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.
Which one of the following codons act as start codon in protein synthesis?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → AUG
A start codon is the codon where translation begins. It is AUG, and it also codes for the amino acid methionine. A translational unit of mRNA lies between the start codon and a stop codon.
(1) AUU — Incorrect. It is an ordinary codon that codes for isoleucine.
(2) AUG — Correct. It is the start codon.
(3) AUC — Incorrect. It is an ordinary codon that codes for isoleucine.
(4) AUA — Incorrect. It is an ordinary codon that codes for isoleucine.
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:
Translation – …
