The Cell Cycle

Question 1

List the stages of the cell cycle

Answer 1

Mitosis (M) phase
Interphase- G1, S, G2

Question 2

What is the role of Cyclins in the Cell cycle

Answer 2

-family of proteins cyclins A,B and E
-activate cyclin dependent kinases (CDKs) which act in conjunction with cyclins
-Upon completion of cell cycle, cyclins and CDKs are degraded: the residues of used molecules are taken up by cytoplasmic care taker protien; ubiquitin

Question 3

What occurs during G0?

Answer 3

• the resting phase
• cell is neither dividing nor preparing to divide

Question 4

What occurs during G1?

Answer 4

-cells respond to growth factors
-irreversible -primary point at which it must choose whether or not to divide
-retinoblastoma protein (Rb) acts as a tumour suppressor by inhibiting the transcription of genes that code
for proteins needed for DNA replication
-Phosphorylation by G1 cyclin-CDK inhibits
the retinoblastoma protein (Rb) allowing transcription of the genes that code for proteins needed for DNA replication.
-most of the molecular machinery required to complete another cell cycle is generated
- controlled by cyclin E and CDKs
- time 2-100 hours (very variable)

Question 5

What occcurs during S phase?

Answer 5

-DNA replication (takes place in chromosomes)
-individual chromosome are not visible
-promoted by cyclin A and CDKs
-Begins 8 hours after the end of mitosis and take about 7 to 8 hours to complete

Question 6

What occurs during G2 phase?

Answer 6

• the cell prepares for division
• asseses DNA damage
• this period ends with the breakdown of the nuclear membrane and the onset of chromosome condensation
• controlled by cyclin B and CDKs
• 2-4 hours

Question 7

List the stages of Mitosis

PMAT

Answer 7

Prophase
Metaphase
Anaphase
Telophase

Question 8

Prophase

Answer 8

-chromosomes (splits longitudinally) are visualised each consisting of two sister chromatids attatched at a centromere
-spindle microtubules extend from the centrosome by polymerisation and attach to chromosomes via their kinetochores in the centromere region.
-1.5 hours

Question 9

Metaphase

Answer 9

-chromosomes are aligned at
the metaphase plate (equator of the spindle)
-20 mins

Question 10

Anaphase

Answer 10

-Centromeres split longitudinally and chromatids separate to form 2 new chromosomes.
-Spindle fibres contract (exerting a centrifugal force in centromere region), each pair of newly formed chromosomes separates and migrates to the opposite pole of the spindle

Question 11

Telophase

Answer 11

-It is a reorganization period
-The daughter chromosomes are enveloped by new nuclear membrane and nucleolus appears
-The chromosomes become uncoiled and their individual identity is lost.
-The cytoplasm divides and two complete cells are formed
-In the process of cytokinesis, the cleavage furrow develops around the equator region of achromatic spindle.

Question 12

What is non-disjunction?

Answer 12

non-disjunction:
where one daughter cell receives extra chromosomes and the other daughter cell is deficient in this chromosome
why:
-Due to abnormal function of spindle apparatus
-One or more chromosomes fail to migrate properly in anaphase

Question 13

**

What is Isochromosome?

Answer 13

the centromere splits transversely instead of longitudinal division, leading to formation of two daughter chromosome of unequal length.