Lecture 11 - DNA damage & repair Flashcards
(41 cards)
What is the structure of dsDNA (double-stranded DNA)?
- Sugar (Deoxyribose)
- Phosphate
- Nucleotide (base)
What are the DNA bases?
- Adenine (A)
- Thymine (T)
- Cytosine (C)
- Guanine (G)
Describe DNA base hydrogen bonding
A-T = 2 hydrogen bonds (weaker)
C-G = 3 hydrogen bonds (stronger)
What are purines?
Double ring:
- A
- G
What are pyrimidines?
Single ring:
- T
- C
- U
Describe genetic code
- 32 trillion cells
- each cell contains 2 meters of DNA
- 3 billion DNA base pairs per cell
What is genome stability?
An individual cell can experience up to 1 million DNA damage events per day.
- 32 trillion cells
- From the moment you were a single cell
- Constantly happening
What are examples of sequence changes?
- Damage due to reactive oxygen species
- Thymine dimers
- Deamination
- Depurination
What are structural changes?
- Localised structural changes
- Large structural changes
- Consequences
What are examples of Localised structural changes?
- Single strand breaks
- Double strand breaks
What is an example of large structural changes?
Chromosomal rearrangements e.g. translocations
What are consequences of structural changes?
Point mutations
- silent (different DNA sequence but same amino acid)
- Missense (different amino acid results)
- Nonsense (forms premature stop codon)
Insertions/deletions/duplications
How can reactive oxygen damage lead to DNA damage?
- can be byproducts of cellular metabolism (mitochondria)
- or generated by radiation exposure
- Reactive oxygen species (ROS) react with DNA bases
- Changes their chemistry (oxidized bases)
- Disrupt base pairing
- Can also attack the DNA backbone, causing breaks
What is thymine dimers?
- a type of DNA damage where thymines get stuck together
- occur when covalent links form between neighbouring thymine dimers
- inhibits DNA replication
What is deamination?
- the loss of the amine (NH2) group from cytosine bases
- changes to Uracil - not found in DNA
- 100 bases per cell per day
- Affects DNA replication
- Polymerase can mistake uracil for thymidine (T)
- causes and G-A switch in the new sequence (incorrect)
- can be repaired via BER (base excision repair)
If deamination occurs & the base becomes uracil, this can lead to base change - however it can be repaired by base excision repair (BER)
What is depurination?
- spontaneous loss of adenine or guanine bases
- 5000 bases lost per cell per day
- become removed from the DNA backbone
- causes a loss of genetic information
- decreases stability in this region - generally not good
- can be repaired via base excision repair (BER)
What occurs with single-strand breaks (SSBs)?
- naturally occur during many forms of DNA repair (e.g. base-excision repair)
- can also be caused by other factors
- one of the most common lesions in cells
- interferes with DNA replication & transcription
- easily repaired by the cell
- using a range of different SSB repair pathways
- can develop into more severe damage if not repaired efficiently
- e.g. can become a double strand break (DSB)
A lot of redundancy - one pathway can do another (very conserved)
What occurs with double strand breaks (DSBs)?
- ionizing radiation & carcinogens can directly break the DNA backbone on both strands
- can be caused by unresolved stalled replication forks
- also occur naturally during meiosis & recombination
- very serious
- cell cycle is often arrested
- disrupts replication/transcription
- can lead to large genome rearrangements
- a significant DNA damage response is triggered
- Repaired primarily through non-homologous end joining (NHEJ) or homologous recombination (HR)
- if not resolved, normally triggers apoptosis
- if significant, can impact tissue function.
Describe how large chromosomal abnormalities can occur
- failure to repair DBSs properly cause large chromosomal abnormalities
- or improper telomere regulation
- deletion of entire chromosome regions or translocations
- this is where DSBs at different genomic sites are incorrectly joined together
- causes genomic regions to be the incorrect place
- e.g. t(9;22) translocations, also known as the Philadelphia chromosome - causes many problems
How can sequence change cause DNA damage?
Damage or incorrect repair can lead to point mutations
- silent (different DNA sequence but same amino acid)
- missense (different amino acid results)
- nonsense (forms premature stop)
DNA also experiences insertions/deletions/duplications
- nucleotides are gained or lost from a sequence
- causes a frameshift
- can cause STOP codons to be abnormally present
- can disrupt the whole sequence (frameshift)
- or add additional information
Explain mutations in disease
- genome instability & unresolved damage lead to mutations
- mutations in coding regions of the genome lead to altered protein function
- can cause a loss of function
- can cause abnormal gain & functions (dominant negatives) - proliferation signals
- disrupt normal cellular physiology & pathways
- leads to abnormal cellular function
Mutations that cause disease are classed as PATHOGENIC
Mutations may have no effect are classed as BEGIGN
Mutation & genetic variation however are drivers of evolution
How can genome instability & accumulation of mutations lead to disease?
Some diseases can be caused by a single nucleotide mutations - e.g. Sickle cell disease (Single A>T change).
Some require multiple mutations - e.g. Cancer (Leukaemia)
Mutations can make cells more susceptible to genomic instability - e.g. mutation in a DNA repair gene… lead therefore to more mutations.
What causes DNA damage & mutations?
- Radiation (from sun)
- Reactive oxygen
- Chemical carcinogens (smoking)
- some cells don’t regenerate - therefore mutations will accumulate
Different cells face different environment & different types of damage
What are exogenous sources of genome instability?
- Non-ionizing radiation (UV)
- Ionizing radiation (X-ray, gamma rays, etc)
- Thermal damage
- Alkylating agents (tobacco smoke, chemicals)
- Chemotherapy drugs (Cisplatin)
- Viruses (Influenza virus)
- Plant/fungal toxins
- Excess hormones
