Problem 64 - A Complete Resource Book in Chemistry for JEE Main

Question

The base found in DNA but not in RNA is (a) cytosine (b) guanine (c) thymine (d) adenine

Step-by-Step Solution

Answer

The base found in DNA but not in RNA is (c) thymine.

Step 1: Understand DNA and RNA

DNA (Deoxyribonucleic Acid) and RNA (Ribonucleic Acid) are nucleic acids found in cells. Both contain bases known as nucleotides, but they are composed differently. DNA contains four major bases: adenine (A), thymine (T), cytosine (C), and guanine (G), while RNA contains adenine (A), uracil (U), cytosine (C), and guanine (G).

Step 2: Identify the Unique Base

Thymine is unique to DNA, while uracil replaces thymine in RNA. RNA does not contain thymine; instead, it pairs adenine with uracil.

Step 3: Compare Options

Review the given options: - (a) cytosine: found in both DNA and RNA - (b) guanine: found in both DNA and RNA - (c) thymine: unique to DNA, not found in RNA - (d) adenine: found in both DNA and RNA

Step 4: Select the Correct Answer

Since thymine is the base found in DNA but not in RNA, the correct choice is the option where thymine is listed.

Key Concepts

Nucleotide Bases

Nucleotide bases are the building blocks of DNA and RNA, acting as the letters of the genetic alphabet. These small molecules join together to form long chains which store the genetic information of living organisms.

In DNA, there are four nucleotide bases:

• Adenine (A)
• Cytosine (C)
• Guanine (G)
• Thymine (T)

In RNA, these nucleotide bases are largely the same, except for one key difference. Instead of thymine, RNA contains:

• Uracil (U)

This swap of thymine for uracil is a primary distinction between DNA and RNA, enabling them to perform different roles in the cell.
Adenine pairs with thymine in DNA and with uracil in RNA, demonstrating the flexibility and adaptability of nucleic acids. Understanding the differences in nucleotide bases is crucial for grasping their distinct functions.

DNA Structure

DNA, short for deoxyribonucleic acid, has a unique structure crucial to its role in storing genetic information. The DNA molecule is a double helix, resembling a twisted ladder. The "rungs" of this ladder are made of paired nucleotide bases, specifically adenine pairing with thymine, and cytosine with guanine.

The "sides" of the ladder are composed of alternating sugar (deoxyribose) and phosphate groups, contributing to its sturdy backbone. This arrangement allows DNA to hold extensive genetic data in a compact form that can be reliably replicated and passed on during cell division.

The double helix structure is stabilized by hydrogen bonds between the nucleotide bases, making the DNA molecule both strong and flexible. This stability is essential for the accuracy of genetic information transfer, as any errors during replication can lead to mutations.

RNA Structure

RNA, or ribonucleic acid, features a structure that contrasts with DNA in several ways, reflecting its distinct functions. Unlike DNA, RNA is single-stranded, which allows it to fold into complex shapes necessary for its role in gene expression and regulation.

This single strand of RNA is composed of the same sugar-phosphate backbone seen in DNA, but with ribose sugar instead of deoxyribose. Its nucleotide bases carry the genetic code that is used to synthesize proteins. Instead of thymine, RNA contains uracil, which pairs with adenine during protein synthesis. This swap entails a small but significant molecular change that affects how RNA interacts with other molecules.

Various types of RNA, such as messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA), each have unique structures and roles within the cell. This versatility allows RNA to carry out diverse tasks from conveying genetic instructions to catalyzing biochemical reactions. Understanding RNA's structure is key to grasping its critical role in the expression of genes.