CLASS 12 BIOLOGY
Chapter: Molecular Basis of Inheritance
Part 1 – Introduction, DNA as Genetic Material, Structure
of DNA & RNA
Author: Mahadeb Bar
Learning Objectives
After studying this chapter, students will be able to:
Explain the molecular basis of inheritance.
Describe the structure and functions of DNA and RNA.
Understand why DNA is considered the genetic material.
Identify the different types of RNA.
Explain the packaging of DNA inside cells.
1. Introduction
The molecular basis of inheritance explains how genetic information is stored, copied, and
transmitted from one generation to the next. The hereditary material contains the instructions
needed for growth, development, metabolism, and reproduction.
The study of DNA, RNA, genes, and chromosomes forms the foundation of modern genetics
and biotechnology.
2. Genetic Material
A genetic material is a molecule that stores and transmits hereditary information.
To act as genetic material, a molecule should:
Store biological information accurately.
Replicate itself during cell division.
Express information through proteins.
Undergo occasional mutations to create variation.
,Both DNA and RNA can act as genetic material, but in most organisms DNA is the primary
genetic material.
3. DNA as the Genetic Material
DNA (Deoxyribonucleic Acid) is the hereditary material in almost all living organisms.
The Hershey–Chase experiment (1952) demonstrated that DNA, not protein, carries genetic
information in bacteriophages.
Reasons Why DNA Is the Genetic Material
It is chemically stable.
It replicates accurately.
It stores large amounts of genetic information.
It can undergo mutation, leading to evolution.
It controls protein synthesis through gene expression.
4. Structure of DNA
The structure of DNA was proposed by James Watson and Francis Crick in 1953, based on
X-ray diffraction data produced by Rosalind Franklin and Maurice Wilkins.
Main Features
DNA consists of two antiparallel polynucleotide strands.
The strands form a double helix.
Each nucleotide contains:
o A deoxyribose sugar
o A phosphate group
o A nitrogenous base
Nitrogenous Bases
Purines:
Adenine (A)
Guanine (G)
Pyrimidines:
Thymine (T)
Cytosine (C)
, Base Pairing Rule
Adenine pairs with Thymine (A = T)
Guanine pairs with Cytosine (G ≡ C)
This complementary pairing is known as Chargaff's Rule.
5. Components of DNA
Each DNA nucleotide contains:
1. Deoxyribose sugar
2. Phosphate group
3. Nitrogenous base
The sugar and phosphate form the backbone, while the bases project inward and pair through
hydrogen bonds.
6. DNA Double Helix
Important characteristics:
Right-handed double helix
Diameter: 2 nm
Distance between two base pairs: 0.34 nm
One complete turn: 3.4 nm
Approximately 10 base pairs per turn
7. RNA (Ribonucleic Acid)
RNA is usually single-stranded and plays a major role in protein synthesis.
Components
Ribose sugar
Phosphate group
Nitrogenous bases:
o Adenine (A)
o Guanine (G)
o Cytosine (C)
o Uracil (U) instead of Thymine
Chapter: Molecular Basis of Inheritance
Part 1 – Introduction, DNA as Genetic Material, Structure
of DNA & RNA
Author: Mahadeb Bar
Learning Objectives
After studying this chapter, students will be able to:
Explain the molecular basis of inheritance.
Describe the structure and functions of DNA and RNA.
Understand why DNA is considered the genetic material.
Identify the different types of RNA.
Explain the packaging of DNA inside cells.
1. Introduction
The molecular basis of inheritance explains how genetic information is stored, copied, and
transmitted from one generation to the next. The hereditary material contains the instructions
needed for growth, development, metabolism, and reproduction.
The study of DNA, RNA, genes, and chromosomes forms the foundation of modern genetics
and biotechnology.
2. Genetic Material
A genetic material is a molecule that stores and transmits hereditary information.
To act as genetic material, a molecule should:
Store biological information accurately.
Replicate itself during cell division.
Express information through proteins.
Undergo occasional mutations to create variation.
,Both DNA and RNA can act as genetic material, but in most organisms DNA is the primary
genetic material.
3. DNA as the Genetic Material
DNA (Deoxyribonucleic Acid) is the hereditary material in almost all living organisms.
The Hershey–Chase experiment (1952) demonstrated that DNA, not protein, carries genetic
information in bacteriophages.
Reasons Why DNA Is the Genetic Material
It is chemically stable.
It replicates accurately.
It stores large amounts of genetic information.
It can undergo mutation, leading to evolution.
It controls protein synthesis through gene expression.
4. Structure of DNA
The structure of DNA was proposed by James Watson and Francis Crick in 1953, based on
X-ray diffraction data produced by Rosalind Franklin and Maurice Wilkins.
Main Features
DNA consists of two antiparallel polynucleotide strands.
The strands form a double helix.
Each nucleotide contains:
o A deoxyribose sugar
o A phosphate group
o A nitrogenous base
Nitrogenous Bases
Purines:
Adenine (A)
Guanine (G)
Pyrimidines:
Thymine (T)
Cytosine (C)
, Base Pairing Rule
Adenine pairs with Thymine (A = T)
Guanine pairs with Cytosine (G ≡ C)
This complementary pairing is known as Chargaff's Rule.
5. Components of DNA
Each DNA nucleotide contains:
1. Deoxyribose sugar
2. Phosphate group
3. Nitrogenous base
The sugar and phosphate form the backbone, while the bases project inward and pair through
hydrogen bonds.
6. DNA Double Helix
Important characteristics:
Right-handed double helix
Diameter: 2 nm
Distance between two base pairs: 0.34 nm
One complete turn: 3.4 nm
Approximately 10 base pairs per turn
7. RNA (Ribonucleic Acid)
RNA is usually single-stranded and plays a major role in protein synthesis.
Components
Ribose sugar
Phosphate group
Nitrogenous bases:
o Adenine (A)
o Guanine (G)
o Cytosine (C)
o Uracil (U) instead of Thymine
