Stage Of Transcriptions.

Stage Of transcriptions:

There are three stages of transcriptions,

Initiation :

The binding of the enzyme RNA polymerase to DNA is the prerequisite for the transcription to start. The specific region on the DNA when the enzyme binds is known as promoter  region. There are two base sequence on the coding DNA strand which the sigma factor of RNA polymerase can recognized for initiation of  transcription

• Pribnow box: this consists of 6 nucleotide base, located on the left side about 10 base away from the starting point of transcription.
• The -35 sequence: this is the second recognition site in the promoter region of DNA. It contains a base sequence TTGACA, which is located about 35 base away or the left side from the site of transcription start.

Elongation:

As the holoenyme ,RNA polymerase recognizes the promoter region , the o factor is released and transcription proceeds. RNA is synthesized from 5` end to 31 end antiparallel to the DNA template. RNA polymase utilizes ribonucleotide triphosphates for the formation of RNA for the additional of each nucleotide to the growing chain , a pyrophosphate moiety is released.

The neucleotide base sequence is newly synthesized RNA is identical to the coding strand and complementary to the template strand.

RNA polymerase differs from DNA polymerase in two aspects. No primer is required for RNA polymerase and , further this enzyme does not possess end or exonuclease activity. Due to lack of the latter function, RNA polymerase has no ability to repair the mistake in the RNA synthesized. This is in contrast to DNA replication which is carried out with high fidelity . Mistakes of RNA synthesized are less dangerous since they are not transmitted to the daughter cell.

Termination:

Termination are two types,

1)    RHO(P) dependent termination :  A specific protein named P factor , binds to the growing RNA or weakly to DNA, and in the bound state it acts as Atpase and terminates transcription ans release RNA. The P factor is also responsible for the dissociation of RNA polymerase from DNA.

2)     RHO(p) independent termination: the termination in this case is brought about by the formation of hairpins of newly synthesized RNA. This occurs due to the presence of palindromes. The presence of palindromes in the base sequence of DNA template synthesized RNA folds to form hairpins. That causes termination of transcription.