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? (a) \(2^{\text {nd }}\) (b) \(3^{\mathrm{rd}}\) (c) \(4^{\text {th }}\) (d) \(1^{\text {st }}\)

Step-by-Step Solution

Answer

The hydroxyl group on the 2nd carbon atom differentiates RNA and DNA.

Step 1: Understand the Structure of DNA and RNA

DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are both nucleic acids that contain a sugar-phosphate backbone. DNA contains deoxyribose sugar, while RNA contains ribose sugar.

Step 2: Identify Differences in Sugar Components

Ribose is a 5-carbon sugar that has hydroxyl groups (-OH) on the 2nd, 3rd, and 4th carbons. Deoxyribose, found in DNA, is similar to ribose but lacks an -OH group on the 2nd carbon; instead, it has a hydrogen atom.

Step 3: Analyze Which Carbon Has the Difference

The presence or absence of the -OH group on the 2nd carbon of the sugar is what differentiates RNA and DNA. RNA has the -OH on the 2nd carbon of ribose, while DNA does not have it in deoxyribose.

Step 4: Conclusion Based on Analysis

Given the information, the presence or absence of the hydroxyl group on the 2nd carbon atom of the sugar distinguishes RNA from DNA. Thus, the correct option is the 2nd carbon.

Key Concepts

Hydroxyl group in nucleic acids

Nucleic acids, like DNA and RNA, are fundamental to genetic information storage and transfer. A critical difference between these two molecules lies in the presence of the hydroxyl (-OH) group. This seems trivial but dramatically affects their chemistry and role within our cells. In RNA, specifically the ribose sugar, there is an -OH group on the second carbon atom. In contrast, DNA's sugar, deoxyribose, lacks this hydroxyl group on the same carbon, having a hydrogen atom instead.

This small structural detail influences the overall stability and function of DNA and RNA:

• The absence of the -OH group makes DNA more chemically stable, as it is less prone to hydrolysis. This stability ensures DNA's effectiveness in long-term storage of genetic information.
• The presence of the -OH group in RNA makes it more reactive and versatile. Hence, RNA plays varied roles like protein synthesis and regulation of gene expression.

Understanding these differences helps explain the distinct functionalities of DNA and RNA in living organisms.

Structure of ribose and deoxyribose

Ribose and deoxyribose are both five-carbon sugars, which are essential components of nucleic acids. Their names indicate some structural differences vital to distinguishing between DNA and RNA.

Ribose, found in RNA, has the following features:

• A five-carbon ring structure known as a pentose.
• Hydroxyl groups (-OH) attached to the second, third, and fourth carbon atoms.

These hydroxyl groups make ribose an essential part of the RNA backbone, conferring RNA's stability and function.

Deoxyribose, present in DNA, is slightly different:

• Also a five-carbon ring but lacks the hydroxyl group on the second carbon. This is replaced by a hydrogen atom.
• This lack of an -OH group is why it's termed "deoxy," meaning "without oxygen."

The structural difference, though subtle, forms the basis for the different roles RNA and DNA play in cells.

Nucleic acid sugar component differences

The sugars in DNA and RNA are primarily responsible for their differences in function and stability. The primary sugar in RNA is ribose, which has an extra hydroxyl group in comparison to deoxyribose in DNA. This subtle difference significantly influences how each molecule behaves:

• **Stability**: The extra -OH group in ribose makes RNA less stable and more prone to degradation. Conversely, DNA's deoxyribose without the 2' hydroxyl is more stable, which is crucial for storing genetic information over an organism's life.
• **Flexibility and function**: The presence of the hydroxyl group in RNA contributes to its ability to fold into complex three-dimensional shapes. This allows RNA to perform diverse functions that DNA cannot, such as catalysis and regulatory roles.

In summary, while these sugar component differences might seem minor, they fundamentally distinguish the roles of DNA and RNA in the biology of living organisms.