Cell Cycle

Question 1

name the stages in the cell cycle

Answer 1

G1, S phase, G2, M phase and G0

Question 2

what happens at the G1 checkpoint?

Answer 2

• beginning of cell cycle
• preparation phase of the cycle
• preparing to enter S phase
• check for DNA damage and extracellular damage

Question 3

what happens in S phase?

Answer 3

DNA synthesis

Question 4

what happens at the G2 checkpoint?

Answer 4

• cells with duplicated chromosomes held together by centromere
• another preparation phase
• 2x number of organelles
• increase amount of cytoplasm
• checks DNA damage and if DNA replicated correctly

Question 5

what happens in the M phase?

Answer 5

-mitotic division

Question 6

what is G0?

Answer 6

the resting phase

• cell cycle re-entry not possible
• maintain in G0 unless stimulations to divide

Question 7

what is interphase?

Answer 7

all stages except M phase

Question 8

what are the phases of mitosis?

Answer 8

• prophase
• prometaphase
• metaphase
• anaphase
• telophase
• cytokineses

Question 9

what happens in prophase?

Answer 9

• chromosomes condense
• centromeres move to opposite poles
• mitotic spindles form

Question 10

what happens in prometaphase?

Answer 10

• breakdown of nuclear envelope

- chromosomes attach to mitotic spindle

Question 11

what happens in metaphase?

Answer 11

• centrosome on opposite ends

- chromosome most condensed and line up on equator of mitotic spindle

Question 12

what occurs at anaphase?

Answer 12

• sister chromatids separate synchronously

- each new daughter chromosome move to opposite spindle pole

Question 13

what happens at telophase?

Answer 13

• chromosomes arrive at spindle poled
• chromosomes decondense
• nuclear envelope reforms

Question 14

what is the last phase of mitosis and what happens?

Answer 14

• cytokineses

- cytoplasm divides to form 2 daughter cells

Question 15

what are the 3 requirements for mitosis

Answer 15

1) each phase occurs once
2) phases must be in the right order
3) phases must be non overlapping

Question 16

what is the mitotic spindle and what is its fuction?

Answer 16

a microtubule machine that assembles during prophase and separates duplicated chromosomes

Question 17

what are the 3 mitotic spindles?

Answer 17

• astral microtubules
• kinetichore microtubules
• interpolar microtubules

Question 18

what happens at cytokinesis?

Answer 18

• contractile ring divides cells by two
• cytoskeletal structure of actin myosin
• ring contracts to form cleavage furrow

Question 19

how are cell organelles distributed un cell division?

Answer 19

• relies on organelles to be scattered randomly in cytoplasm

- number of copies of organelles are roughly equal

Question 20

what is it called when meiosis goes wrong and what happens?

Answer 20

• non disjunction
• failure of homologous to separate in MI or MII
• non disjunction at MI: autosomal chromosomes
• non disjunction at MII: sex chromosomes

Question 21

what are the consequences of non disjunction?

Answer 21

-usually the death of foetus or dies soon after

Question 22

what are some non viable sex chromosome monosomies

Answer 22

sex chromosomes:

• XO (Turner’s syndrome)
• XXX (Triple X syndrome)
• XXY (Klinefelter’s syndrome)

Question 23

what are the cell cycle regulators and what do they do?

Answer 23

Cyclin dependent kinases (Cdks)
-enzymes that phosphorylate the 
target proteins 
-become active when bound to a 
corresponding cyclin 

Cyclins
-regulators of Cdks
-different cyclins are produced at
each phase of the cell cycle

Question 24

what re the basic principles of cell cycle control?

Answer 24

• Cdk levels fairly stable throughout cell cycle
• cyclin levels vary as part of cell cycle
• Cdk bound to Cyclin is active and phosphorylates a ‘target’ protein
• Cdk activation triggers the next step in the cell cycle, such as the entry into the S or M phase
• Cyclin degradation terminates Cdk activity

Question 25

what normally happens at the G1 checkpoint to allow the cell to enter the S phase?

Answer 25

• induction/expression of Cyclin D which associates with CDK 4
• binding/ activation of Cdk4
• phosphorylation of pRB by CDK4
• release and activation of E2F
• S phase gene transcription

Question 26

what happens at the G1 checkpoint if there is damage to the DNA?

Answer 26

• p53 (tumour suppression gene) is degraded quickly as it is unstable, and maintained at very low levels
• phosphorylated (active) p53 is not degraded
• active p53 promotes transcription of genes that induce cell cycle arrest,
• it binds to the promoter region of the p21 gene and stimulates p21 expression
• p21 binds and inhibits G1/S-Cdk/S-Cdk complexes
• cell arrests in G1 (allowing time to repair the damaged DNA)

Question 27

what are the two families of Cdk Inhibitors (CKIs)?

Answer 27

Inhibitor of Kinase 4 family (INK4):
-specifically inhibit G1 CDKs (e.g. CDK4)

CDK Inhibitory Protein/Kinase Inhibitory Protein
(CIP/KIP) family
-inhibit all other CDK-cyclin complexes (late G1, G2 & M)

-gradually sequestered by G1 CDKs thus allowing
activation of later CDKs

Question 28

how does the misregulation of the cell cycle cause cancer?

Answer 28

• cells escape the normal cell cycle checkpoint, leading to uncontrolled progression through the cell cycle
• many genes that regulate the cell cycle (eg. p53, p21) are often mutated in human cancers