Elongation of RNA Chain

Elongation of RNA Chain

Elongation of RNA Chain - Chain elongation takes place by addition of activated ribonucleoside triphosphates (ATP, UTP, GTP and CTP) to one strand of the DNA template. For each nucleotide added to the growing RNA chain a pyrophosphate (PPi) is given off. This is rapidly hydrolysed to inorganic phosphate (Pi). The synthesis of the RNA chain is energized by expenditure of energy in the form of pyrophosphates.

For each nucleotide monomer added to the chain two high energy phosphates are expended.The entire reaction may be summarized as follows:

DNA template +

Activated ribonucleoside triphosphates (ATP+GTP CTP+UTP)

RNA
------->
polymerase

DNA template +

RNA polymer (AMP,GMP CMP,UMP)+

4NJ
P~P

Elongation of the RNA chain takes place by means of the core enzyme which moves along the DNA template

1. Dissociation of the sigma factor. After commencement of chain elongation the sigma factor dissociates from the core enzyme. It can now be used by another polymerase molecule. The release of the sigma factor may be taking place either because it is no longer required or because its presence might hinder effective movement of the enzyme along the DNA template.

2. RNA chains grow in the 5'-->3' direction. If RNA chains are synthesized in the 5'-->3' direction, then the first nucleotide should have a triphosphate group. If, on the other hand, the chain grows in the three 3'-->5' direction then the triphosphate group would be on the nucleotide at the growing end. It has been found that the triphosphate group is attached to the first nucleotide.

This shows that chain growth takes place in the 5'-->3' direction. During chain elongation the region of DNA that has been transcribed regains its double helical conformation as the next region of DNA unwinds. The newly formed RNA chain has a triphosphate group at its 5' end and a free hydroxyl group at the 3' end

3.Only one DNA strand of a gene transcribes mRNA. In double stranded DNA a given gene is transcribed from only one of the two strands. The transcribed RNA is complementary to only one of the two strands. All the transcribing genes need not, however, be on one strand of the DNA double helix. One gene may transcribe mRNA from one strand while another transcribes from the other strand. When both strands of the DNA double helix transcribe mRNA, the transcription is said to be symmetrical.

In a circular bacterial chromosome this would mean clockwise transcription on one strand and counterclockwise transcription on the other. In some cases RNA may be transcribed exclusively by one strand. Such transcription is said to be asymmetrical

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