MOLECULAR BASIS OF INHERITANCE
Complete NCERT Study Guide & Exam Resource | Class 12 Biology
1. 🌱 Introduction: The Search for Genetic Material
Nucleic Acids (DNA & RNA) are the building blocks of genetic information. Most organisms use
DNA as genetic material; however, some viruses (e.g., HIV, Tobacco Mosaic Virus) use RNA.
Historical Experiments
• Griffith’s Transforming Principle (1928): Suggested that S strain (virulent) bacteria
could "transform" R strain (non-virulent) into virulent ones even when heat-killed.
• Avery, MacLeod, and McCarty (1933-44): Proved that the transforming substance
was DNA (by using Proteases, RNases, and DNases).
• Hershey-Chase Experiment (1952): Used Bacteriophages and radioactive isotopes
(³⁵S for protein, ³²P for DNA) to conclude that DNA is the genetic material.
2. 🧬 Structure of DNA (Chemical Basis)
Chargaff's Rule: In double-stranded DNA,
the ratio between Adenine (A) and Thymine
(T) and Guanine (G) and Cytosine (C) are
constant and equal. [A+G] = [T+C].
A = T (2 H-bonds) | G ≡ C (3 H-bonds)
DNA Double Helix Model
Salient features of Watson-Crick Model:
1. Two polynucleotide chains with anti-parallel polarity (5'→3' and 3'→5').
Biology Class 12: Molecular Basis | Page 1
, 2. Backbone is made of sugar-phosphate; bases project inside.
3. Right-handed helix with pitch of 3.4 nm and 10 bp per turn.
Biology Class 12: Molecular Basis | Page 2
Complete NCERT Study Guide & Exam Resource | Class 12 Biology
1. 🌱 Introduction: The Search for Genetic Material
Nucleic Acids (DNA & RNA) are the building blocks of genetic information. Most organisms use
DNA as genetic material; however, some viruses (e.g., HIV, Tobacco Mosaic Virus) use RNA.
Historical Experiments
• Griffith’s Transforming Principle (1928): Suggested that S strain (virulent) bacteria
could "transform" R strain (non-virulent) into virulent ones even when heat-killed.
• Avery, MacLeod, and McCarty (1933-44): Proved that the transforming substance
was DNA (by using Proteases, RNases, and DNases).
• Hershey-Chase Experiment (1952): Used Bacteriophages and radioactive isotopes
(³⁵S for protein, ³²P for DNA) to conclude that DNA is the genetic material.
2. 🧬 Structure of DNA (Chemical Basis)
Chargaff's Rule: In double-stranded DNA,
the ratio between Adenine (A) and Thymine
(T) and Guanine (G) and Cytosine (C) are
constant and equal. [A+G] = [T+C].
A = T (2 H-bonds) | G ≡ C (3 H-bonds)
DNA Double Helix Model
Salient features of Watson-Crick Model:
1. Two polynucleotide chains with anti-parallel polarity (5'→3' and 3'→5').
Biology Class 12: Molecular Basis | Page 1
, 2. Backbone is made of sugar-phosphate; bases project inside.
3. Right-handed helix with pitch of 3.4 nm and 10 bp per turn.
Biology Class 12: Molecular Basis | Page 2
