NCERT line-by-line

1. TITLE

NCERT says "There is a clear cut division of labour" for RNA Polymerases — Here's how NEET tests it

2. THE NCERT STATEMENT

NCERT EXACT LINE:
"The RNA polymerase I transcribes rRNAs (28S, 18S, and 5.8S), whereas the 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)."
— NCERT Class 12, Chapter 5, Page 111

3. WHY THIS LINE MATTERS

  • How many times has NEET tested this? This specific division of labour in eukaryotic transcription is one of the most frequently tested concepts in genetics, appearing almost every alternate year as a direct or matching question.
  • What makes this statement tricky? Students easily confuse which polymerase transcribes which specific RNA. The trickiest part is remembering that while most rRNAs (28S, 18S, 5.8S) are transcribed by Polymerase I, the 5S rRNA is a unique exception transcribed by Polymerase III.
  • What do students typically misremember? Students often assume that RNA Polymerase II transcribes mature mRNA directly. However, NCERT strictly states that it transcribes the precursor of mRNA, known as hnRNA. They also frequently mix up snRNA (small nuclear RNA) with hnRNA.

4. MCQ 

According to NCERT, the process of transcription in eukaryotes exhibits a clear-cut division of labour. Which of the following statements is correct?
A) RNA polymerase I transcribes rRNAs (28S, 18S, and 5.8S).
B) RNA polymerase II transcribes the precursor of mRNA, the small nuclear RNA (snRNA).
C) RNA polymerase III transcribes only tRNA and has no role in ribosomal RNA transcription.
D) A single RNA polymerase is responsible for the transcription of all types of RNA in eukaryotes.

5. ANSWER + LINE BREAKDOWN

Correct Answer: A

Break down the NCERT line:
- "RNA polymerase I transcribes rRNAs (28S, 18S, and 5.8S)" means Pol I is strictly dedicated to forming the major ribosomal RNA subunits that build the ribosome.
- "RNA polymerase III is responsible for transcription of tRNA, 5srRNA, and snRNAs" means Pol III handles a mix of smaller structural and functional RNAs, including transfer RNA and the unique 5S ribosomal RNA.
- "RNA polymerase II transcribes precursor of mRNA, the heterogeneous nuclear RNA" means Pol II creates the raw, unprocessed transcript (hnRNA) containing both exons and introns, which will later be spliced into mRNA.
- The key word is "precursor" because RNA Polymerase II does not synthesize functional, mature mRNA directly; post-transcriptional processing (capping, tailing, and splicing) is required first.

6. HOW NEET TWISTS THIS

Show 3 ways NEET can ask the same fact:
- As direct question: "In eukaryotes, which RNA polymerase transcribes the precursor of mRNA (hnRNA)?"
- As match the following: You will be asked to match "RNA Pol I, RNA Pol II, RNA Pol III" in Column I with "tRNA, hnRNA, 28S rRNA" in Column II.
- As an assertion-reason: "Assertion: RNA Polymerase I transcribes all types of ribosomal RNAs in eukaryotes. Reason: There is a clear-cut division of labour among the three RNA polymerases." (Here, the assertion is false because 5S rRNA is transcribed by Pol III, not Pol I).

7. MEMORY LOCK

MEMORIZE:
Pol I = rRNAs (the large ribosomal ones)
Pol II = hnRNA (precursor to mRNA)
Pol III = tRNA, 5S rRNA, snRNA

Mnemonic: "1, 2, 3 = R, M, T"
- 1 makes RRNA
- 2 makes MRNA (hnRNA)
- 3 makes TRNA (plus 5S and snRNA)

8. RELATED NCERT LINES

Also remember these from the same topic:
- "There is single DNA-dependent RNA polymerase that catalyses transcription of all types of RNA in bacteria."
- "The primary transcripts contain both the exons and the introns and are non-functional."

9. NCERT REFERENCE

NCERT Class 12, Chapter 5 (Molecular Basis of Inheritance), Page 111, Paragraph 1

10. CTA

All important NCERT lines from this chapter → Genetics and Evolution Guide