23

What happens during the transition out of metaphase?

Answer 23

Spindle assembly checkpoint
Make sure all chromatids are bound to a microtubules
And that the microtubules are from different poles

Question 24

How does the cell communicate that the checks have been completed?

Answer 24

Each sister chromatid needs to signal

Proteins (CENP-E and BUB protein kinases) dissociate from kinetochore when chromosomes are properly attached.

Question 25

How may aneuploidy occur?

Answer 25

Mis-attachment of microtubules to kinetochores

Aberrant centrosome/DNA replication (multiple rounds of replication)

Question 26

What happens of the cell is not big enough or their is DNA damage?

Answer 26

1. Cell cycle arrest
   - at check points (G1 and spindle check point)
   - can be temporary (following DNA repair)
2. Programmes apoptosis
   - DNA damage too great cannot be repaired
   - Chromosomal abnormalities
   - Toxic agents

Question 27

How can tumour cells avoid check points?

Answer 27

Growth factors at G1 checkpoint induces hyper proliferation
May block G2 checkpoint, mitosis started when cell is not ready
Block Metaphase checkpoint

Question 28

How else may tumours deregulate cells?

Answer 28

Block exit from cell cycle into G0 phase

Cells repeat cycle immediately

Question 29

What triggers a cell to enter the cell cycle and divide?

Answer 29

Growth factors

Question 30

What happens during a signalling cascade?

Answer 30

Réponse to extracellular factors
Signal amplification
Signal integration
Modulation by other pathways

Question 31

What happens during protein kinase cascades?

Answer 31

Sequence of phosphorylation and activation of subsequent kinases
Signal reversed by phosphotases
Leads to amplification, diversification and opportunity fro regulation

Question 32

What dis c-Myc protein do?

Answer 32

Transcription factor
Peaks when growth factors are present
It is an oncogene- over expressed in many tumour cells

Question 33

What are cyclin-dependent kinases?

Answer 33

Present in proliferating cells throughout cycle
Activity regulated by interaction of cyclins and phophorylation
4 types: 1,2,4+6

Question 34

What do cyclins do?

Answer 34

4 types: A,B,D+E
Transiently expressed at specific points in cell cycle
Regulated at level of expression
Synthesised then degraded