The Cell Cycle

Question 1

List the main phases of the cell cycle and correlate them with the changes in DNA content that occur throughout the cycle:

Answer 1

G1 phase - DNA content remains constant at 2n meaning there is a single set of chromosomes
S phase - DNA content increases from 2n to 4n as each chromosome is copied
G2 phase - check for DNA replication errors
M phase - phase of cell division, cytokinesis occurs

Question 2

Summarise cell cycle control system driven by cyclin-dependent kinases (Cdk)

Question 3

Correlate different cyclin-Cdk complexes with each phase of the cell cycle

Question 4

Identify reversible phosphorylation of proteins as the central signalling mechanism employed by the subsystems of cell cycle control

Question 5

Recognise the importance of transcriptional control of the cell cycle through regulated
expression of genes involved in cycle progression and DNA replication.

Question 6

Discuss cycle control through proteolysis directed by the ubiquitin-ligases APC/C and SCF

Question 7

Outline roles of Cdk inhibitory proteins and transcriptional suppressor protein Rb1

Question 8

Describe eukaryotic chromosomes:

Answer 8

Function - genetic information storage
46 chromosomes (23 homologous pairs)
Chromatin - when chromosomes aren’t dividing
Chromatids - duplicated chromosomes
Centrosome - kinetochore and microtubule spindle attachment

Question 9

Describe interphase:

Answer 9

G1 phase - increase in size
S phase - DNA replication, Two sister strands of DNA produced
G2 phase - Organelles double
New cytoplasm forms
Mitotic structure forms

Question 10

What is the G0 phase ?

Answer 10

Resting or quiescent phase in the cell cycle where cells exit the regular cycle of division
Cells in the G₀ phase are metabolically active but do not actively prepare for DNA replication or cell division
Quiescence is a reversible state in which cells can re-enter the cell cycle in response to specific signals
Sensence - cells remain in G₀ and cannot re-enter the cell cycle

Question 11

What are the stages of M phase ?

Answer 11

Prophase
Prometaphase
Metaphase
Anaphase
Telophase
Cytokinesis

Question 12

What are the 3 types of spindle used in metaphase ?

Answer 12

Aster microtubules
Kinetochore microtubules
Interpolar microtubules

Question 13

Describe metaphase:

Answer 13

Centrosomes localise to opposite ends of the cell
Chromosomes line up on the metaphase plate
Microtubules finish attaching to kinetochores

Question 13

Describe prophase and prometaphase:

Answer 13

Prophase;
Chromatin begins to condense and centrosomes duplicate
Nuclear envelope begins to dissolve
Mitotic spindle begins forming
Duplicated chromosomes condense under the control of protein complexes called condensins regulated by M-Cdk

Prometaphase;
Chromosomes completely duplicate
Microtubules begin to attach to kinetochores
Non-kinetochore microtubules begin to extend

Question 14

Describe anaphase

Answer 14

Chromosomes break apart
Sister chromatids are pulled apart toward opposite poles
The cell elongates as non-kinetochore microtubules lengthen
Shortest stage

Question 15

Describe telophase

Answer 15

Nuclei and nuclear envelope reform
Chromosomes de-condense
Spindles are depolymerised
Nucleoli reappear
Contractile ring starts to form
Cytokinesis overlaps with telophase

Question 16

Describe cytokinesis

Answer 16

Division of the cytoplasm

Question 17

Describe the effects of DNA damage:

Answer 17

Can block cell cycle progression
p53 (transcriptional regulator) activates p21, a Cdk inhibitor protein
If DNA damage isn’t repaired, p53 can activate cell death (apoptosis)