🧬 Molecular Basis of Inheritance (Class 12 Biology)
📚 Complete NTA NEET Syllabus Notes (Hindi + English)
NCERT Line-by-Line | PYQ Focus | Learn With Afsa
🌟 Chapter Introduction
English
Inheritance means transmission of characters from parents to offspring. At molecular level, DNA stores, replicates and expresses genetic information.
हिंदी
वंशागति (Inheritance) का अर्थ है माता-पिता से संतान में लक्षणों का स्थानांतरण। आणविक स्तर पर DNA आनुवंशिक सूचना को संग्रहित, प्रतिलिपित और व्यक्त करता है।
1️⃣ The Search for Genetic Material
Griffith's Transformation Experiment (1928)
Organism: Streptococcus pneumoniae
S-Strain
- Smooth
- Virulent
- Disease causing
R-Strain
- Rough
- Non-virulent
Observation
Live S → Mouse dies
Live R → Mouse lives
Heat-killed S → Mouse lives
Heat-killed S + Live R → Mouse dies
Conclusion
Some transforming principle transferred genetic information.
Avery, MacLeod and McCarty (1944)
Removed proteins, RNA and DNA one by one.
Only when DNA was destroyed, transformation stopped.
Conclusion
✅ DNA is transforming principle.
📌 NCERT Highlight
First direct evidence that DNA is genetic material.
🎯 PYQ Focus
Who proved transforming principle?
✅ Avery, MacLeod & McCarty
Hershey-Chase Experiment (1952)
Used:
Bacteriophage
Radioactive Labels:
³²P → DNA
³⁵S → Protein
Conclusion
Only DNA entered bacterial cells.
✅ DNA is genetic material.
🎯 PYQ Focus
Most repeated NEET question.
2️⃣ RNA as Genetic Material
Organisms having RNA as genetic material
✅ Tobacco Mosaic Virus (TMV)
✅ QB Virus
NCERT Highlight
RNA acts as both genetic material and catalyst.
3️⃣ Structure of Polynucleotide Chain
Each nucleotide contains:
- Nitrogenous Base
- Pentose Sugar
- Phosphate Group
Nitrogen Bases
Purines
Adenine (A)
Guanine (G)
Double ring
Pyrimidines
Cytosine (C)
Thymine (T)
Single ring
Nucleoside
Sugar + Base
Nucleotide
Sugar + Base + Phosphate
🎯 PYQ Focus
Difference between nucleoside and nucleotide.
4️⃣ DNA Double Helix Structure
Proposed by
James Watson
Francis Crick
(1953)
Based on:
Rosalind Franklin's X-ray diffraction data.
Features
Double Helix
DNA consists of two strands.
Antiparallel
One strand:
5' → 3'
Other strand:
3' → 5'
Complementary Base Pairing
A = T
G ≡ C
Hydrogen Bonds
A-T → 2
G-C → 3
Measurements
Diameter = 20 Ã…
Distance between base pairs = 3.4 Ã…
One turn = 34 Ã…
10 bp/turn
Chargaff's Rule
A = T
G = C
Purines = Pyrimidines
🎯 PYQ Focus
Higher GC content → More stable DNA
5️⃣ DNA Packaging
Length of DNA in one human cell:
≈ 2.2 meters
Packed inside nucleus.
Histone Proteins
Positively charged
Rich in:
Lysine
Arginine
Histone Octamer
2H2A
2H2B
2H3
2H4
Nucleosome
DNA wrapped around histone octamer.
Basic unit of chromatin.
Chromatin
Nucleosomes combine to form chromatin.
Euchromatin
Lightly packed
Transcriptionally active
Heterochromatin
Densely packed
Transcriptionally inactive
6️⃣ DNA Replication
Definition
Formation of identical DNA molecules.
Semiconservative Replication
Proposed by:
Watson & Crick
Proved by:
Meselson & Stahl
Meselson-Stahl Experiment
Organism:
E. coli
Isotopes:
¹⁵N
¹⁴N
Conclusion:
Each daughter DNA contains:
One old strand + One new strand
Enzymes
Helicase
Unwinds DNA
DNA Polymerase
Adds nucleotides
Ligase
Joins fragments
Direction
5' → 3'
🎯 PYQ Focus
Replication is semiconservative.
7️⃣ Transcription
Definition
DNA → RNA synthesis
Transcription Unit
Promoter
Starting site
Structural Gene
Contains genetic information
Terminator
Stops transcription
Coding Strand
5' → 3'
Does not participate directly.
Template Strand
3' → 5'
Used for RNA synthesis.
Enzyme
RNA Polymerase
RNA Processing
Only in Eukaryotes
Capping
Added at 5' end
Tailing
Added at 3' end
Splicing
Removal of introns
Joining of exons
Introns
Non-coding sequences
Exons
Coding sequences
🎯 PYQ Focus
Mature mRNA contains only exons.
8️⃣ Types of RNA
mRNA
Messenger RNA
Carries genetic information.
tRNA
Transfer RNA
Carries amino acids.
rRNA
Ribosomal RNA
Forms ribosomes.
Most abundant RNA.
tRNA Structure
Clover leaf shape
Anticodon Loop
Recognizes codon.
Amino Acid Acceptor End
Carries amino acid.
🎯 PYQ Focus
Most abundant RNA?
✅ rRNA
9️⃣ Genetic Code
Definition
Language through which information is translated into proteins.
Properties
Triplet
3 bases = 1 codon
Universal
Same in almost all organisms
Degenerate
Many codons code same amino acid
Unambiguous
One codon = One amino acid
Start Codon
AUG
Methionine
Stop Codons
UAA
UAG
UGA
🎯 PYQ Focus
AUG = Start codon
🔟 Translation
Definition
Protein synthesis from mRNA.
Site
Ribosome
Steps
- Activation
- Initiation
- Elongation
- Termination
NCERT Highlight
Translation occurs in ribosomes.
1️⃣1️⃣ Regulation of Gene Expression
Lac Operon
Proposed By
Jacob & Monod
Organism
E. coli
Components
Regulatory Gene
Promoter
Operator
Structural Genes
Absence of Lactose
Operon OFF
Presence of Lactose
Operon ON
Inducer
Lactose
🎯 PYQ Focus
Lac Operon is inducible operon.
1️⃣2️⃣ Human Genome Project (HGP)
Goal
Sequence complete human genome.
Important Facts
Genome Size
3.2 Billion Base Pairs
Genes
20,000–25,000
99.9% DNA same in all humans
Only 1.5% DNA codes proteins.
Achievements
Human genome sequencing completed.
Gene mapping possible.
Disease studies improved.
🎯 PYQ Focus
Genome size = 3.2 billion bp
1️⃣3️⃣ DNA Fingerprinting
Developed By
Sir Alec Jeffreys
Principle
VNTR
(Variable Number Tandem Repeats)
Uses
Forensic Science
Paternity Testing
Crime Investigation
Identification of Individuals
NCERT Highlight
Based on DNA polymorphism.
🚨 Most Important NCERT Lines
✅ DNA is the genetic material in most organisms.
✅ RNA acts as genetic material in some viruses.
✅ DNA strands are antiparallel.
✅ Replication is semiconservative.
✅ rRNA is most abundant RNA.
✅ AUG is start codon.
✅ UAA, UAG, UGA are stop codons.
✅ Lac Operon is inducible.
✅ Human genome contains 3.2 billion bp.
✅ DNA Fingerprinting is based on VNTR.
🎯 NEET PYQ Hot Topics (Highest Priority)
🔥 DNA Structure
🔥 Chargaff Rule
🔥 Histone Octamer
🔥 Meselson-Stahl Experiment
🔥 Replication Enzymes
🔥 Template vs Coding Strand
🔥 RNA Processing
🔥 Genetic Code
🔥 AUG / Stop Codons
🔥 tRNA Structure
🔥 Lac Operon
🔥 Human Genome Project
🔥 DNA Fingerprinting
🧠One-Line Final Revision
DNA → Genetic Material → Structure → Packaging → Replication → Transcription → RNA Processing → Genetic Code → Translation → Lac Operon → Human Genome Project → DNA Fingerprinting
