Molecular Basis of Inheritance

Syllabus Unit: Genetics and Evolution

Why it matters

Chapter overview

If Mendel's laws describe the pattern of inheritance, molecular biology explains the machinery underneath it: how DNA stores information, copies itself with near-perfect fidelity, and is read out through transcription and translation to build every protein in the body. This topic follows that exact information flow — DNA structure and replication, the genetic code and transcription, translation, and finally how gene expression is switched on and off — the molecular core of the Central Dogma that NEET tests in fine mechanistic detail.

Biology Syllabus Genetics and Evolution 18%–20% weightage

Live Animation1 / 4

1UnwindingHelicase opens the helix

2PrimingPrimase adds RNA primer

3SynthesisDNA polymerase extends 5'→3'

4SealingLigase joins Okazaki fragments

1DNA Structure & Replication

DNA's double-helix structure directly explains how it stores information and how it copies itself with near-perfect accuracy.

Learning Objectives

Describe the Watson-Crick double helix model: antiparallel strands, base pairing, sugar-phosphate backbone.
State Chargaff's rule and explain complementary base pairing (A=T, G≡C).
Explain semi-conservative replication and the evidence from the Meselson-Stahl experiment.
Name the key enzymes of replication: helicase, DNA polymerase, primase, ligase.

Key Terms

Double HelixWatson and Crick's 1953 model: two antiparallel DNA strands wound around each other, held together by hydrogen bonds between complementary bases.

Complementary Base PairingAdenine pairs with Thymine via 2 hydrogen bonds; Guanine pairs with Cytosine via 3 hydrogen bonds.

Semi-conservative ReplicationEach new DNA molecule retains one original (parental) strand and one newly synthesised strand.

DNA PolymeraseThe enzyme that synthesises a new DNA strand by adding nucleotides complementary to the template, only in the 5' to 3' direction.

James Watson and Francis Crick proposed the **double helix** structure of DNA in 1953, building on Rosalind Franklin's X-ray diffraction data and Erwin Chargaff's base-composition rule (A = T, G = C in any organism's DNA).

The structure

Two polynucleotide strands wind around a common axis, running antiparallel (one 5'→3', the other 3'→5').
The sugar-phosphate backbone runs on the outside; nitrogenous bases project inward and pair across the two strands.
Adenine pairs only with Thymine (2 H-bonds); Guanine pairs only with Cytosine (3 H-bonds) — this is why G-C rich DNA has a higher melting temperature.
One full turn of the helix spans about 3.4 nm and contains 10 base pairs.

Semi-conservative replication

DNA replicates by using each strand as a template for a new complementary strand, so each daughter molecule has one old and one new strand — this is **semi-conservative** replication, proven by the Meselson-Stahl experiment using 15N/14N density-labelled E. coli DNA.

Helicase unwinds the double helix at the replication fork, breaking hydrogen bonds.
Primase lays down a short RNA primer to give DNA polymerase a starting point.
DNA polymerase III adds nucleotides only in the 5'→3' direction, so the leading strand is made continuously and the lagging strand in Okazaki fragments.
DNA ligase seals the nicks between Okazaki fragments into a continuous strand.

Common MistakeBecause DNA polymerase only works 5'→3', replication of the lagging strand is necessarily discontinuous (Okazaki fragments) — a favourite NEET conceptual question.