Question

Which of the following regarding DNA is not true?

Options

• Complementary base pairs consist of one purine and one pyrimidine.

• ‘Side rails’ of the DNA double helix are made of alternating molecules of phosphate and ribose.

• Complementary base pairs are joined by hydrogen bonds.

• The amount of thymine equals the amount of adenine.

Answer 1

‘Side rails’ of the DNA double helix are made of alternating molecules of phosphate and ribose.

Explanation:

The “side rails” or backbone of the DNA double helix are composed of alternating phosphate groups and deoxyribose sugars, rather than ribose. Ribose is a pentose sugar found exclusively in RNA, distinguished by having a hydroxyl (-OH) group at the 2’ carbon position, whereas DNA’s deoxyribose lacks this oxygen atom. This chemical difference is fundamental because it makes DNA more stable for long-term genetic storage. The other options are scientifically accurate: purines always pair with pyrimidines to maintain a constant width; these pairs are held together by hydrogen bonds; and Chargaff’s Rule ensures that the amount of adenine is always equal to thymine.