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Answer any two of the following : (i) ...

Answer any two of the following :
(i) Explain the role of DNA dependent RNA polymerase in initiation, elongation and termination during transcription in bacterial cell.
(ii) How transcription is a more complex process in eukaryotic cells than that in prokaryotic cells?

Text Solution

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(i) Role of DNA dependent RNA polymerase during transpiration in bacterial cell is given below:
(a) It becomes associated transiently with initiation factor, sigma `(sigma)` and binds to the promoter site on DNA to initiate transcription.
(b) RNA polymerase uses nucleoside triphosphate as substrate and polymerises them in a template-dependent fashion following the base complementary rule in the `5. "to" 3.` direction.
(c) It facilitates the opening of the DNA helix and continues the elongation process.
(d) When polymerase falls off the terminator region
on DNA, the nascent RNA separates.
(e) Association of RNA polymerase with a termination factor is necessary for separation of synthesised RNA.
(ii) Reasons due to which transcription is a more complex process in eukaryotic cells than that in prokaryotic cells are
(a) In the nucleus of a eukaryotic cell, there are three types of RNA polymerases. The RNA polymerase-I transcribes rRNAs (28 S, 18 S and 5.8 S), RNApolymerase-II transcribes the precursor of mRNA called hnRNA and the RNA polymerase-III transcribes tRNA, SrRNA and SnRNAs.
(b) Presence of post transcriptional processing The hnRNA (primary transcript of mRNA) contains both coding sequence called exons and non-coding sequences called intrans. So, it undergoes splicing. In this process, the non-coding sequences (introns) are removed and the coding sequences (exons) are joined together in a defined order.
An additional most transcriptional processing is also involve called capping and tailing.
In capping, unusual nucleotide, methylguanosine triphosphate residues are added at the 5.-end of the hnRNA. In tailing, 200-300 aclenylate residues are added at the 3.-end of the hnRNA.
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