Week 3- Cell Cycle

Question 1

Why do cells divide at different rates?

Answer 1

Embryonic vs stem cell, necessity for renewal, have adaptations (eg tumor cells), some cells don’t replicate (eg neurons and cardiac myocytes)

Question 2

What are some markers that display regulation of cell division?

Answer 2

No of chromosomes, protein levels inside the cell

Question 3

What is the general eukaryotic cell cycle?

Answer 3

1. M phase- mitosis
2. Interphase (G0: cell cycle machinery is dismantled G1: decision point where cell monitors size and external signals)
3. S phase : synthesis of DNA and proteins
4. G2: decision point

Question 4

In which phase of the cell cycle is the cell most vulnerable?

Answer 4

Mitosis as DNA can’t be repaired, gene transcription is silenced etc

Question 5

Describe the structure and function of centrosomes

Answer 5

They are made of 2 centrioles, each of which is a barrel of 9 triplet microtubules . It’s role is to be the microtubule organizing center and to form the spindle during mitosis

Question 6

Describe what happens in prophase

Answer 6

Chromosomes condense (each consists of 2 sister chromatids and a kinetocore)
Duplicated centrosomes move to opposite poles and begin the formation of the spindle

Question 7

How are the spindles formed?

Answer 7

Radial microtubule arrays (ASTERS) form around each centrosome and when they meet polar microtubules form

Question 8

What occurs during metaphase?

Answer 8

Chromosomes line up at the equator of the cell after a microtubule from the opposite pole is captured by the kinetochore

Question 9

What occurs during anaphase?

Answer 9

Cohesin that holds sister chromosomes together breaks down
Chromosome splits into daughter chromosomes, each of which migrates to opposite poles
Centrosomes migrate apart

Question 10

What occurs during telophase?

Answer 10

Daughter chromosomes arrive at opposite poles
Nuclear membrane begins to reform
Contractile ring (made of myosin and actin filaments) begins to form

Question 11

What occurs during cytokinesis?

Answer 11

Contractile ring contracts
Microtubule array dissembles
Chromatin decondenses

Question 12

At what point is the cell cycle checked during mitosis and how?

Answer 12

Transition out of metaphase- check to see if chromosomes are properly lined up at the equator, this is done by BUB proteins. They are attached to kinetochores and only dissociate if the spindle is attached to the kinetochore properly. Once they all dissociate, metaphase and anaphase can occur

Question 13

What happens if there is misattachment of microtubules to the kinetochore?

Answer 13

Aneuploidy eg spindle only attaches to one side of kinetochore

Question 14

What happens if something goes wrong at one of the checkpoints (G0 or metaphase)?

Answer 14

1. Cell cycle arrest- cell cycle stops so eg DNA can be repaired
2. Apoptosis

Question 15

What happens when cells don’t receive external signals?

Answer 15

They go into the G0 phase (quiescent phase).

Exit from this phase is very highly regulated and requires growth factors and intracellular signaling cascades

Question 16

At which point do tumor cells usually exit the cycle?

Answer 16

G0

Question 17

What do signaling cascades involve?

Answer 17

Amplification and integration as well as modulation by other pathways

Question 18

How do protein kinase cascades work?

Answer 18

Kinases often regulate other kinases eg by phosphorylation (can be reversed by phosphotase)

Question 19

How is the cell cycle regulated?

Answer 19

When a certain cyclin binds to a cyclin dependent kinase, a part of the cell cycle is activated. After this the cycling degrade

Question 20

What are the cyclins involved in the cell cycle?

Answer 20

Cyclin A/B/D/E
They are expressed at specific points only
Level of expression is modulated
Degrade soon after synthesis

Question 21

What are the cyclin dependant kinases involved in the cell cycle?

Answer 21

Cak 1/2/4/6
Transiently expressed throughout whole cell cycle
Stimulate synthesis of genes for next cell cycle stage giving the cycle direction and timing