MCBG Session 10 - DNA Repair & Cell Cycle Flashcards
What is cell division?
What occurs during the cell cycle?
What are the stages of the cell cycle + how long do they last?
- The process in eukaryotes where one mother cell becomes two daughter cells.
- Mitosis in somatic cells, Meiosis in germline cells
- G1 - cell content duplication - 10-12 hours
- S - DNA replication - 6-8 hours
- G2 - double check + DNA repair - 3-4 hours
- M - Mitosis - <1 hour
G1, S + G2 make up interphase (23 out of 24 hours).
What other stage can a cell cycle have (somewhere around G1 phase)?
G0 - the stationary or quiescence phase - some cell types can stay in this phase for the duration of their lifespan as they do not need to divide.
How is the cell cycle controlled and what is the consequence of loss of control?
By cell cycle checkpoints (G1 + G2 checkpoints) and CDK/cyclin complexes (don’t need to know in detail). Loss of control can lead to cancer.
DNA damage can occur anywhere on DNA, and can result in single or double strand breaks. Name some exogenous and endogenous sources of DNA damage.
Exogenous = Ionising radiation, UV light, alkylating agents, mutagenic chemicals, free radicals Endogenous = free radicals, replication errors.
Upto 1 million molecular lesions per cell can happen each day - name a few methods of DNA damage,
Double strand breaks, Single strand breaks, Interstrand crosslink, Deamination, Bulky adduct, Intercalating agents etc.
What is DNA replication stress and what are the 3 main ways in which it occurs?
Inefficient replication leading to replication fork slowing, stalling and/or breakage
- Fork slippage (where a nucleotide loops out on the newly synthesised strand and an extra base is added or a nucleotide on the template strand loops out leading to a base being omitted).
- Replication machinery defects (e.g.: DNA polymerase misincorporation)
- Defects in response pathway
How do replication machinery defects lead to DNA replication stress?
DNA polymerase makes a misincorporation mistake once in a million bases, however the proof reading mechanism of DNA makes it 1000 x more efficient. There are a number of machinery defects that result in a proof reading mistake, and DNA replication stress.
How does fork slippage lead to DNA replication stress?
What can fork slippage lead to?
- Newly synthesised strand or the template strand can loop out, resulting in a nucleotide being added on or omitted on the new strand respectively.
- Fork slippage can lead to trinucleotide expansion, e.g.: in Huntington’s disease where >35 CAG repeats in the HTT gene leads to polyglutamine repeats in Huntingtin protein. Mutant huntingtin aggregates in neurones leading to progressive nature of disease.
How does the DNA Damage Response (DDR) work?
What are the 3 potential consequences?
The cellular pathways that sense, signal and repair DNA damage, which have 3 possible outcomes:
1+2 = if DNA damage is too high/persists 3 = If DNA can be repaired
1) Senescence (permanent cell cycle arrest)
2) Apoptosis (cell death)
3) Proliferation (after DNA repair + cell cycle control)
Explain briefly how base excision repair (BER), nucleotide excision repair (NER) and mismatch repair (MR) work?
BER = Incorrect base is removed, and filled with correct base by DNA polymerase and gap sealed by a ligase.
NER = When dimers are produced, the surrounded DNA is opened to form a bubble. Enzymes cut out the damaged bubble region and a DNA polymerase replaces the excised DNA, backbone sealed by a ligase.
MR = When 2 mistmatched bases are recognised (e.g.: G-T), the DNA and its neighbours are cut by exonucleases, and replaced by DNA polymerase, sealed by a ligase.
What is the difference between single strand and double strand DNA breaks?
Single - simple, integrity of DNA still intact and homology of other strand used to repair.
Double - complex, integrity of DNA lost, more-likely error prone + use of homology can only sometimes occur.
What is:
1) Non-homologous end joining
2) homology-directed repair
in terms of DNA repair
1) Broken ends recognised and re-joined via ligase - very high chance of mutations!
2) Repair of DS breaks that results in homologous recombination, such that DNA is repaired with usual genetic sequence.
What is the multi-step cancer model?
DNA replication stress stimulates an accumulation of tumours which can result in a premalignant then malignant cancer cell. However, DDR prevents this carcinogenesis and accumulation of mutations.
What is tumour heterogeneity?
This refers to the existence of different subpopulations of cells that have different biological behaviours within a primary tumour
Name and explain 2 different mechanisms by which a cancer can evolve.
1) Cancer-induced mutagenesis - chemotherapy treatment results in mutations that allow cancer to grow
2) Differential sensitivity - Certain subpopulations of cells resistant to chemotherapy therefore these subpopulation continue to grow and divide even in the face of treatment.
