• 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.
• 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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