Molecular Basis of Inheritance

NCERT Line-by-Line Breakdown for NEET 2026

Unit 2: Genetics and Evolution

In the previous chapter, we learned that the “factor” regulating inheritance is the gene. Here, we explore the nature of this genetic material—DNA (Deoxyribonucleic Acid). We will decode how DNA replicates, how it codes for proteins (The Central Dogma), and the revolutionary Human Genome Project.

1. The DNA Structure

DNA is a long polymer of deoxyribonucleotides. The length is defined by the number of nucleotides (or base pairs).

A. Structure of Polynucleotide Chain

Nitrogenous Base: Purines (Adenine, Guanine) & Pyrimidines (Cytosine, Thymine/Uracil).
Pentose Sugar: Ribose (RNA) or Deoxyribose (DNA).
Phosphate Group: Linked to 5′-OH of sugar via Phosphoester linkage.

Two nucleotides are linked by 3′-5′ Phosphodiester linkage.

B. The Double Helix Model (1953)

Proposed by Watson and Crick (based on X-ray diffraction data by Wilkins & Franklin).

🧬 Salient Features:

Two polynucleotide chains with Anti-parallel polarity (5’→3′ and 3’→5′).
Bases paired by H-bonds: A=T (2 bonds) and G≡C (3 bonds).
Backbone formed by Sugar-Phosphate.
Pitch of helix: 3.4 nm. Rise per base pair: 0.34 nm. (10 bp per turn).

Chargaff’s Rule: For double-stranded DNA, the ratio of A:T and G:C is constant and equals one.

C. Packaging of DNA Helix

In eukaryotes, positively charged basic proteins called Histones (rich in Lysine & Arginine) form an octamer. Negatively charged DNA wraps around it to form the Nucleosome (“Beads-on-string” appearance).

2. The Search for Genetic Material

Scientist(s)	Experiment / Discovery	Conclusion
Frederick Griffith (1928)	Streptococcus pneumoniae (S-strain & R-strain) on mice.	Transforming Principle. Something from heat-killed S-strain transformed R-strain.
Avery, MacLeod, McCarty (1944)	Biochemical characterisation. Used Proteases, RNases, DNases.	Only DNase inhibited transformation. DNA is the genetic material.
Hershey & Chase (1952)	Bacteriophage T2. Used radioactive ³²P (DNA) and ³⁵S (Protein).	Unequivocal proof that DNA is genetic material.

3. Replication (Semi-Conservative)

Proven by Meselson and Stahl using ¹⁵N (Heavy nitrogen) in E. coli.

[Image of DNA replication fork diagram]

Origin of Replication (ori): Site where replication starts.
DNA Polymerase: Main enzyme. Polymerizes only in 5′ → 3′ direction.
Replication Fork: Y-shaped opening.
- Leading Strand: Continuous synthesis (3′ → 5′ template).
- Lagging Strand: Discontinuous (Okazaki fragments) on 5′ → 3′ template. Joined by DNA Ligase.

4. Transcription

Process of copying genetic information from one strand of DNA into RNA.

Transcription Unit: Promoter + Structural Gene + Terminator.

Prokaryotes vs Eukaryotes

Feature	Prokaryotes	Eukaryotes
Genes	Polycistronic (Multiple genes).	Monocistronic (Split genes – Exons & Introns).
Processing	Not required. Translation can begin before transcription ends.	Post-transcriptional modification required (Splicing, Capping, Tailing) to form mature mRNA.

5. Genetic Code & Translation

Genetic Code: Deciphered by Nirenberg, Khorana, and Holley.

Triplet: 61 codons code for amino acids. 3 stop codons (UAA, UAG, UGA).
Unambiguous & Specific: One codon = One amino acid.
Degenerate: Some amino acids coded by multiple codons.
AUG: Dual function (Codes for Methionine + Initiator codon).

Translation (Protein Synthesis)

Occurs in Ribosomes.
1. Charging of tRNA: Amino acid activation.
2. Initiation: Ribosome binds to mRNA at AUG.
3. Elongation: Peptide bond formation (Ribozyme).
4. Termination: Stop codon reached.

6. Regulation of Gene Expression

Lac Operon (Jacob & Monod): Control of lactose metabolism in E. coli.

Inducer: Lactose (Allolactose).
Repressor: Synthesized by i gene. Binds to Operator region preventing transcription.
Mechanism: When Lactose is present, it binds to Repressor → Repressor inactive → RNA Polymerase binds Promoter → Genes z, y, a transcribed (Beta-galactosidase, Permease, Transacetylase).

7. HGP & DNA Fingerprinting

Human Genome Project (HGP)

A 13-year mega project.
Key Findings: 3 billion base pairs. Average gene size 3000 bases. Total genes ~30,000. 99.9% nucleotide bases are exactly same in all people. Less than 2% codes for proteins.

DNA Fingerprinting

Developed by Alec Jeffreys. Used for forensic applications and paternity testing.

Basis: Polymorphism in DNA sequences (VNTRs – Variable Number of Tandem Repeats).
Steps: Isolation → Digestion (Restriction Endonuclease) → Separation (Gel Electrophoresis) → Blotting (Southern Blot) → Hybridization (Probe) → Autoradiography.

📝 Rapid Fire MCQs

Q1. Which of the following is not a stop codon?

A) UAA
B) UAG
C) UGA
D) UGG

Click to check Answer

Answer: D) UGG (Codes for Tryptophan).

Q2. In the Lac Operon, the ‘z’ gene codes for:

A) Beta-galactosidase
B) Permease
C) Transacetylase
D) Repressor