DNA repair and genetic diseases in humans

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 and genetic diseases in humans
Several genetic diseases in human beings are characterized by cellular hypersensitivity to DNA damage. These diseases also cause increased risk of cancer. Four such recessively inherited diseases include the following : (i) Fanconi anaemia (FA); (ii) Xeroderma pigmentosum (XP); (iii) Ataxia telangiectasia (AT) and (iv) Bloom's syndrome (BS); cells from patients with the first three diseases are uniquely sensitive to DNA damage by DNA cross-linking agents, ultraviolet light and ionizing radiations respectively, those with BS are moderately hypersensitive to several agents causing different types of DNA damage. Genes responsible for two of these diseases (FA, XP) have been identified recently by complementation of hypersensitive cell lines using DNA expression cloning technology (see Genetic Engineering and Biotechnology 1.  Recombinant DNA and PCR (Cloning and Amplification of DNA) for cloning). The XP DNA products show significant similarity to the products of the RAD DNA repair gene of budding yeast, and ERCC genes of rodents (Table 26.7), which are involved in the nucleotide excision repair showing that the essential enzymes involved in DNA metabolism have been conserved through evolution. In future more work on the molecular mechanism of these diseases involving DNA damage will be done.

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*XPAC (and so on) is the term for the gene encoding a protein that corrects XP-A.
+ Rodent complementation groups are numbered. The human genes correcting the defect of rodent mutants are designated ERCC (for excision repair cross complementing) genes, the number referring to the number of the corrected group.
++ Level and type of homology not conclusive.