Chapter 6 (Molecular Basis of Inheritance)

the genius April 07, 2026

Molecular Basis of Inheritance: Class 12 Biology Chapter 6 Full Guide

Pichle chapter mein humne padha ki "Factors" (Genes) characters ko control karte hain. Is chapter mein hum un 'Factors' ke molecular structure yani DNA ke baare mein baat karenge. Yeh chapter NEET aur Board exams ke liye "Gold Mine" hai kyunki isse har saal kaafi zyada sawal aate hain.

1. DNA: The Genetic Material

DNA (Deoxyribonucleic Acid) ek lamba polymer hai jo Nucleotides se bana hota hai.

DNA ka Structure (Watson & Crick Model):

  1. Double Helix: DNA do ladiyon (strands) se bana hota hai jo aapas mein coiled hoti hain.

  2. Backbone: Yeh Sugar aur Phosphate se bani hoti hai.

  3. Nitrogenous Bases: * Purines: Adenine (A) aur Guanine (G).

    • Pyrimidines: Cytosine (C) aur Thymine (T).

  4. Pairing: A hamesha T ke saath judta hai (2 Hydrogen bonds), aur G hamesha C ke saath (3 Hydrogen bonds).

2. Packaging of DNA Helix

Ek insaan ki cell mein DNA ki lambai lagbhag 2.2 meters hoti hai. Itne bade DNA ko ek chhote se nucleus mein pack karne ke liye Histone Proteins ki zaroori hoti hai.

  • DNA (negatively charged) Histone Octamer (positively charged) ke charon taraf lapta hota hai, jise Nucleosome kehte hain.

  • Yahi aage chalkar Chromatin aur Chromosomes banate hain.

3. Search for Genetic Material

Yeh prove karne ke liye ki DNA hi genetic material hai, teen main experiments hue:

  1. Griffith’s Transforming Principle: Mice aur Streptococcus pneumoniae par experiment.

  2. Avery, MacLeod, and McCarty: Griffith ke experiment ka chemical nature bataya.

  3. Hershey-Chase Experiment: Bacteriophage ka use karke prove kiya ki DNA hi genetic material hai (Sulphur vs Phosphorus).

4. RNA World aur Replication

RNA (Ribonucleic Acid) pehla genetic material tha. Lekin DNA zyada stable hai, isliye evolution ke dauran DNA ne jagah le li.

DNA Replication:

DNA se apne jaisa dusra DNA banana.

  • Yeh Semi-conservative hota hai (Matlab ek strand purana hota hai aur ek naya).

  • Ise Messelson and Stahl ne prove kiya tha.

  • Ismein DNA Polymerase enzyme sabse main role nibhata hai.

5. Central Dogma of Molecular Biology

Francis Crick ne bataya ki information ka flow aise hota hai:

DNA $\rightarrow$ RNA $\rightarrow$ Protein

A. Transcription (DNA se RNA banna):

DNA ki ek strand se RNA ki copy banana. Ismein RNA Polymerase enzyme kaam aata hai.

  • Introns: Non-coding parts (jinhe nikal diya jata hai).

  • Exons: Coding parts.

B. Translation (RNA se Protein banna):

mRNA mein chhupi "Code" ki language ko amino acids (protein) mein badalna. Yeh Ribosomes ke andar hota hai.

Getty Images

6. Genetic Code

Amino acids ko code karne wale triplets (3 bases) ko Codons kehte hain.

  • Total 64 codons hote hain.

  • AUG start codon hai.

  • UAA, UAG, UGA stop codons hain.

7. Regulation of Gene Expression (Lac Operon)

Har waqt saare genes kaam nahi karte. Jacob aur Monod ne E. coli mein Lac Operon model diya.

  • Jab lactose (sugar) maujood hota hai, tabhi genes on hote hain taaki lactose ko digest kiya ja sake.

8. Human Genome Project (HGP) aur DNA Fingerprinting

  • HGP: Insaan ke poore DNA ki sequence maloom karne ka "Mega Project".

  • DNA Fingerprinting: Alec Jeffreys ne ise shuru kiya tha. Iska use apradhiyon (criminals) ko pakadne aur paternity tests (baap-bete ka rishta) ke liye kiya jata hai. Ismein VNTRs ka use hota hai.

Conclusion

Chapter 6 lamba zaroori hai lekin agar aap iska flow samajh lein (DNA $\rightarrow$ RNA $\rightarrow$ Protein), toh yeh aapke liye sabse scoring chapter ban jayega. DNA Replication aur Lac Operon ke diagrams ki practice zaroor karein.

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