Which of the following is not a component of DNA?

The correct answer is Option (1) → Thiamine

Option (1) Thiamine — Not a component of DNA. Thiamine is vitamin B₁, not a nitrogenous base. The base of DNA is thymine; the option adds letters to its name and turns it into a vitamin. Thiamine sounds like thymine, but a vitamin has no place in DNA.

Option (2) Adenine — A component of DNA. Adenine is a purine, found in both DNA and RNA. In DNA it pairs with thymine.

Option (3) Guanine — A component of DNA. Guanine is the other purine. It pairs with cytosine.

Option (4) Cytosine — A component of DNA. Cytosine is a pyrimidine common to DNA and RNA.

NCERT Extract:
Structure of Polynucleotide Chain – “… Let us recapitulate the chemical structure of a polynucleotide chain (DNA or RNA). A nucleotide has three components – a nitrogenous base, a pentose sugar (ribose in case of RNA, and deoxyribose for DNA), and a phosphate group. There are two types of nitrogenous bases – Purines (Adenine and Guanine), and Pyrimidines (Cytosine, Uracil and Thymine). Cytosine is common for both DNA and RNA and Thymine is present in DNA. Uracil is present in RNA at the place of Thymine.”