Steps Involved in Transcription

 Transcription is a heterocatalytic function of DNA which involves transfer of coded information from DNA through the synthesis of RNA over the template of DNA. Only one of the strand of DNA called sense strand (master strand) is involved.
  The segment of DNA involved in transcription is cistron. It has a promoter region where initiation begins and a terminator region where transcription ends.

Enzyme involved in transcription is RNA polymerase. It is single in prokaryotes. There are three types of RNA polymerase in eukaryotes,
I for 28s, 18s & 5.8s RNA.
II for mRNA and SnRNA and
III for tRNA, 5s RNA & Sc RNA.

RNA polymerase has five polypeptides — σ, α, β, β' and ω).

σ or sigma factor recognises the promoter region while the remaining or core enzyme takes part in transcription.
A rho or ρ factor is needed for termination of transcription.

Steps involved in transcription

a] Activation of Ribonucleotides :

With the help of phosphorylase, energy and phosphoric acid, ribo- nucleoside monophosphates are changed into triphosphates ATP, GTP, CTP & UTP.

b] Cistron :

It has a promoter region where transcription begins and a terminator region where transcription ends.
Promoter region possesses RNA polymerase recognition as well as RNA-polymerase binding sites.
Sigma factor or RNA polymerase gets attached to promoter region of cistron. The two DNA strands separate with the help of unwindase and helix destabilising proteins.
One of them function as sense strand and takes part is transcription. Transcription proceeds in 5' to 3' direction.

c] Base Pairing :

Ribonucleoside triphosphate come to lie opposite complementary nitrogen bases of sense strand (A opposite T, C Copposite G, U opposite A and G opposite C).
Pyrophosphatase hydrolyses two phosphates from cach activated nucleotide. This releases energy.

d] Strand Formation : 

RNA polymerase establishes phosphodiester bonds between adjacent ribonucleotides.
Sigma factor σ recognises.
Core enzymes moves along the sense strand till it reaches terminator region and separates from DNA template in the presense of rho (ρ) factor.
Terminator region contains a stop signal made of 4-8 A (poly A tail) nucleotides.

e] Separation of RNA :

rho (ρ) factor has ATP ase activity. It helps in separating the newly formed RNA or primary transcript from the sense strand of DNA.

f] Duplex Formation :

With the release of primary transcript the sense and antisense strands of DNA re-establish their hydrogen linkages and form duplex. Unwindase and helix destablising proteins are released.

g] Post-Transcriptional Processing :

Primary transcript is often called heterogenous or hn RNA as it is generally bigger than the functional RNA.

1. Cleavage : hn RNA is broken to form smaller pieces. Ribonuclease-P (RNA enzyme) separates 5-7 t RNA precursors from primary transcript.
2. Splicing : Introns or intervening sequences of nonessential nature are removed by nucleases. Ribozyme (RNA enzyme) is one such enzyme.
3. Terminal Additions : Nucleotides are added at the ends of RNA for specific functions, e.g. CCA at 3'ends of tRNA, cap nucleotides at 5'ends of m RNA or poly A nucleotides at 3'end of m RNA.
4. Nucleotide Modifications : Certain nucleotides are methylated, ethylated, deaminated, etc. to produce different chemicals like inosine, methyl cytosine, dihydrouracil, pseudouracil, etc.

The promoters of prokaryotic genes are simple. They generally consist of about 40 bp of DNA that contain two elements :

• a TATA box, located 10 bp upstream from the transcription initiation site and
• a second element, located 35 bp in the 5' direction upstream.

Eukaryotic promoters are more complex and usually consist of several elements. One set of elements ensures that transcription initiates at the correct site. The elements in this set, which of ten include a TATA box, which are usually found within 100-200 bp of transcription start site. Another set of clements, usually located further 5' from the transcription initiation site, can enhance or silence transcription. These work with, or serve as tissue specific or signal response elements.

Ribosomal RNA is also synthesized as a large 45s precursor molecule that is processed into the mature 28s, 18s and 5.8s fomms.