Damage, Repair and Recombination
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What are the forms of DNA damage?
Deamination Depurination Pyrimidine dimers DMA breaks
Types of DNA repair mechanisms
1) Base excision repair 2) Nucleotide Excision repair 3) Homologous recombination machinery
What is the most frequent spontaneous form of DNA damage?
Hydrolytic depurination and deamination of bases
What is depurination?
Releases guanine and adenine from DNA
What is deamination?
Converts cytosine to an altered DNA base, uracil Normally takes place in double helical DNA
Base Excision Repair repairs what?
Depurination Deamination
Describe the steps of how deaminated or depurinated bases are repaired
1) The damaged base is removed by DNA glycosylase 2) The sugar phosphate with the missing base is cut out by the action of apurinic/apyrimidic endonuclease and a phosphodiesterase 3) The gap is filled with DNA polymerase and DNA ligase 4) DNA polymerase extends the primer; template junction 5) DNA ligase seals the nick
How can UV irradiation cause DNA damage?
Causes the formation of pyrimidine dimers which can arrest DNA replication or cause mis-reading of the DNA sequence by DNA polymerase Pyrimidine dimers can form between C and T neighbours
Describe the steps of nucleotide excision repair
1) Excision nuclease cleaves a single stranded DNA segment containing the defect 2) DNA helicase removes damaged segment 3) DNA polymerase extends primer:template junction 4) DNA ligase seals the nick
Defective nucleotide excision repair machinery cause which disorder?
Xeroderma Pigmentosum
What is the effect of xeroderma pigmentosum?
Renders patients extremely sensitive to light induced skin cancer by the sin
Which genes are complicated in xeroderma pigmentosum?
XPA,XPC,XPD,XPF,XPG These all encode proteins involved in the nucloetide excision repair pathway for the correction of UV induced DNA lesions
Double stranded breaks usually occur due to what?
By ionising radiation-type of mutagen
What are the two repair mechanisms used to repair double stranded breaks?
Non-homologous end joining Homologous recombination
What is non homologous end joining?
Enzymes alert to the production of fragmented DNA and they rapidly rejoin free ends of the chromosomes which are colse together Vulnerable to sticking the wrong peices of DNA together as it doesnt take into account the fragments sequence
What is homologous recombination?
Thought of as a last line of defence since single stranded breaks produced by Base Excision Repair are vulnerable to double stranded breaks These structures are more prone to becoming a double stranded break
Mutations in which homologous recombination machinery are linked to cancer?
BRAC2 = breast cancer ATM = leukemia and lymphoma FANC = leukemia