By: Bhuwan Singla : By: Bhuwan Singla DNA Repair CONTENTS : CONTENTS Things that Damage DNA
Types of DNA Damage
Repairing Damaged Bases
Basic mechanism Of Repairing DNA
Base Excision Repair
Nucleotide excision repair
Proteins involved in the DNA repairing
Diseases Things that Damage DNA : Things that Damage DNA Radiation – Cosmic
Chemicals - in the environment alkylating agents
Methyl and ethyl groups added to DNA bases
DNA Replication Accidents
Heat (spontaneous chemical alterations) Types of DNA Damage : Types of DNA Damage All four of the bases in DNA (A, T, C, G) can be covalently modified at various positions.
Mismatches of the normal bases because of a failure of proofreading during DNA replication.
Breaks in the backbone.
Crosslinks Covalent linkages can be formed between bases Other forms of DNA damage : Other forms of DNA damage Deamination - An amino group of Cytosine is removed and the base becomes Uracil
Deamination - An amino group of Adenine is removed and the base becomes Hypoxanthine
Deamination - An amino group of Guanine is removed and the base becomes Hypoxanthine
Depurination - the base is simply ripped out of the DNA molecule leaving a gap (like a missing tooth)… 06_23_Depurination.jpg : 06_23_Depurination.jpg Molecular level view-
Remember these are random events 06_25_mutations.jpg : 06_25_mutations.jpg DNA level view of the same two events as last slide Repairing Damaged Bases : Repairing Damaged Bases Direct Reversal of Base Damage
Base Excision Repair (BER)
Nucleotide Excision Repair (NER)
Mismatch Repair (MMR) Basic mechanism Of Repairing DNA : Basic mechanism Of Repairing DNA Remove damaged region
Ligation Base Excision Repair : Base Excision Repair Presence of the Uracil in DNA is a great example of this type
Special enzymes replace just the defective base
1 snip out the defective base
2 cut the DNA strand
3 Add fresh nucleotide
4 Ligate gap DNA repair by base excision : DNA repair by base excision A base-specific DNA glycosylase detects an altered base and removes it
AP endonuclease and phosphodiesterase remove sugar phosphate.
DNA Polymerase fills and DNA ligase seals the nick Nucleotide Excision : Nucleotide Excision Same as Base Excision Except that
It recognizes more varieties of damage
Remove larger segments of DNA (10 -100s of bases) Nucleotide excision repair : Nucleotide excision repair a large multienzyme compound scans the DNA strand for anomalities
upon detection a nuclease cuts the strand on both sides of the damage
DNA helicase removes the oligonucleotide
the gap is repaired by DNA polymerase and DNA ligase enzymes Mismatch Repair (MMR) : Mismatch Repair (MMR) Mismatch repair deals with correcting mismatches of the normal bases; that is, failures to maintain normal base pairing (A?T, C?G)
Recognition of a mismatch requires several different proteins including one encoded by MSH2.
Cutting the mismatch out also requires several proteins, including one encoded by MLH1. How does the MMR system know which is the incorrect nucleotide? : How does the MMR system know which is the incorrect nucleotide? In E. coli, certain adenines become methylated shortly after the new strand of DNA has been synthesized. The MMR system works more rapidly, and if it detects a mismatch, it assumes that the nucleotide on the already-methylated (parental) strand is the correct one and removes the nucleotide on the freshly-synthesized daughter strand. How such recognition occurs in mammals is not yet known. Mismatch Repairing Mechanism : Mismatch Repairing Mechanism Proteins involved in the DNA repairing of E. coli. : Proteins involved in the DNA repairing of E. coli. Recombinational Repair : Recombinational Repair This type of repair is much more complicated than is excision repair, and requires many more gene products. The products of a number of these repair genes are induced by radiation damage, and therefore this type of repair requires protein synthesis before it can function. Because of its complexity, this type of repair makes mistakes Postreplication Repair (recombinational DNA repair) : Postreplication Repair (recombinational DNA repair) The dots indicate lesions in the DNA.
DNA synthesis proceeds up to a lesion and then skips past the lesion, leaving a gap in the daughter strand.
Filling of the daughter strand gaps with DNA from parental strands by a recombinational process that requires a functional recA gene.
Gaps in the parental strands are repaired by repair replication Recombinational Excision Repair : Recombinational Excision Repair This process occurs in the part of the chromosome that was replicated prior to irradiation, i.e., where two sister duplexes were present before irradiation. After the excision of the lesion, the resulting gap is filled by the same recombinational process just described for postreplication repair The recombinational repair of excision gaps in E. coli. : The recombinational repair of excision gaps in E. coli. UV radiation-induced lesions are produced in both the replicated and unreplicated portions of the genome
The gaps produced by excision in the unreplicated portion are repaired by the classical methods of nucleotide excision repair
The gaps produced in the replicated portion of the chromosome are repaired by a recombinational process that requires both recA and recF (C-D) Diseases : Diseases cellular ultraviolet sensitivity
Werner syndrome (premature aging, retarded growth)
Bloom syndrome (sunlight hypersensitivity)
colon cancer Slide 23: THANKS