Class 12 Biology Chapter 6 Molecular Basis of Inheritance Notes
6.1 The DNA (Structure and Components)
DNA is the genetic material in most living organisms. It stores and transmits hereditary information.
Components of DNA:
Nitrogenous bases
Purines: Adenine (A), Guanine (G)
Pyrimidines: Cytosine (C), Thymine (T)
Sugar: Deoxyribose
Phosphate group
Nitrogenous Bases vs Nucleosides:
Nitrogenous bases: Adenine, Thymine, Cytosine, Uracil
Nucleosides: Cytidine, Guanosine
Chargaff’s Rule (Base Composition)
A = T
G = C
Purines = Pyrimidines
👉 If Cytosine = 20%
→ Guanine = 20%
→ Remaining 60% = A + T
→ Adenine = 30%
Complementary Base Pairing
A pairs with T
G pairs with C
This property helps in replication and transcription.
DNA Double Helix Model
Proposed by James Watson and Francis Crick.
Key Features:
Double stranded
Anti-parallel strands
Hydrogen bonding between bases
Sugar-phosphate backbone outside
6.2 Search for Genetic Material
DNA is the genetic material in most organisms, but RNA acts as genetic material in some viruses.
Hershey–Chase Experiment
Conducted by Alfred Hershey and Martha Chase (1952).
DNA labelled with ³²P
Protein labelled with ³⁵S
Only DNA entered bacterial cells
Proved DNA is genetic material
6.3 RNA World Hypothesis
RNA was the first genetic material
RNA could:
Act as genetic material
Act as catalyst (ribozymes)
Later replaced by DNA due to stability
6.4 DNA Replication (Semi-Conservative)
Why Semi-Conservative?
Each daughter DNA has:
One old strand
One new strand
Proposed by Watson and Crick
Experimentally proved by Matthew Meselson and Franklin Stahl
Types of Nucleic Acid Polymerases
DNA-dependent DNA polymerase → DNA synthesis
DNA-dependent RNA polymerase → Transcription
RNA-dependent DNA polymerase → Retroviruses
RNA-dependent RNA polymerase → RNA viruses
6.5 Transcription
Definition:
DNA-directed synthesis of RNA.
Key Points:
Occurs on template strand (3′ → 5′)
RNA synthesized in 5′ → 3′ direction
Coding strand has same sequence as mRNA (except T → U)
Promoter:
Start site of transcription
Contains TATA box
Binding site for RNA polymerase
6.6 Genetic Code
Properties:
Triplet code
Universal
Degenerate
Non-overlapping
Start codon: AUG
Stop codons: UAA, UAG, UGA
6.7 Translation (Protein Synthesis)
Role of Ribosome:
Binding of mRNA
Peptide bond formation (peptidyl transferase)
Role of tRNA:
Transfers amino acids to ribosome
Has anticodon region
Differences (Exam Favourite)
mRNA vs tRNA
mRNA: Carries genetic message
tRNA: Transfers amino acids
Template vs Coding Strand
Template strand: Used for transcription
Coding strand: Same sequence as mRNA
Repetitive DNA vs Satellite DNA
Repetitive DNA: Repeated sequences
Satellite DNA: Highly repetitive, shows polymorphism
6.8 Regulation of Gene Expression – Lac Operon
Lactose acts as inducer
Inactivates repressor
β-galactosidase breaks lactose
When lactose finishes → operon shuts off
6.9 Human Genome Project (HGP)
Why Mega Project?
Huge data generation
International collaboration
Advanced bioinformatics required
Goals:
Identify all human genes
Determine DNA sequence
Store data digitally
6.10 DNA Fingerprinting
DNA fingerprinting identifies individuals using VNTRs.
Applications:
Paternity disputes
Crime detection
Genetic disease identification
Evolutionary studies
Important Definitions (1–2 Lines)
Promoter: Initiates transcription
tRNA: Transfers amino acids
Exons: Coding regions of DNA
Polymorphism: Genetic variation
Bioinformatics: Computational analysis of biological data
6.11 Summary (Quick Revision ✨)
DNA is genetic material
Replication is semi-conservative
Transcription & translation form central dogma
Gene expression is regulated
HGP revolutionised genetics
DNA fingerprinting has wide applications
✍️ Top 10 Short Question–Answers
Q1. What is DNA?
Answer: DNA is the genetic material that stores and transmits hereditary information.
Q2. Name the nitrogenous bases present in DNA.
Answer: Adenine, Guanine, Cytosine and Thymine.
Q3. State Chargaff’s rule.
Answer: In DNA, Adenine = Thymine and Guanine = Cytosine.
Q4. Who proposed the double helix model of DNA?
Answer: James Watson and Francis Crick.
Q5. Which experiment proved that DNA is genetic material?
Answer: Hershey–Chase experiment.
Q6. What is semi-conservative replication?
Answer: Each daughter DNA has one old strand and one new strand.
Q7. What is transcription?
Answer: DNA-directed synthesis of RNA.
Q8. Name the start codon of genetic code.
Answer: AUG.
Q9. What is the function of tRNA?
Answer: It transfers amino acids to ribosome during protein synthesis.
Q10. What are VNTRs used for?
Answer: DNA fingerprinting and individual identification.
📝 Long Answer Questions
Q1. Describe the structure of DNA.
Answer:
DNA is a double-stranded molecule with the following features:
Double helical structure
Two anti-parallel strands
Sugar-phosphate backbone on the outside
Nitrogenous bases on the inside
Complementary base pairing (A–T, G–C)
Hydrogen bonds hold the strands together
Q2. Explain the Hershey–Chase experiment.
Answer:
The experiment was conducted to identify genetic material.
DNA of bacteriophage labelled with ³²P
Protein coat labelled with ³⁵S
Only DNA entered bacterial cell
Protein remained outside
Conclusion: DNA is the genetic material.
Q3. Explain semi-conservative replication of DNA.
Answer:
In semi-conservative replication:
DNA strands separate
Each strand acts as template
New complementary strand is synthesized
Each daughter DNA has one parental and one new strand
This was experimentally proved by Meselson and Stahl.
📝 PYQs (Previous Year Questions)
🔹 1️⃣ Very Short Answer Questions (1 Mark)
What is the genetic material in most living organisms?
Name the sugar present in DNA.
State Chargaff’s rule.
Who proposed the double helix model of DNA?
Which experiment proved DNA is the genetic material?
Name the radioactive isotope used to label DNA in Hershey–Chase experiment.
What is the start codon of genetic code?
Name any one stop codon.
What is transcription?
What are VNTRs used for?
🔹 2️⃣ Short Answer Questions (2–3 Marks)
Write the components of DNA.
Differentiate between purines and pyrimidines.
Explain Chargaff’s base pairing rule with an example.
Why is DNA called a better genetic material than RNA?
What is semi-conservative replication?
Name the types of nucleic acid polymerases and their functions.
Define transcription and mention the direction of RNA synthesis.
What are the properties of genetic code?
What is RNA world hypothesis?
Write any two applications of DNA fingerprinting.
🔹 3️⃣ Long Answer Questions (3–5 Marks)
Describe the structure of DNA with a labelled diagram.
Explain the Hershey–Chase experiment and its conclusion.
Describe the Meselson and Stahl experiment.
Explain transcription in prokaryotes.
Describe translation with the role of ribosome and tRNA.
Explain the lac operon model of gene regulation.
Write a note on Human Genome Project.
Explain the process of DNA fingerprinting and its applications.
🔹 4️⃣ Difference-Based PYQs (Very Important)
DNA vs RNA
Purines vs Pyrimidines
Template strand vs Coding strand
mRNA vs tRNA
Repetitive DNA vs Satellite DNA
🔹 5️⃣ Assertion–Reason / Concept-Based PYQs
DNA replication is called semi-conservative. Justify.
Genetic code is degenerate. Explain.
Lac operon is an inducible operon. Give reason.
RNA viruses do not require DNA-dependent DNA polymerase. Explain.
DNA fingerprinting is unique for every individual. Justify.
🔹 6️⃣ Diagram / Flowchart Based PYQs
Draw a labelled diagram of DNA double helix.
Diagrammatic representation of lac operon.
Flowchart showing central dogma of molecular biology.
Diagram showing Meselson–Stahl experiment.
❓ FAQs (5)
FAQ 1. Why is DNA more stable than RNA?
Because DNA has double-stranded structure and deoxyribose sugar.
FAQ 2. Why is RNA considered first genetic material?
Because RNA could act as genetic material and catalyst.
FAQ 3. What is the role of promoter in transcription?
It is the binding site for RNA polymerase to start transcription.
FAQ 4. Why is genetic code called degenerate?
Because more than one codon codes for the same amino acid.
FAQ 5. What is the importance of DNA fingerprinting?
It helps in crime detection, paternity disputes and genetic studies.
| Chapter No. | Chapter Name | Visit |
|---|---|---|
| 1 | Reproduction in Organisms | Visit |
| 2 | Sexual Reproduction in Flowering Plants | Visit |
| 3 | Human Reproduction | Visit |
| 4 | Reproductive Health | Visit |
| 5 | Principles of Inheritance and Variation | Visit |
| 6 | Molecular Basis of Inheritance | Visit |
| 7 | Evolution | Visit |
| 8 | Human Health and Disease | Visit |
| 9 | Strategies for Enhancement in Food Production | Visit |
| 10 | Microbes in Human Welfare | Visit |
| 11 | Biotechnology: Principles and Processes | Visit |
| 12 | Biotechnology and its Applications | Visit |
| 13 | Organisms and Populations | Visit |
| 14 | Ecosystem | Visit |
| 15 | Biodiversity and Conservation | Visit |
| 16 | Environmental Issues | Visit |
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