Molecular Basis of Inheritance | Class 12 Biology Notes

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The Molecular Basis of Inheritance explains how DNA stores, replicates, and transfers genetic information from one generation to the next. This chapter is very important for NEET Biology and Class 12 CBSE exams, as many questions are directly asked from it.


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🔹 What is Genetic Material?

Genetic material is the substance that carries information from one generation to the next.

Griffith’s Experiment (1928): Discovered the “Transforming Principle” using Streptococcus pneumoniae bacteria.

Avery, MacLeod, and McCarty (1944): Proved DNA is the genetic material.

Hershey and Chase Experiment (1952): Confirmed DNA is the genetic material using bacteriophages.


👉 DNA is the molecule of inheritance.

High CPC keywords: DNA as genetic material, Griffith experiment, Hershey Chase experiment, class 12 genetics notes


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🔹 Structure of DNA

Discovered by Watson and Crick (1953).

Double helix model: Two strands of DNA run anti-parallel.

Nucleotides: Made of sugar (deoxyribose), phosphate, and nitrogen bases (A, T, G, C).

Base pairing: A–T, G–C (complementary).


High CPC keywords: DNA structure, double helix model, nucleotides in DNA, Watson and Crick


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🔹 Packaging of DNA

In prokaryotes: DNA is circular and not packed in histones.

In eukaryotes: DNA is wrapped around histone proteins forming nucleosomes (beads-on-string model).



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🔹 DNA Replication

Process of making an exact copy of DNA.

Semi-conservative (proved by Meselson and Stahl experiment).

Steps:

1. Unwinding by helicase


2. Formation of replication fork


3. Synthesis of new strands (leading & lagging strand)


4. DNA ligase joins Okazaki fragments




👉 This ensures accurate transfer of genetic material.

High CPC keywords: DNA replication, semi-conservative replication, Meselson Stahl experiment, NEET biology


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🔹 RNA and Types

RNA is single-stranded and helps in protein synthesis.

mRNA: Carries genetic code.

tRNA: Brings amino acids.

rRNA: Forms ribosomes.



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🔹 Central Dogma of Molecular Biology

Proposed by Francis Crick:
DNA → RNA → Protein

This explains how genetic information flows.


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🔹 Transcription (DNA → RNA)

Takes place in the nucleus.

RNA polymerase reads DNA and forms mRNA.

In eukaryotes, mRNA undergoes splicing, capping, and tailing.



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🔹 Translation (RNA → Protein)

Takes place in the ribosomes.

mRNA codons are read by tRNA anticodons.

Amino acids are joined to form proteins.


👉 This is how DNA controls traits by forming proteins.

High CPC keywords: transcription, translation, protein synthesis, genetic code, RNA


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🔹 Regulation of Gene Expression

Genes can be switched ON/OFF.

Lac Operon (in bacteria): Classic example of gene regulation.



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🔹 Human Genome Project

International project (1990–2003).

Sequenced entire human DNA.

Helped in medical research, disease diagnosis, and biotechnology.



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🔹 NEET & CBSE Exam Tips

Practice diagrams of DNA, transcription, translation.

Revise experiments (Griffith, Hershey-Chase, Meselson-Stahl).

Learn genetic code properties (triplet, universal, degenerate).

Solve NEET previous year questions.


👉 Example NEET Question:
Q: DNA replication is called semi-conservative because?
Ans: Each daughter DNA has one old and one new strand.


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Final Words

The Molecular Basis of Inheritance chapter connects DNA, RNA, and proteins with traits. For Class 12 Biology and NEET, focus on experiments, diagrams, and genetic code. Mastering this will give you an edge in competitive exams.

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