Topic 10 Flashcards
Describe the cell cycle and its phases
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
Explain the checkpoint controls
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
What are the exogenous sources of DNA damage?
ionising radiation UV alkylating agents mutagenic chemicals anti cancer drugs free radicals
What are the endogenous sources of DNA damage?
free radicals
replication errors
What is replication stress?
inefficient replication leads to replication fork slowing, stalling and / or breakage
What is proofreading in replication?
DNA polymerase checks for mistakes in the new strand and corrects it if there is a mistake
What are the replication machinery defects that can occur? Or rather, what can cause the defects?
problems in DNA polymerase clamp loader sliding clamp DNA helical and DNA primase topoisomerase (prevents supercoiling of the DNA)
How does fork slippage happen and what does it lead to?
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
What is Huntington’s disease?
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
What are defects in response pathways?
another type of replication stress
What is the DNA damage response?
the cellular pathways that sense, signal and repair DNA damage
What happens if DNA damage levels are too high or persist?
senescence
permanent cell cycle arrest
dies sooner but still has function, stops dividing
apoptosis
cell death
What is the ideal scenario for the outcome of the DNA damage response?
to repair DNA and maintain function
proliferation
after DNA repair and cell cycle control
Describe base excision repair
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
Describe nucleotide excision repair
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
Describe mismatch repair
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
Describe a single strand break
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
Describe a double strand break
integrity of DNA molecule lost
more likely error prone
use of homology may be possible
Explain non- homologous end joining and homologous- directed repair
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
How is cancer formed? Describe the multi step model
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
Describe tumour heterogeneity
a tumour is not a single clone, it is many sub clones
What are the two ways that cancer can evolve after chemotherapy?
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)
What are synthetic lethality strategies?
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
