What is the primary difference between a nucleoside and a nucleotide?

The distinction lies in the presence of a phosphate group. A **nucleoside** is formed when a nitrogenous base (purine or pyrimidine) is covalently linked to a pentose sugar (ribose or deoxyribose) via an N-glycosidic bond. It lacks a phosphate group. A **nucleotide**, on the other hand, is a nucleoside with one or more phosphate groups attached to the sugar molecule, typically at the 5' carbon. Nucleotides are the monomeric units of nucleic acids, while nucleosides are precursors to nucleotides.

Why is DNA more stable than RNA?

DNA's greater stability stems from two main structural features. Firstly, the deoxyribose sugar in DNA lacks a hydroxyl group at the 2' carbon, making it less reactive and less susceptible to hydrolysis compared to the ribose sugar in RNA, which has a 2'-OH group. Secondly, DNA typically exists as a double helix, where two complementary strands are stabilized by numerous hydrogen bonds between base pairs, providing structural rigidity and protection against enzymatic degradation. RNA, being mostly single-stranded, is more prone to degradation.

What are Chargaff's rules and why are they important?

Chargaff's rules, formulated by Erwin Chargaff, state that in a double-stranded DNA molecule, the amount of adenine (A) is always equal to the amount of thymine (T), and the amount of guanine (G) is always equal to the amount of cytosine (C). Consequently, the total amount of purines (A+G) equals the total amount of pyrimidines (T+C). These rules were crucial in the elucidation of the DNA double helix structure by Watson and Crick, as they indicated the specific complementary base pairing (A with T, G with C) that holds the two DNA strands together.

What are the different types of RNA and their primary functions?

There are three main types of RNA involved in protein synthesis: **Messenger RNA (mRNA)** carries the genetic code from DNA in the nucleus to the ribosomes in the cytoplasm, serving as a template for protein synthesis. **Transfer RNA (tRNA)** acts as an adaptor molecule, bringing specific amino acids to the ribosome and matching them to the codons on mRNA. **Ribosomal RNA (rRNA)** is a major structural and catalytic component of ribosomes, where the actual protein synthesis takes place. Other types like snRNA, miRNA, and siRNA are involved in gene regulation and RNA processing.

How are nucleotides linked together to form a polynucleotide chain?

Nucleotides are linked together by **phosphodiester bonds**. This is a strong covalent bond formed between the 5'-phosphate group of one nucleotide and the 3'-hydroxyl group of the sugar of the adjacent nucleotide. During this process, a molecule of water is removed (dehydration synthesis). This repetitive linkage creates a sugar-phosphate backbone, which forms the structural framework of the nucleic acid strand, with the nitrogenous bases projecting inwards (in DNA) or outwards (in single-stranded RNA).

Nucleic Acids

Biology

NEET UG

Version 1Updated 21 Mar 2026

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Nucleic acids are biopolymers, essential macromolecules, and one of the four major classes of organic compounds found in all known forms of life. They are composed of monomeric units called nucleotides, which are linked together by phosphodiester bonds to form long polynucleotide chains. The two primary types of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA primaril…

Quick Summary

Nucleic acids are fundamental biological macromolecules responsible for storing, transmitting, and expressing genetic information. They are polymers made of repeating monomer units called nucleotides.

Each nucleotide comprises a pentose sugar (deoxyribose in DNA, ribose in RNA), a nitrogenous base (Adenine, Guanine, Cytosine, Thymine in DNA; Adenine, Guanine, Cytosine, Uracil in RNA), and a phosphate group.

Nucleotides link via phosphodiester bonds to form polynucleotide chains. DNA (Deoxyribonucleic Acid) is typically a double helix, with two antiparallel strands held by complementary base pairing (A-T, G-C) through hydrogen bonds, serving as the genetic blueprint.

RNA (Ribonucleic Acid) is usually single-stranded and comes in various forms (mRNA, tRNA, rRNA), playing crucial roles in gene expression and protein synthesis. The central dogma describes the flow of genetic information from DNA to RNA to protein.

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Key Concepts

Nucleoside vs. Nucleotide Formation

Understanding the sequential assembly is crucial. A **nucleoside** is formed first when a nitrogenous base…

Phosphodiester Bond and Polarity

The phosphodiester bond is the backbone of nucleic acids. It's a covalent link between the 3'-hydroxyl group…

Complementary Base Pairing and Hydrogen Bonds

The stability and function of DNA rely heavily on specific base pairing. Adenine (A) always pairs with…

Monomer — Nucleotide

Nucleotide = Sugar + Base + Phosphate

Nucleoside = Sugar + Base

DNA Sugar — Deoxyribose (no -OH at 2')

RNA Sugar — Ribose (with -OH at 2')

DNA Bases — A, T, G, C

RNA Bases — A, U, G, C (Uracil replaces Thymine)

Base Pairing (DNA) — A=T (2 H-bonds), G≡C (3 H-bonds)

Chargaff's Rules — In dsDNA, A=T, G=C; A+G = T+C

Bonds — N-glycosidic (Base-Sugar), Phosphodiester (Sugar-Phosphate backbone), Hydrogen (between base pairs)

DNA Structure — Double helix, antiparallel strands (5' to 3', 3' to 5')

RNA Types — mRNA (messenger), tRNA (transfer), rRNA (ribosomal)

To remember the purines and pyrimidines:

'Pure As Gold': Purines are Adenine and Guanine.

'CUT the Py': Cytosine, Uracil, Thymine are Pyrimidines.