The Cell Cycle

Question 1

Describe the 4 phases of the cell cycle.

Answer 1

• G0- exit phase where cell leaves cell cycle and needs a signal to reenter (eg cancer)
• G1-Gets ready for DNA replication. Each chromosome has a single chromatid (not duplicated yet.
• S-Each chromosome duplicated to contain two joined chromatids
• G2- cell prepares for metaphase and cell division (contains twice the amount of G1 DNA)
• Cytokinesis

Question 2

Describe what happens in prophase, metaphase, anaphase and telophase.

Answer 2

Prophase:

• Chromatin condenses into chromosomes
• Nuclear envelope breaks down.
• Centrioles separate and move to poles
• Spindle formation

Metaphase:

• Chromosomes align to equator
• Each sister chromatid linked to opposite centromere

Anaphase:
-Chromatids pulled apart by mictitibule shortening and centrosome movement

Telophase:

• Decondense chromosomes
• Nuclear envelope forms

Question 3

What are kinetochores?

Answer 3

The areas of the chromosome to which the mitotic spindles attach. Associated with the centromere.

Question 4

Why is each specific phase procedure followed by a ‘check point’?

Answer 4

• Check for damage to DNA (to destroy cell and prevent it going any further in the cell cycle). -Check that favourable environmental condition are available.
• Check for correct mitotic spindle attachment

Question 5

What is flow cytometry?

Answer 5

Measuring the DNA content per cell. This tells you which stage the cell is in.

G1 phase has a relative amount of DNA of 1, G2 phase has a relative number of 2 and the DNA in the S phase is somewhere between 1 and 2.

Question 6

What are checkpoints controlled by? What do these enzymes do?

Answer 6

Controlled by cell cycle kinases. Cell cycle kinases are called CYCLIN DEPENDENT KINASES (CDKs)

They phosphorylate proteins.

Question 7

What are CDKs and what are they activated by?

Answer 7

They are cyclin dependent kinases which control the cell checkpoints.

They are activated by cyclins.

Question 8

Describe the 3 types of CDKs/cyclins.

Answer 8

• G1/S cyclin activates CDK in G1 which phosphorylate and inactivate proteins that block progression through the restriction point and activate the synthesis of S cyclins
• S cyclins activate S-CDKs which phosphorylate and activate DNA replication proteins and allow entry into the S phase.

Question 9

Describe the levels of CDKs and cyclins throughout the cell cycle.

Answer 9

CDK protein levels are constant, only the cyclin level changes and are present at specific times only.

Question 10

Describe when CDK-cyclin complexes are present throughout the cell cycle.

Answer 10

S-cyclin is synthesised in G1
S-cyclin is destroyed during mitosis
They activate DNA replication proteins and allow entry into the S phase

M-cyclin is synthesised in G2 phase
Triggers preparation for cell division
M-cyclin is rapidly destroyed at end of metaphase

Question 11

Describe what these check for:
G1-CDK
G1/S-CDK and S-CDK
M-CDK
APC/C

Answer 11

G1-CDK- favourable extracellular environment

G1/S-CDK and S-CDK-DNA damage

M-CDK- unreplicated DNA and DNA damage

APC/C- chromosome unattached to spindle