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Section: Genetics » Chemistry of the Gene » Synthesis, Modification and Repair of DNA
 
 
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  DNA repair
 
     
 
Content
⇒ Chemistry of the Gene 2.  Synthesis, Modification and Repair of DNA
⇒ DNA replication: general features 
⇒ Semi-conservative DNA replication in E. coli
⇒ Semi-conservative replication of chromosomes in eukaryotes
⇒ Semi-discontinuous DNA replication
⇒ Unidirectional and bidirectional DNA replication
⇒ RNA primers in DNA replication
⇒ Regulation of DNA replication by anti-sense RNA primer
⇒ Prokaryotic DNA polymerases
⇒ Eukaryotic DNA polymerases
⇒ Replicons for DNA replication
⇒ DNA replication in prokaryotes 
⇒ Experimental approaches for the study of DNA replication
⇒ Initiation of DNA replication
⇒ Elongation of DNA chain
⇒ Replication fork movement
⇒ Termination of DNA replication
⇒ DNA replication in eukaryotes 
⇒ DNA replication and cell cycle
⇒ Replication origins and initiation of DNA replication (cis and trans-acting elements)
⇒ Comparison of initiation of DNA replication with transcription initiation
⇒ Different steps involved in eukaryotic DNA replication
⇒ Synthesis of telomeric DNA by telomerase
⇒ Models of DNA replication
⇒ Replication fork model
⇒ Rolling circle model of DNA replication
⇒ Mitochondrial DNA replication and D-loops
⇒ RNA directed DNA synthesis (reverse transcription)
⇒ DNA modification and DNA restriction
⇒ DNA repair
⇒ Excision repair systems in E. coli
⇒ An SOS repair system in E. coli
⇒ DNA repair and genetic diseases in humans
DNA Repair
Both in prokaryotic and eukaryotic cells, there are repair enzyme systems to deal with DNA damage, which is caused rather frequently. The changes in DNA leading to damage are broadly divided into two general classes : (i) Single base changes, are corrected through DNA replication leading to changes in DNA sequences; this may be caused by conversion of one base to another (e.g. deamination of 5-methylcytosine to thymine). (ii) Structural distortions may result from a single strand nick, removal of a base or introduction of a covalent link between bases of same or different strands (e.g. formation of thymine dimer due to UV-see Mutations: 3.  Molecular Level (Mechanism)).

A cell may have several systems to deal with DNA damage. These systems include the following : (i) Direct repair involves reversal of the damage; for instance, photoreactivation of pyrimidine dimer involves removal of covalent bonds giving the original structure (no fresh DNA synthesis is involved), (ii) Excision repair involves recognition of a damaged or altered base(s) followed by excision of a sequence including damaged bases. This is achieved by an endonuclease, or an exonuclease. A new stretch of DNA is then synthesized to replace the excised material, (iii) Mismatch repair involves correction of mismatches or pairing between bases which are not complementary. Mismatches may arise either (a) during replication or (b) due to base conversion (e.g. deamination) and are corrected by a process described as error correction during DNA replication. Error correction involves a process that distinguishes between 'new' and 'old' strands (due to methylation), so that the error in the newly synthesized strand is corrected.

 
     






     
     
 
 
     
 
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