Last Updated: May 2026
Molecular Basis of Inheritance (NCERT Class 12 Biology, Chapter 6 in the rationalised syllabus) is one of the highest-yield topics for NEET 2027 — typically 4–6 questions every year. The chapter covers DNA structure, replication, transcription, translation, the genetic code, regulation of gene expression, and the Human Genome Project. NEET examiners reward students who can recall not just the molecular machinery but the experimental landmarks that established it.
Quick Reference Table — DNA Replication Machinery
| Enzyme/Protein | Function |
|---|---|
| Helicase | Unwinds DNA double helix |
| Topoisomerase (DNA gyrase) | Relieves super-coiling ahead of fork |
| Single-strand binding proteins (SSB) | Stabilise unwound single strands |
| Primase | Synthesises short RNA primers |
| DNA polymerase III | Main replicative polymerase (5′→3′) |
| DNA polymerase I | Removes RNA primers, fills gaps |
| DNA ligase | Joins Okazaki fragments |
The Six Foundational Experiments
- Frederick Griffith (1928) — transforming principle in Streptococcus pneumoniae
- Avery, MacLeod, McCarty (1944) — DNA, not protein, is the transforming material
- Hershey-Chase (1952) — radioactive labelling proves DNA is genetic material in T2 phage
- Watson and Crick (1953) — DNA double helix structure (used Rosalind Franklin’s X-ray data)
- Meselson-Stahl (1958) — semi-conservative replication, using ¹⁵N/¹⁴N density gradient
- Nirenberg, Khorana, Holley — cracking the genetic code; 1968 Nobel Prize
DNA Structure — Watson-Crick Model
- Two anti-parallel polynucleotide strands, right-handed double helix
- Diameter 2 nm; one full turn = 3.4 nm = 10 base pairs
- Sugar = deoxyribose; bases = A, T, G, C
- A pairs with T (2 H-bonds); G pairs with C (3 H-bonds)
- Chargaff’s rule: A = T, G = C
Replication — Semi-Conservative and Discontinuous on Lagging Strand
Replication is semi-conservative (one parental + one new strand per daughter). DNA polymerase only works 5′→3′, so the leading strand is continuous and the lagging strand is synthesised as Okazaki fragments joined later by DNA ligase. The origin of replication contains specific sequences recognised by initiator proteins.
Transcription — Three Steps
- Initiation — RNA polymerase binds promoter (with sigma factor in prokaryotes, transcription factors in eukaryotes)
- Elongation — RNA polymerase synthesises mRNA in 5′→3′ direction using the template strand 3′→5′
- Termination — rho-dependent or rho-independent in prokaryotes; complex signalling in eukaryotes
Eukaryotic mRNA processing: 5′ capping (7-methylguanosine), 3′ poly-A tail, splicing (introns removed via spliceosome). Three RNA polymerases — I (rRNA), II (mRNA), III (tRNA + 5S rRNA).
The Genetic Code — Five Properties
- Triplet — 64 codons code 20 amino acids + 1 start (AUG) + 3 stop (UAA, UAG, UGA)
- Degenerate — most amino acids coded by more than one codon (except Met, Trp)
- Universal — same code in nearly all organisms (with minor mitochondrial exceptions)
- Non-overlapping — adjacent codons do not share bases
- Comma-less — read sequentially without gaps
Translation and the Lac Operon
Translation occurs on ribosomes; tRNA carries amino acids; codon-anticodon pairing achieves accuracy. The lac operon (Jacob & Monod, 1961 Nobel) is the classical example of negative inducible regulation in E. coli: lactose binds repressor → repressor releases operator → RNA polymerase transcribes the lac genes.
Human Genome Project — Key Facts
- Started 1990, draft 2001, completed 2003
- ~3 billion base pairs, ~20,000–25,000 genes
- Less than 2% codes for proteins; rest is regulatory/repetitive
- Chromosome 1 has the most genes (~2968), Y chromosome the least
- SNPs: ~1.4 million identified — basis of personalised medicine
40 Practice MCQs — NEET Molecular Basis of Inheritance
[cg_quiz id=”cg-neet-mol-inh-2027″]
Frequently Asked Questions
Why is DNA replication called semi-conservative?
Each daughter DNA molecule contains one parental strand and one newly synthesised strand. Meselson and Stahl (1958) proved this using density-gradient centrifugation with ¹⁵N-labelled DNA.
Why are Okazaki fragments needed?
DNA polymerase only synthesises 5′→3′. On the lagging strand (running 5′→3′ away from the fork), synthesis must occur in short fragments that are later joined by DNA ligase.
What is the role of sigma factor in transcription?
In prokaryotes, sigma factor binds RNA polymerase to recognise the promoter sequence. After initiation, sigma dissociates and the core enzyme proceeds with elongation.
What is the wobble hypothesis?
Crick’s hypothesis explaining degeneracy: the third base of a codon can pair non-strictly with the corresponding tRNA anticodon position, allowing one tRNA to read multiple codons that share the first two bases.
Continue Your NEET 2027 Prep
- Genetics and Inheritance — Mendel’s Laws
- NEET Biology Evolution 2027
- NEET Gurukul Daily Practice Engine
- NEET Free Mock Test
Bottom line: Lock down the six foundational experiments, the seven replication enzymes, the three transcription steps, the five genetic-code properties, and the lac operon. These five clusters cover virtually every NEET question on this chapter.