Transcription is the formation of RNA from DNA.
It occurs in the nucleus during the G1 & G2 stage of the cell cycle.
Of the two strands of DNA only one strand called the template strand takes part in transcription.
A transcription unit in DNA consists of three regions:
- A promoter
- The structural gene
- A terminator.
Structural gene is the part of DNA from which RNA is formed.
- It is polycistronic in prokaryotes and is monocistronic in eukaryotes.
Transcription occurs with the help of DNA dependent RNA polymerase.
Polymerization occurs in 5′ → 3′ direction.
- The 3′ → 5′ strand of DNA acts as the template strand/ anti-sense strand.
- The 5′ → 3′ strand is the coding strand/ sense strand, it has the base sequence similar to the template strand. Only Uracil replaces by Thymine.
All reference points while defining the transcription unit are made with the coding strand.
Lying towards 5′ end (upstream) of the structural gene is the promoter. It is the binding site for RNA polymerase for transcription to start.
- Pribnow box is found in the promoter of prokaryotes.
- Hogness or TATA box is found in the promoter of eukaryotes.
Lying towards 3′ end (downstream) of the structural gene is the terminator, where transcription ends.
Single RNA polymerase synthesizes all type of RNA in prokaryotes.
- RNA polymerase is a holoenzyme made of 5 polypeptide chains: 2α, β, β′ & σ factors.
- Sigma (σ) factor recognizes the promoter to which RNA polymerase binds and the ‘σ’ factor is released immediately after initiation.
- RNA polymerase without ‘σ’ factor is core polymerase.
There are 3 RNA polymerases in eukaryotes:
- RNA polymerase I synthesize 28S, 18S & 5.8S rRNAs.
- RNA polymerase II synthesizes hnRNA (heterogeneous nuclear RNA) that forms mRNA.
- RNA polymerase III synthesizes t RNA, 5S rRNA & small nuclear RNAs.
Transcription occurs in 3 steps:
- Initiation: upon receiving signals RNA polymerase binds to the promoter on DNA by the help of ‘σ’ factor. RNA polymerase separates the two strands of DNA.
- Elongation: Core polymerase moves along the template strand adding ribonucleoside triphosphates (ATP, GTP, CTP, UTP) as substrates in 5′→3′ direction. The RNA formed is complementary to the template strand.
- Termination: it occurs when RNA polymerase reaches a termination signal. Termination occurs by the help of rho (ρ) factor.
Eukaryotes have Split genes, discovered by Sharp. Split genes are structural genes having coding and non-coding sequences.
- Exons are the coding sequences.
- Introns are the non-coding sequences.
The primary RNA formed has both exons and introns and are non-functional. They undergo processing to form functional RNA.
Processing of primary RNA in eukaryotes:
- Splicing: it is the removal of introns and joining of exons. Ribonucleoprotein complexes called Spliceosomes carry out splicing.
- In case of hnRNA, further processing occurs to form mRNA. It includes:
- Capping: addition of methyl-guanosine caps to 5′ end.
- Tailing: polyadenylation occurs at 3′ end. Addition of 200-300 adenylic acids occurs.