Which of the following is/are the bases of DNA?

(A) Adenine
(B) Uracil
(C) Thymine
(D) Cytosine

Choose the correct answer from the options given below:

(A), (C) and (D) only

The correct answer is Option (3) → (A), (C) and (D) only.

Let us dive into the details of the nitrogenous bases in DNA, and why certain bases are present in DNA while others are not.

Structure of DNA

DNA (Deoxyribonucleic acid) is made up of two long chains (or strands) of nucleotides twisted into a double helix. Each nucleotide consists of:

A deoxyribose sugar molecule.

A phosphate group.

A nitrogenous base.

These nitrogenous bases form the genetic code that determines the characteristics of an organism. In DNA, the nitrogenous bases are of two types: purines and pyrimidines.

Nitrogenous Bases of DNA

Adenine (A):

Type: Purine (double-ring structure).

In DNA, adenine pairs with thymine (T) through two hydrogen bonds. This pairing is part of the base-pairing rule that ensures the stability of the DNA double helix. Present in both DNA and RNA.

Thymine (T):

Type: Pyrimidine (single-ring structure).

Thymine pairs with adenine in DNA, forming two hydrogen bonds.

Thymine is specific to DNA and is replaced by uracil (U) in RNA. This is a key difference between the nucleic acids DNA and RNA.

Cytosine (C):

Type: Pyrimidine (single-ring structure).

In DNA, cytosine pairs with guanine (G) through three hydrogen bonds. This bond is stronger due to the three hydrogen bonds, contributing to the stability of the DNA double helix. Present in both DNA and RNA.

Guanine (G):

Type: Purine (double-ring structure).

Guanine pairs with cytosine in DNA, forming three hydrogen bonds.

Though guanine is not listed in the question, it is a fundamental base of both DNA and RNA

Why Uracil is Not in DNA:

Uracil (U):

Type: Pyrimidine (single-ring structure).

Uracil is found only in RNA, where it pairs with adenine. In RNA, thymine is replaced by uracil.

Uracil is not used in DNA because thymine (which is methylated uracil) is more chemically stable, providing better protection against mutations. DNA, being the permanent storage of genetic information, requires more stability, which thymine offers.

Base-Pairing Rules in DNA

DNA bases pair specifically due to hydrogen bonding:

Adenine (A) always pairs with Thymine (T).

Cytosine (C) always pairs with Guanine (G).

This pairing mechanism ensures that the DNA strands are complementary, making it possible for DNA to replicate accurately.

Differences Between DNA and RNA:

DNA contains Adenine (A), Thymine (T), Cytosine (C), and Guanine (G).

RNA contains Adenine (A), Uracil (U), Cytosine (C), and Guanine (G) (thymine is replaced by uracil in RNA).

Conclusion

In DNA, the bases are Adenine (A), Thymine (T), Cytosine (C), and Guanine (G). Since guanine is not in the options, the correct answer is (A) Adenine, (C) Thymine, and (D) Cytosine, which are all present in DNA.

Thus, the correct option is: (A), (C), and (D) only