Biology · Molecular Basis of Inheritance · NEET
There are three nuclear RNA polymerases in eukaryotes: RNA polymerase I, II and III. This is different from prokaryotes, where a single RNA polymerase does all the transcription. This division of work is a very common NEET question, so remember the number three.
RNA polymerase II makes the mRNA precursor. It does not make finished mRNA directly. It first makes hnRNA (heterogeneous nuclear RNA), which is later processed (capping, tailing, splicing) into mature mRNA. So when NEET asks 'which enzyme synthesizes precursor mRNA', the answer is RNA polymerase II.
RNA polymerase I transcribes the rRNAs 28S, 18S and 5.8S. But be careful: the 5S rRNA is NOT made by Pol I. The 5S rRNA is made by RNA polymerase III. NEET loves to test this exact split, so learn 5S rRNA separately.
RNA polymerase III transcribes tRNA, 5S rRNA and snRNAs (small nuclear RNAs). Notice it makes the 'small' and helper RNAs. A simple hook: Pol III does the 'tiny team' — transfer RNA, the small 5S rRNA, and the small nuclear RNAs.
No. Prokaryotes (like E. coli) have only ONE RNA polymerase that makes all types of RNA (mRNA, tRNA, rRNA). Eukaryotes have THREE separate RNA polymerases, each making different RNAs. This one-vs-three difference is a favourite NEET trap.
The 5S rRNA comes from RNA polymerase III, not Pol I. RNA polymerase I makes only the 28S, 18S and 5.8S rRNAs. This tiny detail (5S with Pol III) is a classic NEET distractor, so mark it clearly.
Match the following RNA polymerase with their transcribed products: (a) RNA polymerase I (b) RNA polymerase II (c) RNA polymerase III; (i) tRNA (ii) rRNA (iii) hnRNA
What is the role of RNA polymerase III in the process of transcription in eukaryotes?
Which of the following enzymes synthesizes precursor mRNA?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
RNA polymerase I makes rRNAs (28S, 18S, 5.8S). RNA polymerase II makes hnRNA (the precursor of mRNA). RNA polymerase III makes tRNA, 5S rRNA and snRNAs. All three work inside the nucleus of eukaryotic cells.
RNA polymerase II is the key one for protein-coding genes, because it makes the mRNA precursor (hnRNA). This hnRNA is processed into mRNA, which carries the code for proteins. So Pol II is directly linked to gene expression.
No. Prokaryotes have only a single RNA polymerase that makes mRNA, tRNA and rRNA all together. The three-enzyme system (I, II, III) is found only in eukaryotes.
All three are nuclear enzymes, meaning they work inside the nucleus. RNA polymerase I mostly works in the nucleolus (where ribosomal RNA is made), while Pol II and Pol III work in the nucleoplasm.
snRNAs (small nuclear RNAs) are transcribed by RNA polymerase III, according to NCERT. They help in splicing. Do not confuse snRNA (Pol III) with hnRNA (Pol II).
