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.
If a double stranded DNA has 40 percent of guanine, calculate the percent of adenine in the DNA.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 10 %
In double stranded DNA the bases pair in a fixed way: adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). So the ratios A : T and G : C are each equal to one. That means A = T and G = C.
G = 40 %, so C = 40 %
G + C = 40 + 40 = 80 %
A + T = 100 − 80 = 20 %
A = T, so A = 20 / 2 = 10 %
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:
Structure of Polynucleotide Chain – …
