Question
Describe the Watson and Crick model of DNA. What are the key structural features of the double helix, and what is the significance of Chargaff’s rules in this model?
Solution — Step by Step
DNA is a double helix — two polynucleotide strands wound around a common axis. Each strand is made of nucleotides (deoxyribose sugar + phosphate + nitrogenous base), connected by phosphodiester bonds between the 3’ carbon of one sugar and the 5’ carbon of the next.
The two strands run in opposite directions — one goes 5’→3’ and the other goes 3’→5’. This antiparallel arrangement is non-negotiable for proper base pairing. Think of it like two parallel roads with traffic moving in opposite directions — the geometry only works this way.
The nitrogenous bases face inward and pair specifically:
- Adenine (A) pairs with Thymine (T) — 2 hydrogen bonds
- Guanine (G) pairs with Cytosine (C) — 3 hydrogen bonds
This is why Chargaff observed that in any DNA sample, [A] = [T] and [G] = [C]. Watson and Crick’s model explained why this ratio holds: it’s enforced by the complementary base pairing geometry.
The helix has a uniform diameter of 2 nm. Each complete turn (one full rotation) spans 3.4 nm and contains exactly 10 base pairs. This means adjacent bases are 0.34 nm apart. These numbers appear directly in NEET and board questions — memorise them.
- Diameter of DNA = 2 nm
- Rise per base pair = 0.34 nm
- Pitch (one full turn) = 3.4 nm
- Base pairs per turn = 10
Because the two strands spiral together unevenly, the helix has two types of grooves — a wider major groove and a narrower minor groove. Transcription factors and proteins that regulate gene expression bind to the major groove because it exposes more base-pair information. This is tested in NEET in the context of gene regulation.
Why This Works
The double helix is a masterpiece of chemical logic. The sugar-phosphate backbone stays on the outside, keeping the hydrophilic part in contact with the aqueous cellular environment. The bases are tucked inside — hydrophobic and protected from random chemical reactions.
The G-C pair has 3 hydrogen bonds versus A-T’s 2. This means DNA strands rich in G-C content are harder to separate (higher melting temperature). This fact appears in NEET as a reasoning question: “DNA from organisms in hot springs has higher G-C content” — now you know why.
The antiparallel complementary structure is also the entire basis of DNA replication. Each strand acts as a template, and because A always pairs with T and G always pairs with C, the sequence is perfectly copied every time.
Alternative Method (For Remembering the Model)
If a question gives you a partial DNA sequence and asks you to complete it, work systematically:
Given strand (5’→3’): T-A-C-G-G-A
The complementary strand runs 3’→5’, so pair each base:
5' — T A C G G A — 3' (original)
3' — A T G C C T — 5' (complementary)To write the complementary strand in the conventional 5’→3’ direction: T-C-C-G-T-A. This comes up in board practicals and NEET 2023.
PYQ shortcut: If a question says “the ratio of purines to pyrimidines in a double-stranded DNA is:”, the answer is always 1:1 — because every purine (A or G) pairs with exactly one pyrimidine (T or C). This is directly from Chargaff’s rules.
Common Mistake
Students often write that A-T has 3 hydrogen bonds and G-C has 2 — it’s the other way around. G-C has 3 bonds (that’s why it’s stronger and needs more energy to separate). A common trick question in NEET shows a “melting curve” and asks which segment is G-C rich — it’s the one that melts at a higher temperature. Getting the bond numbers swapped will cost you here.