Problem 173 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following bases is not present in DNA ? (a) Cytosine (b) Thymine (c) Quinoline (d) Adenine

Step-by-Step Solution

Answer

Quinoline is not present in DNA.

Step 1: Identify DNA Bases

DNA is composed of four nucleotide bases: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G). These are the fundamental building blocks of DNA strands.

Step 2: Examine Given Options

Review the list of options provided in the question: (a) Cytosine, (b) Thymine, (c) Quinoline, (d) Adenine. Compare each option with the known DNA bases.

Step 3: Eliminate Known DNA Bases

Cytosine, Thymine, and Adenine are all bases known to be part of DNA. Thus, options (a), (b), and (d) can be eliminated as they are present in DNA.

Step 4: Identify the Odd One Out

The remaining option, (c) Quinoline, is not a DNA base. Quinoline is not one of the four nucleotide bases of DNA.

Key Concepts

Nucleotide

Nucleotides are the basic building blocks of DNA and RNA molecules. Each nucleotide is made up of three important components: a nitrogenous base, a five-carbon sugar molecule, and one or more phosphate groups.
In DNA, the sugar is deoxyribose, and the series of nucleotides forms the DNA strand through strong covalent bonds.
These covalent bonds link the phosphate group of one nucleotide to the sugar of the next.

The nitrogenous bases are crucial, as they allow the DNA to store genetic information.

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

These bases pair in a specific way, with Adenine always pairing with Thymine, and Cytosine always pairing with Guanine. This pairing holds the two strands of DNA together in its double helix shape.

Adenine

Adenine is one of the four nitrogenous bases found in DNA. Structurally, it is categorized as a purine, which is a type of molecule characterized by a two-ring structure.
Adenine's structure enables it to form hydrogen bonds with Thymine – its complementary base, creating a base pair that is essential for the stability of DNA.

In the context of the genetic code, Adenine plays a vital role in coding for various amino acids through processes like transcription and translation.

• Forms a base pair with Thymine (A-T)
• Involved in key genetic processes

Adenine is also a part of molecules like ATP (adenosine triphosphate), crucial for cellular energy transfer.

Thymine

Thymine is another essential base in DNA, complementing Adenine to form stable base pairs. It belongs to the class of molecules known as pyrimidines, which are structured with a single ring.
This single-ring structure allows for the distinct pairing with Adenine by forming two hydrogen bonds.

One of Thymine's unique roles is in providing the genetic instructions carried by the DNA in cellular activities.

• Always pairs with Adenine (T-A)
• Contributes to DNA's double helix structure

Thymine is distinct to DNA and is replaced by Uracil in RNA, which is a key difference between these two nucleic acids.

Cytosine

Cytosine, like Thymine, is a pyrimidine base, characterized again by that single-ring structure.
In DNA, Cytosine pairs specifically with Guanine, forming three hydrogen bonds that contribute to the DNA’s structural integrity.

This base pairing is key to the genetic coding mechanism, allowing Cytosine to play a pivotal role in the transcription process where genetic information is copied from DNA to RNA.

• Pairs with Guanine (C-G)
• Integral to DNA’s information storage and stability

Cytosine’s integrity is vital, and errors or modifications in these base-pair matchings can lead to mutations, sometimes causing significant biological consequences.