Nucleic acid structure — DNA double helix, base pairing, Chargaff's rules

Question

What is the structure of DNA? How does complementary base pairing work, and what do Chargaff’s rules tell us about nucleotide composition?

(NEET, CBSE 12 — DNA structure and Chargaff’s rules are tested in both chemistry biomolecules and biology molecular basis of inheritance)

Solution — Step by Step

Each nucleotide consists of three parts:

Nitrogenous base — purines (Adenine, Guanine) or pyrimidines (Cytosine, Thymine in DNA / Uracil in RNA)
Pentose sugar — deoxyribose (DNA) or ribose (RNA)
Phosphate group

Nucleotides connect via phosphodiester bonds between the 3’-OH of one sugar and the 5’-phosphate of the next, forming the sugar-phosphate backbone. The bases project inward like steps of a ladder.

The two strands of DNA are held together by hydrogen bonds between complementary bases:

Adenine (A) pairs with Thymine (T) — 2 hydrogen bonds
Guanine (G) pairs with Cytosine (C) — 3 hydrogen bonds

This pairing is highly specific because of the geometry: a purine always pairs with a pyrimidine, maintaining a uniform width of the double helix (2 nm).

GC-rich regions are more thermally stable than AT-rich regions because GC has 3 H-bonds vs 2 for AT. This is why the melting temperature ( $T_m$ ) of DNA increases with GC content.

Rule 1: In any DNA molecule:

[A] = [T] \quad \text{and} \quad [G] = [C]

Rule 2: Therefore:

\frac{[A] + [G]}{[T] + [C]} = 1 \quad \text{(purines = pyrimidines)}

Rule 3: The ratio $[A+T]/[G+C]$ is species-specific (varies between organisms but is constant within a species).

If a DNA has 30% adenine, then thymine = 30%, and guanine = cytosine = 20% each (since total = 100%).

These rules were established experimentally by Erwin Chargaff BEFORE Watson and Crick proposed the double helix — they were a crucial clue for the structure.

Key structural features of B-DNA (the most common form):

Right-handed helix with two antiparallel strands (one runs 5’ to 3’, the other 3’ to 5’)
Diameter: 2 nm (20 angstroms)
Pitch (one complete turn): 3.4 nm, containing 10 base pairs per turn
Distance between base pairs: 0.34 nm
Major groove and minor groove — proteins bind in the major groove to read the base sequence without unwinding the DNA

flowchart TD
    A["DNA Nucleotide"] --> B["Base + Sugar + Phosphate"]
    B --> C["Bases: A, T, G, C"]
    C --> D["Base Pairing Rules"]
    D --> E["A = T (2 H-bonds)"]
    D --> F["G ≡ C (3 H-bonds)"]
    E --> G["Chargaff's Rules"]
    F --> G
    G --> H["Purines = Pyrimidines"]
    G --> I["A+T/G+C ratio is species-specific"]
    H --> J["Double Helix Structure"]
    J --> K["Antiparallel, right-handed"]
    J --> L["10 bp per turn, 3.4 nm pitch"]

Why This Works

Complementary base pairing is the foundation of all molecular biology — it explains DNA replication (each strand serves as a template), transcription (RNA is synthesised complementary to the template strand), and the stability of the double helix. Chargaff’s rules are a direct mathematical consequence of base pairing: if every A pairs with T and every G pairs with C, then their amounts must be equal.

Common Mistake

A very common numerical error: if told “A = 20%”, students calculate G as 30%. The correct calculation: A = T = 20%, so A + T = 40%, remaining = 60%, split equally between G and C, giving G = C = 30%. The mistake happens when students forget that Chargaff’s rule applies to the double-stranded DNA as a whole. In single-stranded DNA or RNA, $[A] \neq [T]$ and $[G] \neq [C]$ — Chargaff’s rules DO NOT apply to single strands.

For NEET numericals: if given the percentage of any one base, you can find all four. Just remember $A = T$ and $G = C$ , and the total of all four = 100%. Two equations, done.

Question

Solution — Step by Step

Why This Works

Common Mistake

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