Molecular Basis of Inheritance
Read the following passage carefully and answer the given questions.
A nucleotide has three components – a nitrogenous base, a pentose sugar (ribose in case of RNA and deoxyribose in case of DNA), and a phosphate group. There are two types of nitrogenous bases – Purines (Adenine and Guanine), and Pyrimidines (Cytosine, Uracil and Thymine). 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). It was George Gamow, a physicist, who argued that since there are only 4 bases and if they have to code for 20 amino acids, the code should constitute a combination of bases. He suggested that in order to code for all the 20 amino acids, the code should be made up of three nucleotides. This was a very bold proposition, because a permutation combination of 43 (4 × 4 × 4) would generate 64 codons; generating many more codons than required.
Heterogeneous nuclear RNA is produced by -
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → RNA polymerase II
In eukaryotes there are three RNA polymerases, and each one makes different RNAs. The first product of RNA polymerase II is the heterogeneous nuclear RNA (hnRNA). It has both exons and introns, so it is not yet functional. It is spliced to remove the introns, and this gives the mRNA.
(1) RNA polymerase I — Incorrect. It transcribes the rRNAs (28S, 18S and 5.8S).
(2) RNA polymerase II — Correct. It transcribes the precursor of mRNA, which is the hnRNA.
(3) RNA polymerase III — Incorrect. It transcribes tRNA, 5srRNA and snRNAs.
(4) DNA polymerase — Incorrect. It makes DNA during replication. It does not make RNA.
Note: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
NCERT Extract:
Transcription Unit and the Gene – “… (i) There are at least three RNA polymerases in the nucleus (in addition to the RNA polymerase found in the organelles). There is a clear cut division of labour. 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).”