Witch of the following is not a purine ?
guanine
cytosine
thymine
none of the above

Cytosine and thymine

Explanation:

Purines- Adenine, guanine

Pyrimidines- cyosine, thymine, uracil (in RNA)

Cytosine and thymine.

Explanation:

DNA is present as genetic material in all the living organism except some viruses. The structure of DNA contains a nitrogenous base, a pentose deoxyribose sugar and a phosphate group.

The nitrogenous base of DNA are characterized into purines and pyrimidines. The pyrimidines are six member ring and contains thymine and cytosine. Purines are nine member rings and contain adenine and guanine.

Thus, the correct answer is option (2) and (3).

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).