Problem 169 - A Complete Resource Book in Chemistry for JEE Main

Question

The presence or absence of hydroxyl group on which carbon atom of sugar differentiates RNA and DNA? [2011] (a) \(2^{\text {nd }}\) (b) \(3^{\text {rd }}\) (c) \(4^{\text {th }}\) (d) \(1^{\text {st }}\)

Step-by-Step Solution

Answer

Option (a) \( 2^{\text{nd}} \) carbon atom.

Step 1: Understand the Structure of Ribose and Deoxyribose

To differentiate between RNA and DNA, we need to look at the structure of the sugars they contain. RNA contains ribose sugar, whereas DNA contains deoxyribose sugar. Both these sugars are pentoses, meaning each has a five-carbon skeleton.

Step 2: Identify the Presence/Absence of Hydroxyl Group

Ribose, the sugar in RNA, has a hydroxyl group (-OH) attached to its 2nd carbon atom. Deoxyribose, the sugar in DNA, lacks this hydroxyl group on the 2nd carbon atom, having a hydrogen atom instead.

Step 3: Associate the Hydroxyl Group with RNA and DNA

The key structural difference that distinguishes ribose in RNA from deoxyribose in DNA is the presence of the hydroxyl group on the 2nd carbon in ribose, compared to its absence in deoxyribose.

Step 4: Select the Appropriate Answer

Based on the analysis above, the carbon atom that differentiates RNA and DNA by the presence or absence of the hydroxyl group is the 2nd carbon atom. Therefore, the correct answer is the option (a) \( 2^{\text{nd}} \).

Key Concepts

Ribose and Deoxyribose structures

The molecular structures of ribose and deoxyribose are central to understanding the differences between RNA and DNA. Both are five-carbon sugars, or pentoses, but they each play distinct roles. The ribose in RNA has a hydroxyl group (-OH) attached to its second carbon atom. This seemingly small group makes ribose reactive and essential for RNA's role in protein synthesis and other biological processes.
Deoxyribose, on the other hand, is found in DNA. This sugar is called "deoxy" because it lacks one oxygen atom compared to ribose. Specifically, deoxyribose has a hydrogen (H) atom instead of a hydroxyl group at the same position on the second carbon. This absence makes deoxyribose less reactive, stabilizing DNA's double-helix structure, crucial for storing genetic information.
In summary, while ribose and deoxyribose appear similar, their differences at the molecular level are fundamental in determining the function and stability of nucleic acids.

Pentose sugars

Pentose sugars, as their name suggests, are sugars consisting of five carbon atoms. These structures are vital components of nucleic acids, where they form part of the backbone. In both RNA and DNA, the pentose sugar is essential for supporting the attachment of nucleotides, the building blocks of genetic material.

• In RNA, the pentose sugar is ribose. Its structure supports the synthesis and various roles RNA plays in carrying out genetic instructions.
• In DNA, the sugar is deoxyribose. Although it supports a similar backbone, the lack of a hydroxyl group on the second carbon helps maintain the integrity of the DNA molecule over time.

The role of pentose sugars cannot be overstated. They help to organize the linear structure of genetic material and facilitate the formation of sequences that code for life processes.

Hydroxyl group in nucleic acids

The hydroxyl group (-OH) plays a critical role in differentiating RNA and DNA. Understanding its chemical properties provides insight into these nucleic acids' functions.

• In ribose, the presence of the hydroxyl group on the second carbon atom allows RNA to be more reactive. This reactivity is necessary for many of RNA's functions, such as being a template for protein synthesis and serving regulatory roles within cells.
• The absence of the hydroxyl group in deoxyribose results in increased stability for DNA. DNA's primary role is to store genetic information securely, making stability a valuable trait.

Thus, the distinction in the hydroxyl group's presence fundamentally contributes to the different roles and properties of RNA and DNA despite their structural similarities.