Problem 57 - Objective Biology for NEET

Question

Identify the nucleoside from the following: A. Adenosine B. Uridylic acid C. Uridine D. Cytidylic acid (a) A and B only (b) A and C only (c) C and D only (d) \(\mathrm{B}\) and \(\mathrm{C}\) only

Step-by-Step Solution

Answer

The correct answer is (b) A and C only. These are the options that represent a nucleoside, which is formed of a nitrogenous base and a sugar only with no phosphate group.

Step 1: Identify the Components of a Nucleoside

Recognize that a nucleoside is made up of a sugar and a nitrogenous base. It does not include any phosphate group.

Step 2: Evaluate Option A

Adenosine is a combination of adenine (an Nitrogenous base) and ribose (a sugar). Therefore, Adenosine is a nucleoside.

Step 3: Evaluate Option B

Uridylic acid, also known as UMP (uridine monophosphate), contains a phosphate group in addition to uracil (a nitrogenous base) and ribose (a sugar). Since it has a phosphate group, it is not a nucleoside but a nucleotide.

Step 4: Evaluate Option C

Uridine is made up of uracil (a nitrogenous base) and ribose (a sugar). It does not have a phosphate group. Therefore, Uridine is a nucleoside.

Step 5: Evaluate Option D

Cytidylic acid (CMP, cytidine monophosphate) contains a phosphate group in addition to cytosine (a nitrogenous base) and ribose (a sugar). Since it has a phosphate group, it is not a nucleoside but a nucleotide.

Key Concepts

Nucleotide Structure

Understanding nucleotide structure is important for grasping how genetic material is built. A nucleotide is composed of three main components: a nitrogenous base, a ribose sugar, and a phosphate group. This combination is essential for forming the building blocks of DNA and RNA. When multiple nucleotides join together, they create long chains, which are your DNA or RNA strands. These chains are crucial for storing and transmitting genetic information. Each component plays a vital role, with the nitrogenous base providing genetic data, the sugar serving as a backbone support, and the phosphate group linking the nucleotides together, ensuring stability of the chain. The phosphodiester bond, which connects the phosphate group to the sugar, is essential for the structure's integrity.

Nitrogenous Bases

Nitrogenous bases are organic molecules with nitrogen atoms that have the ability to form hydrogen bonds. They are a key component of nucleotides. In DNA and RNA, there are two categories of nitrogenous bases: purines and pyrimidines. Purines include adenine (A) and guanine (G), while pyrimidines contain thymine (T), cytosine (C), and uracil (U).

• DNA contains adenine, guanine, cytosine, and thymine.
• RNA contains adenine, guanine, cytosine, and uracil.

The pairing of these bases, such as adenine with thymine and guanine with cytosine in DNA, is crucial for the structure and function of the genetic material. In RNA, uracil replaces thymine and pairs with adenine. These bases store genetic information and play an important role in coding for proteins.

Ribose Sugar

Ribose sugar is a five-carbon sugar that is an essential component of RNA and various nucleotides. In their natural form, ribose sugars are in a ring structure. In RNA, the ribose sugar has a hydroxyl group (-OH) attached to the 2' carbon. This differentiates it from the deoxyribose sugar found in DNA, which lacks an oxygen atom at the same position.

• Ribose sugar's presence in RNA contributes to its more reactive nature compared to DNA.
• Its structure facilitates the diverse functions RNA molecules perform, such as coding, decoding, and regulatory activities within cells.

The ribose sugar, by linking with nitrogenous bases and phosphate groups, forms the backbone of RNA, supporting its structure and function in cellular processes.

Phosphate Group

The phosphate group is a crucial part of nucleotides, contributing to the DNA and RNA's structural stability. Each nucleotide features a single phosphate group which is bonded to the 5' carbon of the sugar molecule. This results in the formation of a phosphodiester bond with the 3' carbon of the neighboring nucleotide. Such bonding forms a sugar-phosphate backbone, a key structural feature in both DNA and RNA.

• The phosphate group carries a negative charge, contributing to the nucleic acids' overall negative charge.
• This charge is vital for interactions with proteins, especially during processes such as transcription and replication.

Without a phosphate group, the molecule forms a nucleoside, not a nucleotide. The presence of the phosphate group thus defines nucleotides, reflecting their role in genetic material.

Nucleotides vs Nucleosides

Nucleotides and nucleosides are both integral components of genetic material substances, but they differ fundamentally. A nucleoside is composed of just two components: a nitrogenous base and a ribose sugar. It lacks a phosphate group, which differentiates it from a nucleotide.

• Nucleotides consist of a nitrogenous base, a ribose sugar, and one or more phosphate groups.
• Nucleotides, with their phosphate group, form the basic structural unit of nucleic acids like DNA and RNA.
• Nucleosides, lacking a phosphate group, represent the free form of the bases and are not involved in forming the nucleic acids directly.

Understanding this distinction is crucial as it influences the function and formation of nucleic acids, affecting genetic coding and information transmission within biological systems.