Introduction to cell biology

Question 1

What is the total human genome and what is it made up of?

Answer 1

3.2 billion base pairs long
- formed of DNA sequences
- 1 chromosome from each of 22 autosome pairs and both sex chromosomes
Also mitochondrial DNA (only 16,000 base pairs long)

Question 2

How are cell numbers controlled?

Answer 2

• controlled by cell-cell communication
• growth factor signals and cell-cell contacts positively regulate cell proliferation - controlling if and when cells divide

Question 3

What processes are in place to ensure new cells are identical?

Answer 3

• processes of growth and division must be tightly controlled
• cell cycle control involved checkpoints and feedback control
• maintains genome integrity which is crucial

Question 4

What are the phases of the cell cycle and their timings?

Answer 4

Interphase:
- G1 phase (about 11 hours)
- S phase (about 8 hours)
- G2 (about 4 hours)
M phase (about 1 hour) - varies based on cell type
- this phase includes mitosis and cytokinesis

For most cells the cycle is 24 hours

Question 5

What are the phases of mitosis?

Answer 5

Prophase: nuclear envelope dissembles and chromosomes condense, becoming visible, centrosomes start forming mitotic spindle
Metaphase: Centrosomes migrate to opposite poles of the cell, chromosomes line up along metaphase plate (equatorial plate)
Anaphase: paired chromatids pulled apart to opposite poles by mitotic spindle attached to kinetochores at the centromeres of the chromosomes
Telophase: nuclear envelope reforms, mitotic spindle disappears and cytokinesis is completed

Question 6

What is Go phase?

Answer 6

quiescent phase- cell is neither dividing or preparing to divide
e.g. lymphocytes are typically not in cycle so are in the Go phase

Question 7

What is happening in the G1 phase?

Answer 7

Period during which cells are responsible to mitogenic growth factors and to TGF-beta
Later in this stage is the restriction point - this checkpoint is responsive to external growth factors however beyond this point the cells is unresponsive

Question 8

What are the cell cycle checkpoints?

Answer 8

G1 restriction point: responds of extracellular signals. Cell irreversibly commits to cell division. Check the cell has sufficient nutrients/reserves and its big enough. Checks for any DNA damage 
G1 checkpoint: Checks for any DNA damage 
Intra-S checkpoint:Checks for any DNA damage 
G2/M checkpoint: Nutrients/ reserves and size are checked. Checks replication is complete and for any DNA damage 
M checkpoint (spindle checkpoint): checks all the spindle are correctly attached at the kinetochores of the centromeres

Question 9

What signals induce a cell to divide?

Answer 9

• death of a nearby cell
• release of growth-promoting hormones, such as human growth hormone (HGH)
• the size of the cell; as a cell grows, it becomes inefficient due to its decreasing surface-to-volume ratio. The solution to this problem is to divide.

Question 10

What enzymes regulate the cell cycle?

Answer 10

Cyclin dependent kinases
- regulate cell cycle checkpoint transitions- they are regulated by feedback
(kinases phosphorylate)

Question 11

What do cycling dependent kinases do to the cell cycle?

Answer 11

they fluctuate in abundance and activity to control the pace of events through the cell cycle
- Cyclin D to Cyclin E to Cyclin A to Cyclin B

Question 12

What do kinase enzymes do and how do CDKs do it?

Answer 12

Kinases activate or deactivate proteins via phosphorylation

• in the cell cycle they provide the signals to drive the checkpoints
• they are activated by CYCLINs which are proteins named due to their cyclical functioning concentrations

Question 13

What cyclin and CDK is involved in the S phase?

Answer 13

Cyclin A and CDK2

- binding of cyclin A to CDK2 increases its enzymatic activity by 400,000 fold

Question 14

What are the pairs of cyclin and CDKs?

Answer 14

Cyclin D and CDK 4+ 6- CYCLIN D is an extracellular signal and is controlled by colony stimulating factor 1 
Cyclin E and CDK2
Cyclin A and CDK2
Cyclin A and CDK1
Cyclin B and CDK1

The cyclin and CDK complexes in one phase lead to activation of the next pair - APART from cyclin D which is controlled by extracellular signals

Question 15

What does the G1 restriction point do?

Answer 15

-ensures cell is large enough to divide and that enough nutrients are available to support the resulting daughter cell
If a cell receives a go-ahead signal at the G1 checkpoint it will usually continue with the cell cycle
If the cells does not receive the go-ahead signal it will exit the cell cycle and switch to a non-dividing state called Go

Question 16

What is the purpose of the G2 checkpoint?

Answer 16

It ensure that DNA replication in the S phase has been completely successfully

Question 17

What is the purpose of the M checkpoint?

Answer 17

ensures all the chromosomes are attached to the mitotic spindle by kinetochore

Question 18

What is the purpose of the intra-S checkpoint?

Answer 18

check for any DNA damage

Question 19

In the absence of growth factors what regulates the G1 restriction point?

Answer 19

retinoblastoma protein (pRB) - its job is to stop the cell cycle
It binds to E2F transcription factor and this stops it being induced and promoting growth
- pRB is a negative regulator of growth

Question 20

How does the cell stop pRB inhibiting cell division?

Answer 20

growth factors which activate cyclin D and CDK4+6 complex phosphorylate pRB causing it to release E2F transcription factor - therefore allowing cell proliferation

Question 21

What does p53 regulate?

Answer 21

Regulates G1/s phase, responding to damaged DNA and shutting the cell cycle down if damage is detected

Question 22

What is p53 mechanism of action?

Answer 22

DNA damage activates p53 which subsequently activates p21 which is an inhibitor of CDK therefore preventing continuation into S phase

Question 23

What are pRB and p53?

Answer 23

Tumour suppressor genes and are most commonly lost genes in cancer
- primary negative regulators of cell growth

Question 24

What does p53 do if the DNA damage is too severe?

Answer 24

induces the cell to kill itself by apoptosis - can also make the cell senescent

Question 25

What happens with p53 loss of function?

Answer 25

unrestrained replication of damaged DNA= genome instability / resistance to apoptosis

Question 26

What is TGF-beta?

Answer 26

tumour suppressor gene

• strongly increases expression of p15(INK4B) which inhibits cyclin D CDK4 +6
• more weakly induces p21

Question 27

What are some other inhibitors of cyclin D?

Answer 27

p16 (INK4A), p15(INK4B), p18(INK4C), p19(INK 4D)

Question 28

How can extracellular signals control cell cycle?

Answer 28

Mitogens can bind to cell surface receptors and can activate AKT/PKB signalling pathway which causes phosphorylation and localisation of p21 and p27 to the cytoplasm, preventing them entering the nucleus and inhibiting cell cycle

Question 29

What are the consequences of checkpoint failure?

Answer 29

1) proliferation of cells in absence of Bfs
2) replication of damaged DNA
3) segregation of incompletely replicated chromosomes
4) division of cells with wrong numbers of chromosomes
GENOME INSTABILITY = increases rate of mutations