Question

(a) Describe the process of transcription in bacteria.

(b) Explain the processing the hnRNA which needs to undergo before becoming functional mRNA eukaryotes. [5]

Answer 1

(a) [3]
Bacterial transcription is the process in which a segment of bacterial DNA is copied into a newly synthesized strand of messenger RNA (mRNA) with use of the enzyme RNA polymerase. It has three steps : Initiation, elongation and termination.

(i) Initiation : DNA dependant RNA polymerase binds to promoter site of DNA at the 5' end and initiates transcription. It associates with initiation sigma ($\sigma$) factor and alters the specificity of RNA polymerase to initiate the transcription.

(ii) Elongation :RNA polymerase uses ribonucleoside triphosphate as substrate, and polymerises in a template depended fashion following the rule of complementarity from the 5' to 3' direction and facilitates opening of the helix and continues elongation.

(iii) Termination : It occurs when termination factor (rho $\rho$) alters the specificity of RNA polymerase. Only a short stretch of RNA remains bound to the enzyme. Once the polymerases reaches the terminator region, the nascent RNA falls off, so also the RNA polymerase. This results in termination of transcription.
(each point 1M)

(b) The precursor of mRNA, i.e., hnRNA, contains both introns and exons. Introns are removed and exons are joined by a process called splicing. The remaining mRNA is processed in two ways : [2]

Capping : An unusual nucleotide (methyl guanosine triphosphate) is added to the 5'-end of hnRNA.

Tailing : Adenylate residues (200-300) are added at 3'-end of hnRNA.

When hnRNA is fully processed, it is known as mRNA, which is transported out of the nucleus.