The Cell Cycle

Question 1

Define the cell cycle

Answer 1

The interval between two successive mitotic divisions resulting in the production of 2 daughter cells

Question 2

List the 2 main phases of the cell cycle

Answer 2

Interphase and M phase

Question 3

List the phases in interphase

Answer 3

G1, S, G2

Question 4

What happens in the G0 phase?

Answer 4

Cell not actively dividing - Quiescence
Cell remains metabolically active but not actively dividing
Cell can either enter G1 phase or die

Question 5

What happens in the G1 phase?

Answer 5

Extra organelles and proteins produced for S phase
Monitoring of external environment
Cells grow in size and prepare to undergo DNA synthesis
Commitment to divide

Question 6

What happens in the S phase?

Answer 6

Each chromosome is duplicated resulting in identical chromosomes, known as sister chromatids
Sister chromatids attach to each other at the centromere

Question 7

What happens in the G2 phase?

Answer 7

Growth more directly involved in preparing for mitosis
Cell organelle replication and microtubule syntheisis

Question 8

What are the 2 phases in the M phase?

Answer 8

Mitosis and Cytokinesis

Question 9

What are the different phases of mitosis?

Answer 9

Prophase, prometaphase, metaphase, anaphase, telophase

Question 10

What occurs in prophase?

Answer 10

Chromosomes condense
Nucleolus disappears
Centrioles move to poles

Question 11

What occurs in prometaphase?

Answer 11

Nuclear membrane dissolves
Chromosomes attach to microtubules and begin moving

Question 12

What occurs in metaphase?

Answer 12

Spindle fibres align to chromosomes along middle of cell nucleus (metaphase plate)
METAPHASE CHECKPOINT

Question 13

What occurs in Anaphase?

Answer 13

Centromere splits separating the paired chromosomes
Chromatids move to opposite sides of cell

Question 14

What occurs in telophase?

Answer 14

Chromatids arrive at opposite poles
New membranes form around daughter nuclei
Chromosomes decondense
Spindle fibres disperse

Question 15

What occurs in cytokinesis?

Answer 15

Plasma membrane around spindle equator becomes indented, forming cleavage furrow
Two cells of equal size produced

Question 16

What is a checkpoint?
What are the 4 checkpoints in the cell cycle? When do they each occur?

Answer 16

Checkpoints are points in the cell cycle where it can be halted if something is wrong to ensure 2 viable daughter cells are produced.
4 Checkpoints:
- Restriction point (G1)
- DNA damge checkpoints (late G1 and G2)
- Metaphase checkpoint (during mitosis)

Question 17

What occurs at the Restriction point?

Answer 17

Cell is committed to cell cycle (sufficient signal from growth factors needed)
1. Growth factor binds to receptor causing activation of pathway through cytosol into nucleus
2. Pathway results in production of cyclin D
3. ATP and cyclin D bind to cdk4/6 activating it
4. Active cdk4/6 phosphorylates RB protein using phosphate from ATP - ADP and cyclin D detach
5. Phosphorylation of RB protein causes it to detach from E2F
6. E2F free to transcribe and make proteins for S phase

Question 18

What occurs at the G2 checkpoint?

Answer 18

Check for DNA damage and allows entry into mitosis
1. Cyclin B levels rise during G2
2. Cyclin B and ATP bind to cdk1 (aka MPF) and activate it
3. Activated cdk1 causes mitosis by:
- phosphorylating lamins which cause break down of nuclear lamina by intermediate filament degradation
- active cdk1 phosphorylates condensins and histones causing chromosome condensation
- active cdk1 phosphorylates microtubule-associated proteins helping with spindle formation

Question 19

What occurs at the metaphase checkpoint?

Answer 19

Delays anaphase until all chromosomes are correctly attached to spindle
- Unattached chromosomes = checkpoint on and anaphase inhibited
-APC (anaphase promoting complexes) are inactive until all spindles are aligned and spindle checkpoint turned off
- When APC is activated cohesin is involved in cleaving sister chromatids

Question 20

What are CDKs? What is their function? How are they regulated? 2 Examples:

Answer 20

CDK = cyclin dependent kinases
- Group of proteins involved in regulating cell cycle by modifying various protein substrates by adding/ removing phosphates when activated by cyclin
Regulation of CDKs:
- cyclins (most important): activator protiens which are up or down regulated depending on cell cycle phase, different cyclins associated with different cdks
- CKIs: small proteins that can inactivate CDKs by forming an inactive complex or acting as competive CDK ligand
Examples:
cdk1/ MPF and cyclin B
cdk4/6 and cyclin D

Question 21

What are tumour suppressors?
Name 2 examples of tumour suppressors and how they function:

Answer 21

Tumour suppressor genes encode normal cellular proteins that inhibit cell proliferation and growth of cell to maintain genome integrity
RB (retinoblastoma protein) block entry to cell cycle by attaching to E2F
p53 detects DNA damage