Molecular Basis of Inheritance – Class 12 Biology Notes | CBSE NCERT Based

 

📚 Chapter: Molecular Basis of Inheritance – Detailed Notes

---

🔸 1. Introduction

This chapter explains how genetic information is stored, replicated, and expressed in living organisms through DNA and RNA.

---

🔹 2. DNA – The Genetic Material

Full form: Deoxyribonucleic Acid

Found in nucleus (eukaryotes), cytoplasm (prokaryotes)

Proved by:

✅ Griffith's Transformation Experiment

✅ Avery-MacLeod-McCarty experiment

✅ Hershey-Chase experiment (Bacteriophage with radioactive sulfur and phosphorus)

---

🔹 3. Structure of DNA

Double helix model by Watson and Crick

Each nucleotide = phosphate + deoxyribose sugar + nitrogenous base

Nitrogenous bases:

Purines: Adenine (A), Guanine (G)

Pyrimidines: Cytosine (C), Thymine (T)

Base pairing: A = T (2 H-bonds), G ≡ C (3 H-bonds)

Antiparallel strands (5′ → 3′ and 3′ → 5′)

---

🔹 4. Packaging of DNA

In prokaryotes: Looped and held by non-histone proteins

In eukaryotes: Histone proteins form nucleosomes

– "Beads on string" structure

---

🔹 5. RNA – The Messenger

Single-stranded

Sugar: Ribose

Bases: A, G, C, U (Uracil) instead of T

Types:

mRNA – Messenger RNA

tRNA – Transfer RNA

rRNA – Ribosomal RNA

---

🔹 6. DNA Replication

Semiconservative model by Meselson and Stahl (N15 and N14)

Enzymes involved:

Helicase: Unwinds DNA

DNA Polymerase: Synthesizes new strand

Ligase: Joins Okazaki fragments on lagging strand

---

🔹 7. Central Dogma

Proposed by Crick

Information flows:

DNA → RNA → Protein

---

🔹 8. Transcription (DNA → RNA)

Promoter: Initiation site

Template strand: Makes RNA

Enzyme: RNA Polymerase

In eukaryotes: hnRNA → mRNA (by splicing, capping, tailing)

---

🔹 9. Genetic Code

Triplet codons (3 bases = 1 amino acid)

64 codons total

20 amino acids

Start codon: AUG (Methionine)

Stop codons: UAG, UAA, UGA

Features: Universal, non-overlapping, degenerate

---

🔹 10. Translation (RNA → Protein)

mRNA carries code

tRNA has anticodon + brings amino acids

rRNA helps in ribosome structure

Process: Initiation → Elongation → Termination

---

🔹 11. Regulation of Gene Expression

In prokaryotes: Lac operon model

Genes turned on/off in presence/absence of lactose

Components:

Regulatory gene (i)

Promoter (P)

Operator (O)

Structural genes (Z, Y, A)

---

🔹 12. Human Genome Project (HGP)

Started in 1990, completed in 2003

Sequenced entire human DNA (~3.3 billion base pairs)

Found ~30,000 genes

Objective: To map and understand all human genes

---

🔹 13. DNA Fingerprinting

Technique used for identity, crime detection, paternity test

Based on VNTRs (Variable Number Tandem Repeats)

Developed by

 Alec Jeffreys

---

📌 Conclusion:

This chapter reveals the molecular secrets of life. Understanding DNA, RNA, and protein synthesis is key to all genetics, medicine, and biotechnology studies.

🖼 Important Diagrams – Molecular Basis of Inheritance

1. ✅ Structure of DNA double helix (Watson & Crick model)

2. ✅ Packaging of DNA (Nucleosome model)

3. ✅ Meselson-Stahl experiment diagram

4. ✅ DNA replication process (Leading/Lagging strands)

5. ✅ Transcription and translation schematic

6. ✅ Genetic code table

7. ✅ Lac operon model

8. ✅ DNA fingerprinting technique

9. ✅ tRNA structure (Clover leaf model)

10.

 ✅ Central Dogma flow chart

✍️ Written by: Shahid – Study with Shahid

🔗 Visit: https://shahidnotes12.blogspot.com