Chain Elongation

Chain Elongation

Chain Elongation - Synthesis of the new DNA strand takes place by addition of DNA nucleotides to the 3'OH group of the last ribonucleotide of the RNA primer. This synthesis takes place in the 5'-->3' direction and is probably catalysed by DNA polymerase III. The newly synthesized DNA chains have primer RNA attached to their 5' ends.

The RNA primer is hydrolysed by the 5'-->3' exonuclease activity of DNA polymerase I. The resulting gap is filled in by DNA nucleotides, again by the catalytic activity of DNA polymerase I. The newly-made DNA is joined by DNA ligase to form high molecular weight DNA (site A).

RNA primers which are joined on both - sides to DNA chains are digested by specific endonucleases, e.g. ribonuclease H. Why are RNA primers necessary? It has been suggested that the laying down of the first few nucleotides during DNA synthesis is more subject to errors than the addition of nucleotides to an already existing polynucleotide chain. Since the RNA primer is removed by polymerase I and then replaced by a DNA segment, any errors in the initial stages of synthesis are eliminated.

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