Q. 26

(a) Explain the p

Answer :


• 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 also facilitates opening of the helix and continues elongation.

• Only a short stretch of RNA remains bound to the enzyme.

• Once the polymerase reaches the terminator region the nascent RNA falls off so also the RNA polymerase terminating the process of transcription.

b) 1.

• 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)

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.)


• 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.


a) T.H. Morgan selected Drosophila melanogaster as a specimen for his experiments because of the following reasons:

• They could be grown on simple synthetic medium in the laboratory.

• They complete their life cycle in about two weeks, and a single mating could produce a large number of progeny flies.

• A clear differentiation of the sexes – the male and female flies are easily distinguishable.

• It has many types of hereditary variations that can be seen with low power microscopes.

b) Morgan hybridised yellow-bodied and white-eyed females, to brown-bodied and red-eyed males, and intercrossed their F1 progeny. He observed that the two genes did not segregate independently of each other, and the F2 ratio deviated very significantly from the 9:3:3:1 ratio that is generally expected when the two genes are independent.

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