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  • (a) Explain the process of transcription in a prokaryote. (b) List only, the two additional complexities observed in the process of transcription in eukaryotes.    

(a) Explain the process of transcription in a prokaryote.

(b) List only, the two additional complexities observed in the process of transcription in eukaryotes.

 

 

 

 
 
 
 
 

Answers (1)

a) The process of copying genetic information from one strand of the DNA into RNA is termed as transcription.

There is single DNA-dependent RNA polymerase that catalyses transcription of all types of RNA in bacteria.

RNA polymerase binds to promoter and initiates transcription (Initiation). It uses nucleoside triphosphates as substrate and polymerises in a template depended fashion following the rule of complementarity.

It somehow also facilitates opening of the helix and continues elongation. 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.

The RNA polymerase is only capable of catalysing the process of elongation. It associates transiently with initiation-factor (σ) and termination-factor (ρ) to initiate and terminate the transcription, respectively. Association with these factors alter the specificity of the RNA polymerase to either initiate or terminate.

b) In eukaryotes, there are two additional complexities –

(i) There are at least three RNA polymerases in the nucleus (in addition to the RNA polymerase found in the organelles). There is a clear cut division of labour. The RNA polymerase I transcribes rRNAs (28S, 18S, and 5.8S), whereas the RNA polymerase III is responsible for transcription of tRNA, 5srRNA, and snRNAs (small nuclear RNAs). The RNA polymerase II transcribes precursor of mRNA, the heterogeneous nuclear RNA (hnRNA).

(ii) The second complexity is that the primary transcripts contain both the exons and the introns and are non-functional. Hence, it is subjected to a process called splicing where the introns are removed and exons are joined in a defined order. hnRNA undergo two additional processing called as capping and tailing. In capping an unusual nucleotide (methyl guanosine triphosphate) is added to the 5'-end of hnRNA. In tailing, adenylate residues (200-300) are added at 3'-end in a template independent manner. It is the fully processed hnRNA, now called mRNA, that is transported out of the nucleus for translation.

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Priyanka Kumari

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