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Molecular Basis of Inheritance — BIOLOGY STD 12 Science — Question

Rajasthan BoardEnglish MediumSTD 12 ScienceBIOLOGYMolecular Basis of Inheritance4 Marks
Question
Explain different types of RNA along with process of transcription.
✓

Answer

→ Type of RNA In bacteria, there are three major types of RNAs:
(i) mRNA (messenger RNA),
(ii) tRNA (transfer RNA), and
(iii) rRNA (ribosomal RNA).
→ All three RNAs are needed to synthesise a protein in a cell.
→ The mRNA provides the template, tRNA brings aminoacids and reads the genetic code, and rRNAs play structural and catalytic role during translation.
→ There is a single DNA-dependent RNA polymerase that catalyses transcription of all types of RNA in bacteria.
Process of Transciption
→ 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.
→ An intriguing question is that how is the RNA polymerases able to catalyse all the three steps, which are initiation, elongation and termination.
→ The RNA polymerase is only capable of catalysing the process of elongation. It associates transiently with initiation-factor (σ) and termination-factor (p) to initiate and terminate the transcription, respectively.
→ Association with these factors alter the specificity of the RNA polymerase to either initiate or terminate.
Image
→ In bacteria, since the mRNA does not require any processing to become active, and also since transcription and translation take place in the same compartment (there is no separation of cytosol and nucleus in bacteria), many times the translation can begin much before the mRNA is fully transcribed.
→ Consequently, the transcription and translation can be coupled in bacteria.

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