Question

Explain the process of transcription in prokaryotes. How is the process different in eukaryotes? [5]

Answer 1

In prokaryotes, only one DNA-dependent RNA polymerase enzyme catalyses the transcription of all types of RNA. Transcription occurs in three stages–initiation, chain elongation and termination.
In the initiation stage, the RNA polymerase along with the initiation factor (denoted by sigma) binds to the promoter sequence in DNA. This helps in the opening of the DNA helix and separates the two DNA strands.
In the elongation stage, the RNA polymerase builds a strand of RNA using one of the DNA strands as a template. This RNA chain growth takes place in the 5' to 3' direction. Once the chain elongation commences, the sigma factor dissociates from the RNA polymerase which can be reused.
In the termination stage, the polymerase along with the termination-factor, represented by rho reaches the terminator region and the newly created mRNA falls off along with the enzyme. [3]

Difference in Eukaryotic Transcription :
(a) In Eukaryotes, the promoter site is recognised by presence of specific nucleotide sequence called TATA box (Hogness Box) on template strand.
(b) In Eukaryotes, RNA polymerase is of 3 types-I, II and III.
(c) Monocistronic structural genes are present which have interrupted coding sequences, i.e., the information on gene is split. The coding sequences are called exons and the non-coding sequences are called introns.
(d) In eukaryotic transcription,there is mRNA processing which includes splicing(introns removed and exons joined),
capping(Addition of 5-methyl guanosine triphosphate at 5' end)
and tailing.(Addition of 200 adenylate residues at 3' end) [2]