Q4. Explain in one or two lines the function of exons. (NCERT Exercise 11 c)
Exons are the coding or expressed sequences of a split gene, the sequences that appear in the mature, processed RNA; the introns that interrupt them are removed by splicing, and the exons are joined in a defined order (NCERT pages 93 and 95). Beyond the NCERT text: some exons are non-coding (NHGRI Talking Glossary, public domain), and exons also include the untranslated stretches at the ends of an mRNA (Wikipedia, “Exon”, facts in our words).
Q5. Briefly describe transcription. (NCERT Exercise 14 a)
Transcription copies the genetic information of one DNA strand, the 3′ → 5′ template, into RNA, using the transcription unit’s promoter, structural gene and terminator. In bacteria one RNA polymerase, with σ, binds the promoter (initiation), polymerises nucleoside triphosphates by complementarity (elongation) and, with the termination-factor ρ, releases the RNA at the terminator (termination); beyond the NCERT text, intrinsic termination needs no ρ. In eukaryotes at least three nuclear RNA polymerases share the work, and the hnRNA is spliced, capped and tailed into mRNA (NCERT pages 91–95).
Q6. Why can translation begin in bacteria before an mRNA is fully transcribed?
In bacteria the mRNA does not require any processing to become active, and transcription and translation happen in the same compartment, as there is no separation of cytosol and nucleus; so the two processes can be coupled (NCERT page 94).
Q7. A coding strand reads 5′-ATGTTCGGA-3′. Write the template strand and the RNA.
Template: 3′-TACAAGCCT-5′. RNA: 5′-AUGUUCGGA-3′, the coding strand’s sequence with U for T (NCERT page 92).
Q8. Which RNA polymerase of the eukaryotic nucleus transcribes snRNAs, and which makes the 18S rRNA?
NCERT: RNA polymerase III transcribes the snRNAs, and RNA polymerase I the rRNAs, 28S, 18S and 5.8S (pages 94–95); give these in an NCERT-based answer. Beyond the NCERT text: most snRNAs are in fact made by polymerase II, with U6 a polymerase III example (Wikipedia, “Small nuclear RNA”, facts in our words).
Q9. Assertion: the structural gene of a bacterial transcription unit is often polycistronic. Reason: a cistron is a segment of DNA coding for a polypeptide. Are both true, and does the reason explain the assertion?
Both are true, but the reason only defines a cistron; it does not explain why bacterial units carry several of them (NCERT page 93).