Enzyme helpful in polymerizing RNA with defined sequences in a templa…

The correct answer is Option (4) → DNA dependent RNA polymerase

Note: The given answer is as per the official NTA Answer Key. However, the correct answer should be Option (2) Polynucleotide phosphorylase, as explained below.

In a cell, RNA is made by copying a DNA template (transcription). To work out the genetic code, scientists needed RNA of known sequence made without any template. Such RNA was made by an enzyme that needs no template.

Why the answer given by NTA is incorrect

Option (1) Mononucleotide phosphorylase — Incorrect. No enzyme of this name is used to make RNA. The prefix has been changed from “poly” to “mono”; the enzyme that builds a polynucleotide is polynucleotide phosphorylase.

Option (2) Polynucleotide phosphorylase — Correct as per NCERT. This is Severo Ochoa’s enzyme. It joins nucleotides into RNA of a chosen sequence without copying any DNA template, which is the enzyme the question describes.

Option (3) RNA polymerase - I — Incorrect. In eukaryotes it transcribes the rRNAs (28S, 18S and 5.8S). Like every RNA polymerase of transcription, it needs a DNA template.

Option (4) DNA dependent RNA polymerase — The answer given by NTA, but incorrect. This is the enzyme of transcription. Its name says it: it depends on a DNA template, which is the opposite of what the question asks.

NCERT Extract: “Providing proof that the codon was a triplet, was a more daunting task. The chemical method developed by Har Gobind Khorana was instrumental in synthesising RNA molecules with defined combinations of bases (homopolymers and copolymers). Marshall Nirenberg’s cell-free system for protein synthesis finally helped the code to be deciphered. Severo Ochoa enzyme (polynucleotide phosphorylase) was also helpful in polymerising RNA with defined sequences in a template independent manner (enzymatic synthesis of RNA).”
Transcription Unit – “… Since the two strands have opposite polarity and the DNA-dependent RNA polymerase also catalyse the polymerisation in only one direction, that is, 5’→3’, the strand that has the polarity 3’→5’ acts as a template, and is also referred to as template strand.”