TRANSCRIPTION

Transcription is the process in which genetic information is transferred from DNA to RNA .

It is the synthesis of m-RNA from DNA .

Transcription  occurs in cytoplasm and in membrane bound in case of prokaryotes and eukaryotes respectively .

Mechanism Of Transcription 

These process of transcription involves five steps 

1- Activation of ribonucleotide 

Ribonucleotides get converted to their activated from .

There are four ribonucleotide like adenosine  monophosphate , guanosine monophosphate  , cytosine monophosphate and Uridine monophosphate .

They get converted to their activated form such as adenosine triphosphate , guanosine triphosphate , cytidine triphosphate and uridine triphosphate through the process of phosphorylation .

2 - Binding to promoter and initiation 

In DNA there are three regions i.e. promoter region expression region and terminator region .

The strat signals on DNA are called promoters .

Promoter regions are at rich region .There are at last 10 bases of A and T before mRNA synthesis .

This region has a common sequence of TATAAT commonly known as TATA box .

RNA polymerase gets bind to this promoter .The start singal is recognised by sigma factor .The only mRNA synthesis initiates .

Sigma factor is required case of prokaryotes where as in eukaryotes ,transcription factors like TFI and TFII are required for recognition of start signals .

After the attachment of RNA polymerase a bubble like structure is formed due to unwinding of helix known as transcription bubble .

Here RNA -DNA hybrid formation initiates .

3- Complementary base pairing 

The activated ribonucleotide come and lie opposite to the complementary base of DNA strands.

Example -u opposite A and c opposite G

4- Elongation of RNA chain 

Temporary hydrogen bond forms between the complementary base pairs of DNA-RNA hybrid .

RNA polymerase forms phosphodiester bond between adjecent nucleotides and RNA sugar phosphate backbone is formed to form a RNA strand .

RNA polymerase copies the DNA sequence accurately .

Only one strand or sense strand .The other DNA strand is not copied and is identical to mRNA  is known as anti-sence strand .

Elongation takes place only in 5' - 3' direction .

It occurs at a speed of 30 nucleotides per second .

The first nucleotide incorporated is always .A or G .

The newly synthesized DNA over template DNA is called RNA - DNA hybrid .

5- Termination 

Transcription of mRNA synthesis is terminated when RNA polymerase reaches terminator region .

Terminator region is GC rich region in prokaryotes and AT rich region in eukaryotes .

Rho factor is responsible for termination .It detaches the RNA polymerase from RNA -DNA hybrid .It also separated newly transcribed RNA from DNA by breaking hydrogen bond .

This type of termination is known as Rho dependent termination .

In Rho independent termination RNA transcription stops when newly synthesized RNA forms GC rich hairpin loop with a long sequence of uracil.