Transcription and RNA Polymerase
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Direct answer
Transcription is DNA-dependent RNA synthesis in the 5' to 3' direction, catalysed by RNA polymerase from a template strand. Prokaryotes manage with a single RNA polymerase that transcribes all RNA types; the sigma factor recognises the promoter to start, and the rho factor terminates some transcripts. Eukaryotes need three nuclear polymerases with divided labour: RNA polymerase I makes rRNA except 5S (28S, 18S, 5.8S), RNA polymerase II makes heterogeneous nuclear RNA — the mRNA precursor — and RNA polymerase III makes tRNA, 5S rRNA and small nuclear RNAs. The primary eukaryotic transcript is then processed: a methyl guanosine cap at the 5' end, a poly-A tail of 200-300 adenylate residues at the 3' end, and splicing that removes introns to yield mature mRNA.
What you must remember
- Prokaryote package: one RNA polymerase; sigma factor for promoter recognition and initiation; rho factor terminates transcription — both named in NCERT.
- Eukaryote division of labour: pol I → rRNAs (28S, 18S, 5.8S); pol II → hnRNA, the precursor of mRNA; pol III → tRNA, 5S rRNA and snRNAs — the single most tested matching pair-set in the chapter.
- Three processing steps: capping at 5' with methyl guanosine, tailing at 3' with 200-300 adenylate residues, and splicing of introns by the spliceosome machinery of snRNPs.
- Strand vocabulary: the template strand is read; the coding strand has the same sequence as the RNA (with uracil for thymine) — a distinction statement-questions exploit.
- No primer needed: unlike DNA polymerase, RNA polymerase can initiate a chain de novo — the contrast NEET loves to set.
- Messenger classes: prokaryotic mRNA is often polycistronic, eukaryotic mRNA typically monocistronic; eukaryotic genes are split, with introns and exons.
- Spliceosome detail: small nuclear RNAs in snRNP complexes recognise splice junctions; the excised intron forms a lariat intermediate in standard accounts.
Follow a eukaryotic gene from promoter to cytoplasm
Start at the promoter, where transcription factors assemble and recruit RNA polymerase II. The enzyme peels the duplex open, reads the template strand, and extrudes hnRNA in the 5' to 3' direction; when it reaches the terminator region, it releases the transcript and dissociates — in prokaryotes, rho may chase the polymerase and pull it off, while eukaryotes rely on polyadenylation signals, per standard accounts. What hangs off the enzyme at this point is unusable: a naked hnRNA with introns. Capping machinery clamps a methyl guanosine on the 5' end, which later signals efficient translation; an endonuclease cuts at the poly-A signal and polymerase adds 200-300 adenylates to the new 3' end, a licence for nuclear export and a shield against degradation; and the spliceosome, built from snRNPs, excises each intron and ligates the exons.
Only now is it mRNA, threaded through a nuclear pore to the ribosome. The practical point for problem-solving: any question about the number of nucleotides in mature mRNA versus the gene must account for introns removed, cap and tail — a favourite quantitative twist. And in prokaryotes, where nucleus is absent, transcription and translation couple in the same cytoplasm, so a polycistronic message can begin to be translated before its own transcription has finished.
Where students lose marks
The pol II slip is the most expensive: candidates write that pol II makes mRNA; strictly it makes hnRNA, which becomes mRNA after processing — matching questions mark the precise word. Second, pol I versus pol III over 5S rRNA: pol I makes the large rRNAs, but 5S rRNA belongs to pol III; the exception is deliberate and constantly tested. Third, introns versus exons: splicing removes introns and joins exons — never the reverse — and the excised material does not reach the ribosome. Keep the "RNA polymerase needs no primer" contrast ready, because it appears as the reason clause in assertion-reason items.
Frequently asked questions
Which RNA polymerase transcribes mRNA precursors in eukaryotes?
RNA polymerase II, which makes hnRNA that is processed into mature mRNA.
Which RNAs do RNA polymerases I and III produce?
Polymerase I makes 28S, 18S and 5.8S rRNA; polymerase III makes tRNA, 5S rRNA and small nuclear RNAs.
What roles do sigma and rho factors play in prokaryotes?
Sigma factor recognises the promoter to initiate transcription, and rho factor terminates transcription of certain genes.
What three modifications convert hnRNA to mRNA?
A 5' methyl guanosine cap, a 3' poly-A tail of 200-300 adenylate residues, and splicing that removes introns.
How does RNA polymerase differ from DNA polymerase in initiation?
RNA polymerase can start a new chain without a primer, whereas DNA polymerase requires a primer with a free 3' end.