The presence or absence of hydroxyl group on which carbon atom of sugar differentiate RNA and DNA:

2nd

The correct answer is option 1. 2nd.

The backbone of both DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) is composed of sugar molecules, specifically ribose in RNA and deoxyribose in DNA. These sugars are pentoses, meaning they have five carbon atoms.

In ribose (the sugar found in RNA), each carbon atom is attached to a hydroxyl (-OH) group except for the second carbon atom, which has two hydrogens (-H) attached to it. This is why it's called "ribose" - "deoxy" refers to the absence of an oxygen atom.

In deoxyribose (the sugar found in DNA), one of the hydroxyl groups on the second carbon atom is replaced by a hydrogen atom. This absence of a hydroxyl group is what distinguishes deoxyribose from ribose.

This structural difference between ribose and deoxyribose is crucial because it affects how the nucleotides polymerize and the stability of the resulting molecule. DNA is more stable than RNA due to the absence of the hydroxyl group on the 2nd carbon atom, which makes it less prone to hydrolysis. Additionally, this difference affects the enzymes that interact with DNA and RNA, influencing their respective roles in genetic information storage (DNA) and protein synthesis (RNA).