Cell replication

Question 1

Which cells divide at different rates?

Answer 1

1. Embryonic vs adult cells: frog early embryo 30 min
2. Complexity of system: yeast 1.5-3 h
3. Necessity for renewal: intestinal epithelial cells around 20h hepatocytes around 1 year
4. State of differentiation: some cells never divide I.e. neurone and cardiac myocytes
5. Tumour cells
   - Appropriate regulation of cell growth rewired (i.e. contact inhibition)

Question 2

What is the relevance of appropriate regulation of cell division?

Answer 2

• Premature, aberrant mitosis results in cell death
• In addition to mutations in oncogenes and tumour suppressor genes, most solid tumours are aneuploid (abnormal chromosome number and content).
• Various cancer cell lines show chromosome instability (loose and gain whole chromosomes during cell division)
• Perturbation of protein levels of cell cycle regulators is found in different tumours - abnormal mitosis
• Contact inhibition of growth
• Attacking the machinery that regulates chromosome segregation is one of the most successful anti-cancer strategies in clinical use

Question 3

What is the cell cycle?

Answer 3

• Orderly sequence of events in which a cell duplicates its contents and divides in two
• Duplication, division, co-ordination

Question 4

What are the different phases in cell cycle?

Answer 4

M phase: mitosis (division)  Nuclear division
 Cell division (cytokinesis)
Interphase: (duplication) DNA
 organelles
 protein synthesis
G0 phase:  cell cycle machinery dismantled
G1 phase (gap): decision point 
S phase: synthesis of DNA/ protein 
G2 phase (gap): decision point

Question 5

What is the S phase?

Answer 5

DNA replication

• Protein synthesis: imitation of translation and elongation increased as well as capacity
• Replication of organelles: centrosomes, mitochondria, Golgi etc) in case of mitochondria, needs to coordinate with replication of mitochondrial DNA

Question 6

What is the centrosome?

Answer 6

DNA replication

1. Consists of two centrioles: barrels of nine triplet microtubules
2. Function: microtubules organising centre (MTOC) biotic spindle

Question 7

Describe prophase 1

Answer 7

condensed chromosomes - each consists of 2 sister chromatids, each with a kinetochore
In prophase, chromatin is condensed down (wrapped around positively charged histones) into chromosomes

Question 8

Describe prophase 2

Answer 8

• Replicated chromosomes condense
• Duplicated centrosomes migrate to opposite sides of the nucleus and organ size the assembly of spindle microtubules
• Mitotic spindle forms outside nucleus between the 2 centrosomes

Question 9

Describe spindle formation

Answer 9

• Radial microtubule arrays (ASTERS) from around each centrosome (MTOC)
• Radial arrays meet
• Polar microtubules form
• Microtubules are in a DYNAMIC state

Question 10

Describe metaphase

Answer 10

-Chromosomes aligned at equator of spindle

Question 11

Describe pro-metaphase I

Answer 11

Early pro metaphase:

• Breakdown of nuclear membrane
• Spindle formation largely complete
• Attachment of chromosomes to spindle via kinetochores

Question 12

Describe pro-metaphase II

Answer 12

Late pro metaphase

• Microtubules from opposite pole is captured by sister kinetochore
• Chromosomes attached to each pole congress to the middle
• Chromosome slides rapidly toward centre along microtubules

Question 13

What is anaphase?

Answer 13

• Paired chromatids separate to form 2 daughter chromosomes

- Cohesion holds sister chromatids together

Question 14

Describe Anaphase A

Answer 14

• Breakdown of cohesion
• Microtubules get shorter
• Daughter chromosomes pulled toward opposite spindle poles

Question 15

Describe Anaphase B

Answer 15

Daughter chromosomes migrate towards poles
Spindle poles (centrosomes) migrate apart

Question 16

Describe telophase

Answer 16

• Daughter chromosomes arrive at spindle
• Nuclear envelope reassembles at each pole
• Assembly of contractile ring

Question 17

What happens when something goes wrong in cell cycle?

Answer 17

E..g. cell not big enough or DNA damage
1. Cell cycle arrest
-At check points (G1 and spindle check point)
-Can be temporary (i.e. following DNA repair)
2. Programmed cell death (apoptosis)
-DNA damage too great and cannot be repaired
-Chromosomal abnormalities
-Toxic agents
Cell cycle progression aborted and cell destroyed

Question 18

What triggers a cell to enter cell cycle and divide?

Answer 18

-In absence of stimulus, cells go into G0 (quiescent phase)
-Most cells in the body which are differentiated to perform specific functions
-Cells are not dormant but are non-dividing
Exit from G0 highly regulated: requires growth factors and intracellular signalling cascades

Question 19

What is signalling through the cell like?

Answer 19

Signalling cascades:

1. Response to extracellular factors
2. Signal amplification
3. Signal integration
4. Modulation by other pathways
5. Regulation of divergent responses

Question 20

Why is mitosis most vulnerable period of cell cycle?

Answer 20

• Cells are more easily killed (irradiation, heat shock, chemicals)
• DNA damage can not be repaired
• Gene transcription silenced
• Metabolism?

Question 21

What is replication of centrosome?

Answer 21

1. Consists of two centrioles:
   Barrels of nine triplet microtubules.
2. Function:
   Microtubules organizing centre (MTOC)
   Mitotic spindle.

Question 22

What is needed for cell cycle checkpoints?

Answer 22

-Requires:
	CENP-E
	BUB protein kinases
-BUBs dissociate from kinetochore when chromosomes are properly attached to the spindle
When all dissociated, anaphase proceeds.
-Monotelic (just one bonded)
Syntelic (same MTOC)
Amphelic (normal)
Merotelic (both MTOCs to one)
-Can be targeted in cancer – generate permanent checkpoint signals to halt development

Question 23

What is signalling with protein kinase cascades?

Answer 23

• Kinases activation leads to signal amplification, diversification and opportunity for regulation
• c-Myc is an oncogene and can make a cell enter the growth phase (stimulated by cancer or GF

Question 24

What are cyclin dependent kinases?

Answer 24

-Cdk1, Cdk2, Cdk4, Cdk6
-Present in proliferating cells throughout cell cycle
-Activity is regulated by:
Interaction with cyclins
Phosphorylation

Question 25

What are cyclins?

Answer 25

-Cyclin A, B, D, E
-Transiently expressed at specific points in the cell cycle
-Regulated at level of expression
Synthesised, then degraded