🌸 Chapter 5
Molecular Basis Of Inheritance
(SUMMARY NOTES )
🌟 1. DNA — The Genetic
Material
🔹 Discovery & Structure
- DNA (Deoxyribonucleic Acid) → carries hereditary
information.
- Discovered by: Friedrich Miescher
(1869) — named “Nuclein”.
- Proved as genetic material by: Avery, MacLeod &
McCarty (1944) and Hershey & Chase (1952).
🔹 Structure (Watson &
Crick Model, 1953)
- Double helix of two antiparallel
polynucleotide chains.
- Backbone: Sugar (deoxyribose) +
phosphate.
- Bases: Adenine (A), Thymine (T),
Cytosine (C), Guanine (G).
- Base pairing rule:
- A ↔ T (2 H-bonds)
- G ↔ C (3 H-bonds)
- Helix dimensions:
- One turn = 3.4 nm (10 base
pairs)
- Distance between two bases =
0.34 nm
🧩 Chargaff’s Rule: [A] = [T], [G] = [C];
purines = pyrimidines.
🧫 2. Experimental Proof of
DNA as Genetic Material
Griffith’s Experiment (1928):
- Streptococcus pneumoniae → S (smooth, virulent) and R
(rough, non-virulent).
- Heat-killed S + live R → dead
mouse.
👉 Transforming principle transferred genetic material.
Avery, MacLeod & McCarty (1944):
- DNA from S strain transformed R
→ S.
- DNA-destroying enzymes (DNase)
prevented transformation → DNA = transforming principle.
Hershey–Chase Experiment (1952):
- Used bacteriophage labeled
with:
- ³²P → DNA
- ³⁵S → Protein
- Only ³²P entered bacterial cell
→ proved DNA is the genetic material.
🧬 3. RNA — The Messenger
Molecule
- Ribonucleic Acid (RNA) — single-stranded.
- Sugar: Ribose
- Bases: A, U (Uracil), G, C
- Types:
- mRNA (messenger RNA) — carries
code.
- tRNA (transfer RNA) — brings
amino acids.
- rRNA (ribosomal RNA) — forms
ribosome structure.
⚙️ 4. DNA Packaging in Prokaryotes
& Eukaryotes
Prokaryotes:
- DNA — negatively charged,
circular, supercoiled with proteins.
Eukaryotes:
- DNA + histone proteins = nucleosome
(bead-on-string structure).
- Nucleosome → solenoid (30 nm
fibre) → chromatin → chromosome.
- Histones: H1, H2A, H2B, H3, H4
(octamer core).
🧩 5. DNA Replication
(Semi-Conservative Model)
Proposed by: Watson & Crick (1953)
Experiment by: Meselson and Stahl (1958) using ¹⁵N and ¹⁴N.
Steps:
1.
Unwinding: Helicase breaks H-bonds.
2.
Primer
binding: RNA
primers added.
3.
Elongation: DNA polymerase adds nucleotides (5′
→ 3′).
4.
Leading
& Lagging strands:
o Leading: continuous
o Lagging: Okazaki fragments
5.
Ligation: DNA ligase joins fragments.
🧠Result: Each new DNA has one parental and
one new strand.
🔤 6. Transcription (DNA →
RNA)
Enzyme: RNA polymerase
Stages:
1.
Initiation: Promoter signals start.
2.
Elongation: Complementary RNA synthesis.
3.
Termination: Terminator signals stop.
In Eukaryotes:
- RNA polymerase I → rRNA
- RNA polymerase II → mRNA
- RNA polymerase III → tRNA
- Post-transcriptional
modifications:
- Capping (5′ end), Tailing (3′
end), Splicing (removal of introns).
💬 7. Genetic Code
- Triplet code: 3 bases → 1 amino acid.
- Start codon: AUG (Methionine)
- Stop codons: UAA, UAG, UGA
- Degenerate: Multiple codons for one amino
acid.
- Universal: Same in all organisms.
🔄 8. Translation (mRNA →
Protein)
Components:
- mRNA — template
- tRNA — adaptor (anticodon
matches codon)
- Ribosomes — site of synthesis
Steps:
1.
Initiation
→ AUG codon starts synthesis.
2.
Elongation
→ Peptide bond formation.
3.
Termination
→ Stop codon ends synthesis.
🧠Enzyme: Peptidyl transferase (rRNA enzyme).
🧬 9. Regulation of Gene
Expression
Operon Concept (Jacob & Monod –
1961):
Example:
Lac Operon in E. coli
- LacZ: β-galactosidase
- LacY: Permease
- LacA: Transacetylase
- Operator: Switches transcription ON/OFF
- Repressor: Inhibits transcription when
lactose absent
- Inducer (lactose): Inactivates repressor → genes
transcribed.
🧩 Keywords: Promoter, Operator, Regulator,
Structural Genes.
🧫 10. Human Genome Project
(HGP)
- Started: 1990 | Completed: 2003
- Goal: Sequence entire human DNA (~3
× 10⁹ base pairs).
- Results:
- Humans have ~20,000–25,000
genes.
- <2% DNA codes for proteins.
- Repetitive sequences — major
part of non-coding DNA.
🧬 11. DNA Fingerprinting
Principle:
Based on polymorphism in DNA sequences (VNTRs —
Variable Number Tandem Repeats).
Technique by: Alec Jeffreys
(1985)
Steps:
1.
DNA
extraction
2.
Restriction
digestion
3.
Gel
electrophoresis
4.
Hybridization
with radioactive probes
5.
Autoradiography
Uses:
- Forensics, paternity disputes,
genetic disorders.
📈 12. Key NCERT Diagrams to
Practice
1.
Watson
and Crick DNA Double Helix
2.
DNA
Packaging in Eukaryote
3.
Semi-Conservative
DNA Replication (Meselson & Stahl Experiment)
4.
Lac
Operon Model
5.
Transcription
& Translation
6.
Human
Genome Project Summary Chart
🧠Quick Revision Table
|
Concept |
Key Points |
Enzyme/Scientist |
|
DNA Replication |
Semi-conservative |
Meselson & Stahl |
|
Transcription |
DNA → RNA |
RNA polymerase |
|
Translation |
RNA → Protein |
Ribosome |
|
Genetic Code |
Triplet, universal |
Nirenberg & Khorana |
|
Lac Operon |
Gene regulation |
Jacob & Monod |
|
HGP |
Genome sequencing |
International project |
|
DNA Fingerprinting |
VNTRs |
Alec Jeffreys |
📖 Important Definitions (as
per NCERT Keywords)
- Gene: Functional unit of inheritance
coding for a polypeptide.
- Codon: Triplet of bases on mRNA
coding for an amino acid.
- Genome: Total genetic material of an
organism.
- Mutation: Sudden heritable change in DNA
sequence.
- Recombinant DNA: Artificially joined DNA from
different sources.
- Exons: Coding sequences in eukaryotic
genes.
- Introns: Non-coding sequences removed
during splicing.
✨ Super Quick NEET + Board Tips
✅ Focus Diagrams: DNA structure, Lac operon,
Transcription-Translation
✅ Memorize Enzymes: Helicase, Polymerase, Ligase, Peptidyl Transferase
✅ Revise Scientists: Griffith, Avery, Hershey–Chase, Meselson–Stahl, Nirenberg,
Jacob & Monod
✅ Read NCERT Lines — keywords like transforming principle, semiconservative,
inducible operon
