The chapter Molecular Basis of Inheritance Class12 Biology Notes is one of the most important and high-weightage chapters in Class 12 Biology. It explains how genetic information is stored, expressed, and transferred from one generation to the next at the molecular level. Concepts like DNA structure, RNA, replication, transcription, translation, genetic code, and regulation of gene expression are crucial not only for CBSE board exams but also for NEET and other competitive exams.
In this chapter, students understand how DNA controls all biological activities and how mutations lead to variation and evolution. This chapter forms the foundation of modern genetics and biotechnology.
Molecular Basis of Inheritance Class12 Biology Notes | NCERT, NEET & Board Exam 2026
1. DNA: The Genetic Material
What is Genetic Material?
A genetic material is a substance that:
- Stores biological information
- Replicates accurately
- Expresses information in the form of traits
- Shows variation through mutation
Why DNA is Genetic Material?
DNA is considered the genetic material in most organisms because:
- It is chemically stable
- It can replicate
- It stores information in the form of nucleotide sequences
- It can mutate slowly and steadily
Exceptions:
- Some viruses (like Tobacco Mosaic Virus) use RNA as genetic material.
2. Structure of DNA (Watson and Crick Model)
Components of DNA
DNA is a polynucleotide chain made of:
- Nitrogenous base (A, T, G, C)
- Deoxyribose sugar
- Phosphate group
Double Helix Model
According to Watson and Crick (1953):
- DNA has two antiparallel strands
- Sugar-phosphate forms the backbone
- Bases pair through hydrogen bonds:
- Adenine pairs with Thymine (2 H-bonds)
- Guanine pairs with Cytosine (3 H-bonds)
Important Features
- Diameter: 2 nm
- One turn: 10 base pairs
- Helical structure gives stability
3. RNA: Structure and Types
What is RNA?
RNA (Ribonucleic Acid) is a single-stranded nucleic acid involved in protein synthesis.
Types of RNA
- mRNA (Messenger RNA)
- Carries genetic information from DNA to ribosome
- tRNA (Transfer RNA)
- Brings amino acids during protein synthesis
- Has anticodon loop
- rRNA (Ribosomal RNA)
- Forms ribosomes
- Site of protein synthesis
4. DNA Replication
Definition
DNA replication is the process by which DNA makes an exact copy of itself before cell division.
Nature of Replication
- Semi-conservative (Meselson and Stahl experiment)
- Each daughter DNA has:
- One parental strand
- One newly synthesized strand
Steps of DNA Replication
- Initiation
- Helicase unwinds DNA
- Origin of replication (Ori)
- Elongation
- DNA polymerase adds nucleotides in 5’ → 3’ direction
- Leading strand: continuous
- Lagging strand: Okazaki fragments
- Termination
- DNA ligase joins fragments
Enzymes Involved
- Helicase
- DNA polymerase
- RNA primase
- Ligase
5. Transcription (DNA → RNA)
Definition
Transcription is the process of synthesis of RNA from DNA.
Steps of Transcription
- Initiation
- RNA polymerase binds to promoter
- Elongation
- RNA strand synthesized in 5’ → 3’ direction
- Termination
- Transcription stops at terminator region
Differences Between Replication and Transcription
| Replication | Transcription |
|---|---|
| DNA → DNA | DNA → RNA |
| Entire genome | Selected genes |
| Uses DNA polymerase | Uses RNA polymerase |
6. Genetic Code
Definition
The genetic code is a sequence of three nucleotides (codon) that codes for one amino acid.
Characteristics of Genetic Code
- Triplet code
- Universal
- Degenerate (more than one codon for same amino acid)
- Non-overlapping
- Commaless
Start and Stop Codons
- Start codon: AUG (Methionine)
- Stop codons: UAA, UAG, UGA
7. Translation (RNA → Protein)
Definition
Translation is the process by which amino acids are linked together to form a protein using mRNA as a template.
Site of Translation
- Ribosomes (cytoplasm)
Steps of Translation
- Initiation
- Ribosome binds to mRNA
- Start codon recognized
- Elongation
- tRNA brings amino acids
- Peptide bond formation
- Termination
- Stop codon reached
- Polypeptide released
8. Regulation of Gene Expression
What is Gene Regulation?
Gene regulation is the process of controlling gene expression so that proteins are made only when required.
Operon Concept (Jacob and Monod)
Lac Operon (in E. coli)
Components:
- Structural genes (lac Z, Y, A)
- Promoter
- Operator
- Regulator gene
When lactose is absent:
- Repressor binds operator
- Transcription stops
When lactose is present:
- Repressor inactivated
- Transcription occurs
9. Mutations
Definition
Mutation is a sudden change in DNA sequence.
Types of Mutations
- Point mutation
- Frameshift mutation
- Deletion
- Insertion
Effects of Mutation
- Genetic disorders
- Variation
- Evolution
10. Human Genome Project (HGP)
Aim
To identify and map all genes in human DNA.
Features
- 3 billion base pairs
- 99.9% DNA same in all humans
- Genes unevenly distributed
Importance
- Understanding genetic diseases
- Personalized medicine
- Biotechnology advancement
11. DNA Fingerprinting
Definition
DNA fingerprinting is a technique used to identify individuals based on DNA patterns.
Applications
- Forensic science
- Paternity testing
- Crime investigation
- Wildlife conservation
Importance of Molecular Basis of Inheritance Class 12 Biology
- Very high weightage in board exams
- Direct questions in NEET
- Foundation of genetics & biotechnology
- Important for future medical and research fields
Conclusion
The chapter Molecular Basis of Inheritance Class 12 Biology explains the fundamental molecular mechanisms behind heredity and variation. Understanding DNA structure, replication, transcription, translation, and gene regulation helps students grasp how life processes are controlled at the molecular level. Mastery of this chapter is essential for scoring high marks in CBSE boards, NEET, and other competitive exams.
Molecular Basis of Inheritance Class 12 Biology
Important Repeated Questions (Board + NEET)
🔴 VERY IMPORTANT LONG ANSWER QUESTIONS (5 Marks)
- Describe the structure of DNA as proposed by Watson and Crick.
(Diagram ke saath likhne par full marks milte hain) - Explain the process of DNA replication in detail.
- Semi-conservative nature
- Enzymes involved
- Leading & lagging strand
- Describe transcription in prokaryotes.
- Initiation
- Elongation
- Termination
- Explain translation with the help of suitable diagrams.
- Role of mRNA, tRNA, ribosomes
- Initiation, elongation, termination
- Explain the lac operon model for gene regulation.
(Most repeated question in CBSE) - What is the genetic code? Explain its main characteristics.
- Describe the Human Genome Project and its significance.
- Explain DNA fingerprinting technique and its applications.
🟠 IMPORTANT SHORT ANSWER QUESTIONS (2–3 Marks)
- Why is DNA considered a better genetic material than RNA?
- What is semi-conservative replication?
- Define transcription unit.
- What are Okazaki fragments?
- What is the role of DNA ligase?
- What is a codon? Give one example.
- Name the start codon and stop codons.
- What is degenerate genetic code?
- Differentiate between transcription and translation.
- What is an operon?
- Write any two differences between DNA and RNA.
- What is mutation? Name its types.
🟡 VERY IMPORTANT 1 MARK QUESTIONS
- Who proposed the double helix model of DNA?
- Which enzyme unwinds DNA during replication?
- Direction of DNA synthesis is ________.
- Name the sugar present in RNA.
- How many hydrogen bonds are present between G and C?
- What is the start codon?
- Name the site of protein synthesis.
- Which RNA carries amino acids?
- Expand HGP.
- Who discovered lac operon?
🔵 MOST REPEATED NEET MCQs (Concept Based)
- Semi-conservative nature of DNA replication was proved by:
Meselson and Stahl - Which enzyme synthesizes RNA primer?
Primase - Which codon codes for Methionine?
AUG - Which RNA has anticodon?
tRNA - In lac operon, structural genes are:
lac Z, lac Y, lac A - Stop codons do not code for any amino acid. (True/False)
True
🔴 DIAGRAM-BASED QUESTIONS (Highly Repeated)
- Labelled diagram of DNA
- Transcription unit
- tRNA structure
- Lac operon
- DNA replication fork
(Diagram = +1 extra impression in boards)
⭐ EXAM TIP (VERY IMPORTANT)
👉 CBSE Board:
- Lac operon
- DNA replication
- Genetic code
- Translation
👉 NEET:
- Enzymes in replication
- Codons
- Operon concept
- Mutation types
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