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Section: Genetics » Chemistry of the Gene » Synthesis, Modification and Repair of DNA
 
 
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  Replicons for DNA replication
 
     
 
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
Replicons for DNA replication
DNA replication in prokaryotes as well as in eukaryotes is achieved in discrete units called replicons, which may vary in a genome from one in bacteria like E. coli, and 500 in yeast to several thousands in animals and plants (Table 26.4). In E. coli there is a single replicon with the origin, identified as a genetic locus oriC (245 bp). The sequence of this locus has been isolated and cloned in plasmids and allows autonomous replication of plasmid DNA just like bacterial chromosome. Isolated and cloned telomeres and centromeres from yeast are also similarly used for preparing artificial yeast chromosomes (YAC). The origins of replication have been identified in bacteria, yeast, chloroplasts and mitochondria. These are A:T rich, a feature which is related to unwinding of DNA to initiate replication. In E. coli there are also termination sites (ter A—F), each consisting of ~ 23 bp. Termination of DNA replication requires product of tus gene (Tus protein or TBP), which recognizes ter sites.

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In a eukaryotic genome, there are many replicons, an individual replicon being small and the rate of replication being much slower. Not all replicons start replication at the same time, but the initiation of replication at different replicons takes place in an ordered manner. There is no evidence of termination sites in eukaryotes (unlike E. coli), but termination is brought about when a termination fork meets another fork proceeding towards it from origin of an adjoining replicon.

 
     






     
     
 
 
     
 
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