DNA Repair And Cell Cycle

Question 1

Cell cycle- g1 phase

Answer 1

Cell content duplication

10-12 hours

Question 2

S phase

Answer 2

DNA replication.

6-8 hours

Question 3

G2 phase

Answer 3

3-4 hours

Double check and repair

Question 4

M phase

Answer 4

Less than an hour

Mitosis

Question 5

What are the exogenous sources of DNA damage

Answer 5

Ionising radiation 
UV
Alkylating got agents 
Mutagenic chemicals 
Anti-cancer drugs 
Free radicals

Question 6

What is dna replication stress

Answer 6

Inefficient replication that leads to replication fork slowing, stalling and or breaks

Question 7

How does DNA polymerase deal with misincorporation

Answer 7

DNA polymerase can sense it has replicated wrong therefore reverses so it can remove the wrong base and put the right one in- proofreading

Question 8

What does fork slippage lead to

Answer 8

Trinucleotide expansion

Huntington’s, spinocerebellar ataxia, fragile X

Question 9

What is senescence

Answer 9

Permanent cell cycle arrest

Wont divide anymore no function

Question 10

Base excision repair

Answer 10

Used when there is the wrong base

• deamination changes base
• wrong base identified and removed leaving a base less nucleotide
• base-less nucleotide removed leaving a small hole in the DNA backbone
• hole is filled with the right base by DNA polymerase and gap is sealed by DNA ligase

Question 11

Nucleotide excision repair

Answer 11

• UV radiation produces a dimer
• once the dimer is detected the surrounding DNA is opened to form a bubble
• enzymes cut the damaged region out
• a DNA polymerase replaced the excised DNA and ligase seals the backbone

Question 12

Mismatch repair

Answer 12

• mismatch is detected in newly synthesised DNA
• new DNA strand is cut and the misread nucleotide and its neighbours are removed by exonuclease activity
• missing patch is replaced with correct nucleotides by a DNA polymerase
• a DNA ligase seals the gap in the DNA backbone

Question 13

Non homologous end joining

Answer 13

Broken ends recognised and protected

Complex formed and damaged ends removed

Broken ends ligated

Error prone

Question 14

What determines the type of DNA double strand break repair that occurs

Answer 14

The stage of the cell cycle that the cell is

Question 15

Homologous directed repair

Answer 15

Firstly, the exposed ends of DNA are resected.

This mechanism utilises a homologous template – which essentially means an undamaged copy of the DNA involved in the double-strand break – e.g. a sister chromatid.

A Holliday Junction then forms between the broken DNA and the DNA template.

The Holliday Junction is then nicked complete the repair.

Question 16

What is clonal expansion

Answer 16

Different clones in 1 tumour

Question 17

What negative effects can chemotherapy have on tumours

Answer 17

Clones of cells in a tumour have differential sensitivity to the chemo- the chemo may work on one cone but nit another so that one will continue to grow- shows why tumours shrink then grow back

Chemotherapy-induced mutagenesis- by trying to kill the clone you can mutate them even more making them more resistant to chemotherapy

Question 18

What are synthetic lethality strategies

Answer 18

If we know that a certain gene doesn’t work to keep the cell alive we can ensure that the only pathway the cell can take i that pathway which would kill the cancer cell by giving treatment that promotes that pathway

Question 19

What are some endogenous causes of mutation in DNA?

Answer 19

DNA replication errors

Free radicals