<< Direct reversal >> :
Cells are known to eliminate three types of damage to their DNA by chemically reversing it.
These mechanisms do not require a template, since the types of damage they counteract can only occur in one of the four bases.
1.thymine dimers:enzyme photolyase
2.methylation of guanine bases:
a.the protein methyl guanine methyl transferase (MGMT)
b.A generalized response to methylating agents in bacteria
is known as the adaptive response
3.certain methylation of the bases cytosine and adenine
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<< Single strand damage >> :
When only one of the two strands of a double helix has a defect, the other strand can be used as a template to guide the correction of the damaged strand.
there exist a number of excision repair mechanisms that remove the damaged nucleotide .
1.Base excision repair (BER), which repairs damage due to a single nucleotide caused by oxidation, alkylation, hydrolysis, or deamination;
2.Nucleotide excision repair (NER), which repairs damage affecting longer strands of 2-30 bases.
This process recognizes bulky, helix-distorting changes such as thymine dimers as well as single-strand breaks (repaired with enzymes such UvrABC endonuclease).
a. A specialized form of NER known as Transcription-Coupled Repair (TCR) deploys high-priority NER repair enzymes to genes that are being actively transcribed;
3.Mismatch repair (MMR), which corrects errors of DNA replication and recombination that result in mispaired nucleotides following DNA replication.
<< Double strand breaks >> :
A type of DNA damage particularly hazardous to dividing cells is a break to both strands in the double-helix.
Two mechanisms exist to repair this damage.
1. non-homologous end-joining (NHEJ) repair
The NHEJ pathway operates when the cell has not yet replicated the region of DNA on which the lesion has occurred.
the enzyme DNA ligase in complex with a required cofactor directly joins the two ends of the broken DNA strands without a template,losing sequence information in the process.
Thus this repair mechanism is necessarily mutagenic.
However, if the cell is not dividing and has not replicated its DNA, the NHEJ pathway is the cell’s only option.
NHEJ relies on chance pairings, or microhomologies, between the single-stranded tails of the two DNA fragments to be joined.
There are multiple independent ”failsafe” pathways for NHEJ in higher eukaryotes.
2. recombinational repair (also known as template-assisted repair or homologous recombination repair).
Recombinational repair requires the presence of an identical or nearly identical sequence to be used as a template for repair of the break.
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