The Mutiblity and Repair of Dna Flashcards
Transition mutation
Pyrimidine to pyrimidine or purine to purine
(T –> C) (A –> G)
Substitution
Are mutations with simple base switches
There are two types:
1. Transitions
2. Transversions
Transversions mutation
Pyrimidine to purine or purine to pyrimidine
(T or C –> A or G) (A or G –> T or C)
Silent mutations
Mutations that does not alter protein function
Missense mutation
Different amino acid; resulting protein may have partial function
Nonsense mutation
Specifies stop codon; truncated, non functional protein
Insertions and deletions, of numbers not divisible by 3, cause a ______________________.
Frameshift mutation
Alternative forms of insertion and deletion?
- Inversions - piece of DNA sequence separates, inverted and reattaches itself
- Transpositions - DNA segments from different chromosome switch
MutS
Protein that repairs DNA
- can go up or down stream
- look for bump out caused by mismatch
MutS mismatch repair in E. Coli (10 + 1)
- A homodimer of MutS recognizes a mismatch-induced distortion in the DNA
2 conformational change occurs to MutS homodimer and ATP binds to each - Causes further kink
- Complex recruits MutL which activates MutH in E. coli
- In eukaryotes and most bacteria, MutL is the endonuclease and it’s homologues nick one strand of DNA near the mismatch
- In E. Coli, MutH is the endonuclease that nicks the strand
- The nick creates and excision entry point for exonuclease activity
- Exonuclease chews strand with kink
- Removes error
- DNA pol fills gap
- Ligase reseals
How does MutS know which nucleotide to replace? (6)
- Dam methylase (E. coli enzyme) methylates A residues in th 5-GATC-3 sequence
- Occurs every 246 bp
- Following replication and before Dam methylase activity, the daughter DNA duplexes are hemimethylated
- In E. Coli, MutH recognizes the the hemimethylated sites,
- but MutH is inactive unless activated by MutL.
- Thus, only areas of mismatch and hemimethylation are repaired by the MutS system
Mutagens
- Alkylation
- Oxidation
- Radiation
Alkylation
Transfer of a methyl or ethyl group to bases and phosphate groups in DNA
Eg. O6-methylguanine, which mispairs with thymine
Oxidation
Reactive oxygen species (ROS)
Eg oxoG which base pairs with A and C
Radiation
260nm wavelength absorbed by bases causes dimerization or adducts
Causes DNA pol to stop during replication
Dimerization
1) Creates bond between two thymine side by side (on same backbone)
2) does not bond to opposite - creates bulge
3) more likely to happen to thymine but also occurs with cytosine
Intercalating Agents
- Flat like DNA bases
- Can do coaxial stacking
- Induce Frameshift mutation
Photoreactivation
- Simple reversal of damage
- After dimerization occurs, whiles in the dark, DNA photolyase binds to dimer and waits for sunlight
- Using same wavelength (260nm) photolyase breaks dimer and restores DNA
Chromosome repair: NHEJ
- A method to repair double stranded DNA breaks (DSB)
- Loss of sequence - mutagenic consequence but allows DNA to be replicated to pass on
Chromosome repair: NHEJ process (6)
- Ku 70/80 heterodimer binds the broken ends of DNA
- And recruits kinase DNA-PKcs
- DNA-PKcs recruits Artimis
- Artimis processes the broken end via exo and endonuclease activity (cleans end - wants blunt ends)
- Ligase binds back together
- Non-hemologous end joining
Translesion synthesis
- Mechanism for bypassing a lesion in DNA
- Allows replication to continue
- Avoids incomplete replicated DNA
Prokaryote Translesion repair
- DNA pol III reaches bulge and dissociated from DNA
- Pol IV or V binds on and can read through lession/bulge
- Pol III binds again and continues after bulge with newly synthesized strand complete
Eukaryote translesion repair - polymerase-switching model
- DNA pol stalls at site of lesion
- Ubiquitination of PCNA occurs
- Causes the recruiting of translesion pol
- Reads through lesion.
- Replication pol replaces the translesion pol and replication continues
Eukaryote translesion repair - gap-filling model
- DNA pol does not stall at site of lesion but skips over leaving a gap
- Translesion polymerase follows behind and fills in the gap
