Topic 10

Question 1

Describe the cell cycle and its phases

Answer 1

Divison
nucelar division (mitosis)
cellular division (cytokinesis)
Interphase
G1, S and G2

G1- 10/12 hours
cell content duplication
S- 6/8 hours
DNA replication
G2- 3/4 hours
double check and repair
M- less than 1 hour
mitosis

G0
stationary phase, comes off G1 and cell decides whether to divide again

Question 2

Explain the checkpoint controls

Answer 2

Mitosis checkpoint 
are all chromosomes properly attached to the mitotic spindle?
G1 checkpoint
is environment favourable?
enter S phase
G2 checkpoint 
is all DNA replicated?
is all DNA damage repaired?
enter mitosis

Question 3

What are the exogenous sources of DNA damage?

Answer 3

ionising radiation
UV 
alkylating agents
mutagenic chemicals
anti cancer drugs
free radicals

Question 4

What are the endogenous sources of DNA damage?

Answer 4

free radicals

replication errors

Question 5

What is replication stress?

Answer 5

inefficient replication leads to replication fork slowing, stalling and / or breakage

Question 6

What is proofreading in replication?

Answer 6

DNA polymerase checks for mistakes in the new strand and corrects it if there is a mistake

Question 7

What are the replication machinery defects that can occur? Or rather, what can cause the defects?

Answer 7

problems in
DNA polymerase
clamp loader
sliding clamp
DNA helical and DNA primase
topoisomerase (prevents supercoiling of the DNA)

Question 8

How does fork slippage happen and what does it lead to?

Answer 8

repetitive DNA
if you have repeated DNA on the new strand, you get a backwards slippage, an insertion mutation and an extra base or set of bases (depending on how many bases are repeated)
if you have repeated DNA on the template strand, you get a forwards slippage, a deletion mutation and a missing base or set of bases

Question 9

What is Huntington’s disease?

Answer 9

due to trinucleotide expansion
CAG repeats in the HTT gene
leads to polyglutamine repeats in the Huntington protein
mutant Huntingtin protein aggregates in neurones affecting mainly basal ganglia
progressive, late onset

Question 10

What are defects in response pathways?

Answer 10

another type of replication stress

Question 11

What is the DNA damage response?

Answer 11

the cellular pathways that sense, signal and repair DNA damage

Question 12

What happens if DNA damage levels are too high or persist?

Answer 12

senescence
permanent cell cycle arrest
dies sooner but still has function, stops dividing

apoptosis
cell death

Question 13

What is the ideal scenario for the outcome of the DNA damage response?

Answer 13

to repair DNA and maintain function

proliferation
after DNA repair and cell cycle control

Question 14

Describe base excision repair

Answer 14

deamination converts x base to y
y is detected and removed, leaving a base less nucleotide
the base less nucleotide is removed, leaving a small hole in the DNA backbone
the hole is filled by the right base by DNA polymerase and the gap is sealed by DNA ligase

Question 15

Describe nucleotide excision repair

Answer 15

UV radiation creates a dimer
once the dimer is detected, the surrounding DNA forms a bubble
enzymes come in and cut out the damaged region in the bubble
DNA polymerase replaces the excised DNA and ligase seals the backbone

Question 16

Describe mismatch repair

Answer 16

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

Question 17

Describe a single strand break

Answer 17

integrity of the DNA molecule remains intact
damage is removed on one strand only
homology of the other strand is used to repair
not error free but not error prone either

Question 18

Describe a double strand break

Answer 18

integrity of DNA molecule lost
more likely error prone
use of homology may be possible

Question 19

Explain non- homologous end joining and homologous- directed repair

Answer 19

non homologous end joining
the broken ends are recognised and protected
formation of complex of proteins and damaged ends removed
broken ends ligated
error prone

homologous directed repair
use homologous pair to fix break
the DNA double strand breaks requires resection of DNA double strand breaks ends. Resection creates 3’-single strand DNA overhangs which then anneal with a homologous DNA sequence. This homologous sequence can then be used as a template for DNA repair synthesis that bridges the double strand break

Question 20

How is cancer formed? Describe the multi step model

Answer 20

loss of the cell cycle control
DNA replication stress stimulates carcinogenesis
DNA damage response prevents carcinogenesis
DNA repair defects stimulate carcinogenesis

normal - premalignant - malignant

normal cell, hyperproliferation, adenoma, carcinoma

Question 21

Describe tumour heterogeneity

Answer 21

a tumour is not a single clone, it is many sub clones

Question 22

What are the two ways that cancer can evolve after chemotherapy?

Answer 22

differential sensitivity (similar to antibiotic resistance)
chemotherapy induced mutagenesis
- chemical induces mutations in another sub clone of the cancer (chemical might destroy one sub clone but promote the production of another)

Question 23

What are synthetic lethality strategies?

Answer 23

when genes A and B are synthetic lethal, the inactivation of gene A (either genetically or by drug) will be lethal to cancer cells having a mutation in gene B but not to normal cells in which gene B is not mutated
inhibition of gene A is selectively lethal to cancer cells with mutations in gene B