The point mutation arise due to change in:

The correct answer is Option (2) → Single base pair of DNA

Option (1) Complete gene — Incorrect. A change in a whole gene is not a point mutation.

Option (2) Single base pair of DNA — Correct. A point mutation is a change in a single base pair of DNA. Sickle-cell anaemia is the classic example.

Option (3) Two base pairs of DNA — Incorrect. A change in two base pairs is not a point mutation. NCERT defines a point mutation as a change in a single base pair of DNA.

Option (4) Three base pairs of DNA — Incorrect. Three base pairs make up one codon, but a change in three base pairs is more than a point mutation. A point mutation, as in sickle-cell anaemia, changes only one base pair.

NCERT Extract:
Mutation – “Mutation is a phenomenon which results in alteration of DNA sequences and consequently results in changes in the genotype and the phenotype of an organism. In addition to recombination, mutation is another phenomenon that leads to variation in DNA. As you will learn in Chapter 5, one DNA helix runs continuously from one end to the other in each chromatid, in a highly supercoiled form. Therefore loss (deletions) or gain (insertion/duplication) of a segment of DNA, result in alteration in chromosomes. Since genes are known to be located on chromosomes, alteration in chromosomes results in abnormalities or aberrations. Chromosomal aberrations are commonly observed in cancer cells. In addition to the above, mutation also arise due to change in a single base pair of DNA. This is known as point mutation. A classical example of such a mutation is sickle cell anemia.”