Question

Thymine is
(a) 5-methyl uracil
(b) 4-methyl uracil
(c) 3-methyl uracil
(d) 1-methyl uracil

Answer 1

Hint: DNA is a double helical structure consisting of two strands made up of nitrogen bases attached with each other with the help of double or triple hydrogen bonds, phosphates, and deoxyribose sugar. But out of 4 types of bases 2 pairs with the other 2 and that is fixed.

Complete answer:
DNA is made up of nucleotide molecules. Every molecule of nucleotide contains one phosphate group, one sugar group, and one nitrogen base. The four types of nitrogen bases present in a nucleotide are adenine (A), thymine (T), guanine (G), and cytosine (C). Thymine is also known as 5-methyl uracil which is unique and in DNA. The nucleotides are attached to each other to form two long strands of DNA that spiral with each other to create a structure called a double helix. So, DNA is said to be the double-helical structure. The bases on one strand pair with the bases on another strand meaning adenine on one strand pairs with the thymine of another strand, and similarly the guanine of one strand pairs with the cytosine of the opposite strand.
The nitrogenous bases are purines and pyrimidines. The purines are adenine and guanine. The pyrimidines are thymine and cytosine. The structure of purines are double-ring-like while in the case of the pyrimidines the structure is composed of carbon and nitrogen and is single ring-like. Purines and pyrimidines pair with each other through the hydrogen bonds. The bond between adenine and thymine may be a hydrogen covalent bond whereas the bond between guanine and cytosine is that hydrogen bond. There is a double hydrogen bond between adenine and thymine and a triple hydrogen bond between guanine and cytosine.

So, the correct answer is ‘5-methyl uracil’.

Note:
Due to exposure to Ultraviolet radiations the thymine bonds with the thymine instead of adenine thus forming a thymine dimer. Formation of thymine dimer causes mutation, and when usually this abnormality occurs our body repairs it. The repair mechanism of thymine dimer in Prokaryotes is known as photoreactivation and in eukaryotes, it is an Excision repair mechanism.