🧬 Structure of DNA – Degree Level Explanation

1. Introduction

DNA (Deoxyribonucleic Acid) is the hereditary material in almost all living organisms. It contains the instructions needed for an organism to develop, survive, and reproduce.

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2. Chemical Composition of DNA

DNA is a polymer made of repeating units called nucleotides. Each nucleotide has three components:

• Nitrogenous Base (Adenine, Thymine, Cytosine, Guanine)
• Deoxyribose Sugar (a 5-carbon pentose sugar)
• Phosphate Group

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3. Nitrogenous Bases

a. Purines (double-ringed):

• Adenine (A)
• Guanine (G)

b. Pyrimidines (single-ringed):

• Cytosine (C)
• Thymine (T)

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4. Base Pairing Rules (Chargaff’s Rule)

• A pairs with T via 2 hydrogen bonds
• G pairs with C via 3 hydrogen bonds

This complementary base pairing enables the double-stranded nature of DNA.

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5. Structure (Watson & Crick Model)

• Double helix: Two antiparallel strands coiled in a right-handed helix.
• Antiparallel: One strand runs 5′ to 3′, the other 3′ to 5′.
• Each turn of the helix spans 10 base pairs and measures 3.4 nm.
• Distance between adjacent base pairs: 0.34 nm.
• Diameter of helix: 2 nm.

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6. Bonding & Backbone

• The sugar-phosphate backbone is held together by phosphodiester bonds.
• The bases point inward and are held together by hydrogen bonds.

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7. Types of DNA (Based on Helix Form)

Type	Helix	Pitch (nm)	Base pairs/turn	Notes
B-DNA	Right-handed	3.4 nm	10	Most common in cells
A-DNA	Right-handed	2.8 nm	11	Dehydrated DNA
Z-DNA	Left-handed	4.5 nm	12	Zig-zag backbone

Structure of DNA

Interactive Double Helix Model