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) \(3^{\text {nd }}\) (c) \(4^{\text {th }}\) (d) \(1^{\text {st }}\)

Step-by-Step Solution

Answer

The hydroxyl group on the 2nd carbon differentiates RNA and DNA; therefore, the answer is (a) 2nd.

Step 1: Understand RNA and DNA Differences

RNA (ribonucleic acid) and DNA (deoxyribonucleic acid) are both nucleic acids found in cells, but they differ in their sugar component. RNA contains the sugar ribose, whereas DNA contains deoxyribose.

Step 2: Examine the Sugar Component

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

Step 3: Determine the Key Carbon Atom

The key difference that distinguishes RNA from DNA is the presence of the hydroxyl group at the 2nd carbon of ribose. Its absence on the 2nd carbon of deoxyribose is what gives DNA its name and character as "deoxy-ribose."

Key Concepts

Nucleic Acids

Nucleic acids are essential molecules that carry genetic information in living organisms. They are the blueprint for the synthesis of proteins and for passing genetic information from one generation to the next.
There are two primary types: DNA and RNA. While both are polymers made up of nucleotides, they serve different functions and have distinct structural differences.

• DNA, or deoxyribonucleic acid, is responsible for storing and transmitting genetic information.
• RNA, or ribonucleic acid, plays a crucial role in translating the genetic code into proteins and can even have enzymatic functions.

The key structural components of nucleic acids involve a nitrogenous base, a sugar molecule, and a phosphate group, forming a nucleotide. Both DNA and RNA share two of the same nitrogenous bases (adenine and guanine), but RNA includes uracil while DNA includes thymine. These differences in structure are foundational to their unique functionalities.

Ribose Sugar

Ribose sugar is a vital component of RNA, differentiating it from DNA. It is a five-carbon sugar that forms the backbone of RNA molecules.
In ribose, each carbon atom is numbered for clarity, with the carbons often termed as 1' (one prime), 2' (two prime), up to 5' (five prime).

• In RNA, the sugar molecule is ribose, which has a hydroxyl group (-OH) attached to each of the 2' and 3' carbon atoms.
• This structure grants RNA greater reactivity and flexibility, contributing to its broader range of roles in the cell.

Understanding the nature of ribose is crucial, as the presence of hydroxyl groups influences the stability and function of RNA. Unlike the ribose in RNA, deoxyribose in DNA lacks the 2' hydroxyl group, which makes DNA more stable and less susceptible to hydrolysis.

2nd Carbon Hydroxyl Group

The 2nd carbon hydroxyl group is a defining structural element that sets RNA apart from DNA at the molecular level. This small chemical difference leads to significant functional differences between the two nucleic acids.
In the sugar ribose of RNA, the presence of the hydroxyl group (-OH) at the 2' carbon is crucial.
This small addition:

• Increases RNA's chemical reactivity, making it less stable than DNA but more versatile.
• Contributes to the ability of RNA to fold into complex structures, allowing it to function not just in coding, but also in catalysis and regulation.
• Explains RNA's role in catalysis and regulation due to its flexible nature and susceptibility to enzymatic cleavage.

In contrast, DNA's sugar, deoxyribose, lacks the hydroxyl group at the 2' carbon. This absence provides DNA with greater resistance to hydrolysis, ensuring its long-term stability as the carrier of genetic information. The term "deoxy" reflects this absence, highlighting the core difference between the two sugars.