Problem 65 - Objective Biology for NEET

Question

Which of the following are purines? (a) Adenine (b) Guanine (c) Cytosine (d) Both (a) and (b)

Step-by-Step Solution

Answer

The correct answer is (d) Both (a) and (b).

Step 1: Identify the purines

Purines are a type of nitrogenous base found in molecules of DNA and RNA. The basic units of purines are Adenine and Guanine.

Step 2: Analyze the options

(a) Adenine is a purine. (b) Guanine is also a purine. (c) Cytosine is a pyrimidine, not a purine. (d) Both (a) and (b) are indeed purines.

Step 3: Select the correct option

After analyzing, we can see that the answer is (d) Both (a) and (b) because both Adenine and Guanine are purines.

Key Concepts

Nitrogenous Bases

Nitrogenous bases are essential components in the structure of nucleic acids, which are the building blocks of life. They are organic molecules that contain nitrogen. There are two main types of nitrogenous bases: purines and pyrimidines.
Purines, like adenine and guanine, have a larger double-ring structure. On the other hand, pyrimidines like cytosine, thymine (in DNA), and uracil (in RNA) have a single-ring structure. These bases pair through hydrogen bonds to form the rungs of the DNA double helix and RNA structures.

• Purines: Adenine (A) and Guanine (G).
• Pyrimidines: Cytosine (C), Thymine (T), and Uracil (U).

The pairing between these bases is specific: Adenine pairs with Thymine in DNA and with Uracil in RNA, while Guanine pairs with Cytosine in both DNA and RNA.

DNA

Deoxyribonucleic acid, or DNA, is the molecule that carries the genetic instructions for life. It is made up of long chains of nucleotides, each consisting of a phosphate group, a sugar molecule (deoxyribose), and a nitrogenous base.
DNA is structured as a double helix, with two strands running in opposite directions. Nitrogenous bases form the core of the double helix, pairing between the strands to hold them together. Specifically, adenine pairs with thymine, and guanine pairs with cytosine, forming base pairs that are crucial for DNA's stability and functionality.
Here are some key features of DNA:

• Double-helix structure.
• Complementary base pairing (A-T, G-C).
• Encodes genetic information for the development and functioning of living organisms.

RNA

Ribonucleic acid, or RNA, plays a vital role in gene expression and the synthesis of proteins. RNA is single-stranded, unlike DNA's double-helix structure. It is composed of nucleotides similar to DNA but with a few differences, particularly in the sugar component and one of the nitrogenous bases.
In RNA, the sugar molecule is ribose instead of deoxyribose, and the nitrogenous base uracil replaces thymine, pairing with adenine. RNA serves multiple functions, including acting as a messenger between DNA and the ribosomes (mRNA), forming part of the ribosome structure (rRNA), and aiding in the protein synthesis (tRNA).
Important aspects of RNA include:

• Single-stranded structure.
• Contains ribose sugar and uracil instead of thymine.
• Involved in protein synthesis and gene regulation.