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

The fundamental difference lies in the presence of a phosphate group. A nucleoside consists only of a nitrogenous base (purine or pyrimidine) covalently linked to a pentose sugar (ribose or deoxyribose). For example, adenosine is a nucleoside. A nucleotide, on the other hand, is a nucleoside with one or more phosphate groups attached to the sugar. So, a nucleotide is essentially a base + sugar + phosphate(s). Adenosine monophosphate (AMP) is an example of a nucleotide. Nucleotides are the monomeric units of nucleic acids.

Why is DNA more stable than RNA?

DNA's greater stability compared to RNA stems from several structural features. Firstly, DNA contains deoxyribose sugar, which lacks a hydroxyl group at the 2' carbon. This 2'-OH group in ribose (found in RNA) makes RNA more susceptible to alkaline hydrolysis. Secondly, DNA is typically double-stranded, allowing for extensive hydrogen bonding between complementary base pairs and hydrophobic stacking interactions between bases, which collectively provide significant stability. RNA is usually single-stranded and, while it can form complex secondary structures, it lacks the consistent double-helical stability of DNA.

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

Chargaff's rules, formulated by Erwin Chargaff, state that in double-stranded DNA, 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 discovery of the DNA double helix structure by Watson and Crick, as they provided the chemical basis for complementary base pairing (A with T, G with C) via hydrogen bonds.

What is the significance of the phosphodiester bond in nucleic acids?

The phosphodiester bond is the covalent linkage that forms the backbone of nucleic acid polymers. It connects the 3'-hydroxyl group of one nucleotide's sugar to the 5'-phosphate group of the next nucleotide. This strong covalent bond is responsible for the structural integrity and stability of the polynucleotide chain. The continuous chain of alternating sugar and phosphate groups forms the 'sugar-phosphate backbone', from which the nitrogenous bases project, carrying the genetic information.

Can RNA also carry genetic information?

Yes, while DNA is the primary genetic material in most cellular organisms, RNA serves as the genetic material in many viruses, such as retroviruses (e.g., HIV) and influenza viruses. In these cases, the viral RNA carries the instructions for viral replication and protein synthesis. Additionally, in cellular organisms, various types of RNA (mRNA, tRNA, rRNA) are essential for the expression of genetic information stored in DNA, acting as intermediaries or functional molecules in protein synthesis.

Nucleic Acids

Chemistry

NEET UG

Version 1Updated 22 Mar 2026

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Nucleic acids are biopolymers, essential to all known forms of life, primarily responsible for the storage and expression of genetic information. They are composed of monomeric units called nucleotides, which consist of a nitrogenous base, a pentose sugar (ribose or deoxyribose), and one to three phosphate groups. The two main types are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA s…

Quick Summary

Nucleic acids are fundamental biomolecules, primarily DNA and RNA, responsible for storing and expressing genetic information. They are polymers made of repeating monomeric units called nucleotides. Each nucleotide consists of three components: a nitrogenous base (Adenine, Guanine, Cytosine, Thymine in DNA; Uracil replaces Thymine in RNA), a pentose sugar (deoxyribose in DNA, ribose in RNA), and a phosphate group.

Nucleotides link together via phosphodiester bonds, forming a sugar-phosphate backbone. DNA typically forms a double helix, with two antiparallel strands held together by hydrogen bonds between complementary base pairs (A-T, G-C).

RNA is usually single-stranded and plays diverse roles in gene expression, including carrying genetic messages (mRNA), forming ribosomes (rRNA), and transferring amino acids (tRNA). Chargaff's rules describe the base composition of double-stranded DNA, stating that A=T and G=C.

Understanding these basic structural and compositional differences is key for NEET.

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Key Concepts

Structure of a Nucleotide

A nucleotide is the fundamental building block of nucleic acids. It's a composite molecule with three…

Phosphodiester Bond Formation

The formation of a polynucleotide chain involves the polymerization of nucleotides through phosphodiester…

Complementary Base Pairing and Hydrogen Bonds

In double-stranded DNA, the two polynucleotide strands are held together by specific complementary base…

Nucleotide: — Base + Sugar + Phosphate

Nucleoside: — Base + Sugar

DNA Sugar: — Deoxyribose (no 2'-OH)

RNA Sugar: — Ribose (with 2'-OH)

DNA Bases: — A, G, C, T

RNA Bases: — A, G, C, U

Base-Sugar Bond: — N-glycosidic bond

Sugar-Phosphate Bond: — Ester bond

Nucleotide-Nucleotide Bond: — Phosphodiester bond (3'-5' linkage)

DNA Structure: — Double helix, antiparallel strands

Base Pairing (DNA): —

A=T

(2 H-bonds),

G equiv C

(3 H-bonds)

Chargaff's Rules (dsDNA): —

A=T

G=C

(A+G)=(T+C)

DNA Stability: — Higher (due to deoxyribose, double helix)

RNA Stability: — Lower (due to ribose 2'-OH, usually single-stranded)

Types of RNA: — mRNA, tRNA, rRNA (main functional types)

To remember the components of a nucleotide: Be Sure to Phosphate! (Base, Sugar, Phosphate). For DNA bases: All Tigers Grow Claws (A-T, G-C). For RNA, just remember 'U' replaces 'T'.