Problem 163 - A Complete Resource Book in Chemistry for JEE Main

Question

In both DNA and RNA heterocyclic base and phosphate ester linkages are at (a) \(\mathrm{C}_{5}\), and \(\mathrm{C}_{2}\), respectively of the sugar molecule (b) \(\mathrm{C}_{2}\), and \(\mathrm{C}_{5}^{2}\) respectively of the sugar molecule (c) \(\mathrm{C}_{1}\), and \(\mathrm{C}_{5}^{3}\) respectively of the sugar molecule (d) \(\mathrm{C}_{5}\), and \(\mathrm{C}_{1}\), respectively of the sugar molecule

Step-by-Step Solution

Answer

The correct answer is (d) C5 and C1 of the sugar molecule.

Step 1: Understanding the structure of the nucleotides

Nucleotides in both DNA and RNA consist of a sugar molecule, a phosphate group, and a nitrogenous base. The sugar used in DNA is deoxyribose, and in RNA, it is ribose.

Step 2: Identifying carbon positions in ribose and deoxyribose

The sugar ring contains five carbons numbered as C1, C2, C3, C4, and C5. The C1 position is where the nitrogenous base (also known as the heterocyclic base) attaches, and the C5 position is the location for the linkage with the phosphate group forming a phosphate ester linkage.

Step 3: Analyzing the given options

Now, we analyze the given options: - (a) incorrectly suggests bases attach at C5 and phosphates at C2. - (b) suggests C2 for bases and C5^2 for phosphates, which is incorrect. - (c) suggests C1 for bases and C5^3 for phosphates, which is incorrect. - (d) correctly suggests bases attach at C1 and phosphates at C5.

Step 4: Selecting the correct answer

The correct positions for heterocyclic base attachment and phosphate ester linkage are C1 and C5, respectively, in both DNA and RNA. Thus, the correct answer is option (d).

Key Concepts

RNA and DNA

RNA and DNA are two essential types of nucleic acids found in all living cells. They play critical roles in genetics and cellular function.

• DNA (Deoxyribonucleic Acid): DNA is the molecule that holds the genetic instructions for the development and functioning of all known living organisms and some viruses. It is typically double-stranded and exists in a helical shape.
• RNA (Ribonucleic Acid): RNA is involved in various cellular processes, including acting as a messenger carrying instructions from DNA for controlling the synthesis of proteins. RNA is usually single-stranded and can fold into complex structures.

One major difference between RNA and DNA is the type of sugar molecules they contain.
In DNA, the sugar is deoxyribose, which lacks an oxygen atom at the 2' carbon compared to ribose in RNA, hence the name "deoxy."
This slight structural difference significantly impacts the stability and function of these molecules.

Phosphate Ester Linkage

The phosphate ester linkage is a key component in the backbone of nucleic acid structures, connecting the nucleotides together. This linkage forms the scaffold that supports the genetic material.
In both DNA and RNA, each nucleotide is composed of:

• A phosphate group
• A sugar molecule (ribose in RNA and deoxyribose in DNA)
• A nitrogenous base

The phosphate group is attached to the 5' carbon of the sugar, creating a bond known as a phosphate ester linkage.
This linkage connects to the 3' carbon of the next nucleotide's sugar, forming a sugar-phosphate backbone.
The consistency of this connection along the chain is crucial for the integrity and function of both RNA and DNA.
It facilitates the formation of long polymer strands necessary for genetic encoding and information transmission.

Carbon Positioning in Sugars

Understanding carbon positioning in sugars is essential for grasping how nucleic acids are structured. In both deoxyribose and ribose, carbon atoms in the sugar ring are numbered from 1 to 5.

• C1 Position: This carbon is where the nitrogenous base attaches. It is pivotal in forming the structure known as the nucleoside.
• C5 Position: The phosphate group attaches here, facilitating the formation of the phosphate ester linkage with another nucleotide.

The configuration of the sugar ring establishes how nucleotides are linked in a linear sequence.
The proper numbering allows accurate formation of the sugar-phosphate backbone, ensuring that genetic information is passed along without structural errors.
Misplacing these connections can lead to malformed proteins and dysfunctional genetic processes.