🧬 Chapter 5: Molecular Basis of Inheritance – Class 12 Biology Notes | NCERT + NEET Focus

 

🌸 Chapter 5

Molecular Basis Of Inheritance

🧬 1. Introduction

Every living organism passes its traits from one generation to the next. This transmission of genetic information is controlled by DNA — the molecule of inheritance.
This chapter explains the structure of DNA, replication, transcription, translation, genetic code, gene expression regulation, Human Genome Project and DNA Fingerprinting.

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🧫 2. The DNA

DNA is a long polymer of deoxyribonucleotides.The length of DNA is usually defined as the number ofnucleotides(or a pair of nucleotide referred to as base pairs) present in it .This is also the characteristic of an organism.

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Discovery

• Friedrich Miescher (1869) discovered a substance called nuclein (now known as DNA) from pus cells.
• Later, DNA was found to be the genetic material in most organisms (except some viruses which have RNA).

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🧩 3. Chemical Structure of Polycucleotide Chain(DNA/RNA):

Both DNA and RNA are polynucleotides, i.e., chains of repeating units called nucleotides.

Each Nucleotide has 3 components:

1.   Nitrogenous base

o   Purines: Adenine (A), Guanine (G)

o   Pyrimidines: Cytosine (C), Thymine (T in DNA) or Uracil (U in RNA)

2.   Pentose sugar

o   Deoxyribose in DNA

o   Ribose in RNA

3.   Phosphate group

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Nitrogenous bases:

There are two types of nitrogenous bases-Purines(Adenine and Guanine), and 

Pyrimidines(Cytosine,Uracil and Thymine)

Cytosine is common for both DNA and RNA and Thymine is present in DNA .Uracil is present in RNA.

A nitrogenous base is linked to the OH of 1`C pentose sugar through a N-glycosidic linkage to form a nucleoside, such as adenosine or deoxyadenosine , guanosine or deoxyguanosine.

When a phosphate group is linked to OH of 5`C of a nucleoside through phosphoester linkage.a corresponding nucleotide is formed.

Two nucleotides are linked through 3`-5` phosphodiester linkage to form a dinucleotide.

More nucleotides can be joined in such a manner to form a polynucleotide chain.

A polymer thus formed has at one end a free phosphate moiety at 5` -end of sugar, which is referred to as  5`-end of polynucleotide chain.

Similarly , at the other end of the polymer the sugar has a free OH of 3`C group which is referred to as  3`-end of polynucleotide chain.

The backbone of a polynucleotide chain is formed due to sugar and phospahtes.

The nitrogenous bases linked to sugar moiety project from the backbone.

Fig : A polynucleotide chain

⚛️ 4. Structure of DNA (Watson and Crick Model – 1953)

Main Features:

• DNA is a double-stranded helical structure.
• Each strand is made of polynucleotide chains.
• The two strands are complementary and antiparallel.
• The strands are held by hydrogen bonds between nitrogenous bases:
• A pairs with T (via 2 H-bonds)
• G pairs with C (via 3 H-bonds)
• Sugar-phosphate backbone lies on the outside; bases face inward.
• Pitch of helix: 3.4 nm
• Distance between two bases: 0.34 nm
• Number of base pairs per turn: 10

🧠 NEET Tip: The total length of DNA in a human cell = 2.2 meters (approx).

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🧪 5. Types of DNA

Type	Strands	Base pairs/turn	Helix	Found in
A-DNA	Double	11	Right-handed	Dehydrated condition
B-DNA	Double	10	Right-handed	Most common form
Z-DNA	Double	12	Left-handed	Rich in GC pairs

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🧬 6. Packaging of DNA

Since the DNA length (~2.2 m) is much longer than the cell nucleus (~6 μm), it must be packaged compactly.

In Prokaryotes (Bacteria):

• DNA is circular and naked, not associated with histones.
• Forms a nucleoid region attached to proteins.

In Eukaryotes:

• DNA is complexed with positively charged histone proteins to form nucleosomes.
• Each nucleosome = DNA + 8 histone proteins (H2A, H2B, H3, H4 ×2 each)
  → Held by linker DNA (~200 bp).

These nucleosomes coil further to form chromatin → condense into chromosomes during cell division.

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🔁 7. The Search for Genetic Material

Griffith’s Experiment (1928)

• Worked on Streptococcus pneumoniae (R and S strains).
• Observation: Heat-killed S strain + live R strain → killed mice.
• Concluded: Some transforming principle converted R → S type.

Avery, MacLeod & McCarty (1944)

• Treated the transforming substance with enzymes that destroy DNA, RNA, or proteins.
• Only when DNA was destroyed, transformation didn’t occur → DNA is the transforming principle.

Hershey & Chase Experiment (1952)

• Used bacteriophage (T2 virus) with radioactive Sulphur (³⁵S) and Phosphorus (³²P).
• ³²P-labeled DNA entered bacteria → proved DNA is genetic material.

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🧬 8. RNA World

• RNA was the first genetic material (can store information + catalyze reactions).
• DNA evolved later — more stable and suitable for long-term storage.

🧠 NEET Tip: RNA acts as both genetic material (in some viruses) and catalyst (e.g., ribozymes).

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🔁 9. Replication of DNA (Semi-conservative Model)

Meselson and Stahl Experiment (1958)

• Used E. coli with heavy nitrogen (¹⁵N) and light nitrogen (¹⁴N).
• After one generation in ¹⁴N, DNA was intermediate → proved semi-conservative replication.

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Process of DNA Replication

1.   Initiation:

o   DNA unwinds by helicase enzyme, forming a replication fork.

2.   Elongation:

o   DNA polymerase adds nucleotides only in 5′ → 3′ direction.

o   Leading strand synthesized continuously.

o   Lagging strand synthesized in short fragments (Okazaki fragments).

3.   Termination:

o   Fragments joined by DNA ligase.

🧠 NEET Tip:

• DNA polymerase III is the main enzyme in prokaryotes.
• Requires primer (RNA) to start synthesis.

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🧬 10. Transcription (DNA → RNA)

Enzyme: RNA Polymerase

Steps:       

1.   Initiation: RNA polymerase binds to promoter region.

2.   Elongation: RNA chain grows complementary to DNA template.

3.   Termination: RNA polymerase detaches at the terminator sequence.

In Eukaryotes:

• 3 types of RNA polymerases:
• RNA pol I: rRNA
• RNA pol II: mRNA
• RNA pol III: tRNA, snRNA

🧠 Post-transcriptional Modifications (in eukaryotes):

• Addition of 5′ cap (methyl guanosine triphosphate)
• Poly-A tail at 3′ end
• Splicing (removal of introns)

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🔡 11. Genetic Code

Features:

• Triplet code: 3 bases = 1 codon → 1 amino acid.
• Degenerate: One amino acid may have multiple codons.
• Unambiguous: Each codon specifies only one amino acid.
• Universal: Same code in all organisms.
• Start codon: AUG (Methionine)
• Stop codons: UAA, UAG, UGA

🧠 NEET Tip: 64 possible codons; 61 code for amino acids, 3 are stop codons.

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⚙️ 12. Translation (Protein Synthesis)

Steps:

1.   Activation: Amino acids attach to tRNA (charging).

2.   Initiation: Ribosome binds to mRNA at start codon (AUG).

3.   Elongation: Peptide bonds form between amino acids.

4.   Termination: Stop codon signals release of polypeptide.

Key Molecules:

• mRNA: Carries codon sequence.
• tRNA: Brings amino acid (anticodon complementary to codon).
• rRNA: Catalytic part of ribosome.

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🔬 13. Regulation of Gene Expression

Lac Operon Model (in E. coli)

• Proposed by Jacob and Monod (1961).

Components:

1.   Structural genes (Z, Y, A) → Enzymes for lactose metabolism.

2.   Promoter (P) → Binding site for RNA polymerase.

3.   Operator (O) → Binding site for repressor.

4.   Regulator gene (i) → Produces repressor protein.

Working:

• In absence of lactose → Repressor binds to operator → Transcription OFF.
• In presence of lactose → Repressor inactivated → Transcription ON.

🧠 NEET Tip: The lac operon is an example of inducible operon.

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🧬 14. Human Genome Project (HGP)

Goals:

• Identify all human genes (~20,000–25,000).
• Determine entire base sequence (3 billion bp).
• Store data for research.

Results:

• Less than 2% DNA codes for proteins.
• Repetitive sequences found in large amount.
• Genes are unevenly distributed.

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🧫 15. DNA Fingerprinting

Principle:

• Based on polymorphism in DNA sequence.
• Uses VNTRs (Variable Number Tandem Repeats).

Steps:

1.   DNA extraction

2.   Restriction enzyme digestion

3.   Gel electrophoresis

4.   Southern blotting

5.   Hybridization with radioactive probes

6.   Autoradiography

Uses:

• Paternity testing
• Criminal identification
• Evolutionary studies

🧠 Scientist: A. J. Jeffreys (1985)

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📚 16. Important Definitions

Term	Definition
Gene	Functional unit of inheritance
Genome	Total genetic material in an organism
Transcriptome	All RNA molecules transcribed from genome
Mutation	Sudden change in DNA sequence
Exon	Coding region of a gene
Intron	Non-coding region removed during splicing

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🧠 17. Key Diagrams (from NCERT)

1.   Watson–Crick Model of DNA

2.   Packaging of DNA (Nucleosome structure)

3.   Semi-conservative replication

4.   Transcription unit

5.   Lac Operon model

(In class, practice labeling these diagrams perfectly; they are often asked for 3–5 marks.)

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🎯 18. Important NCERT Keywords

• Polynucleotide chain
• Nucleosome
• Replication fork
• Okazaki fragment
• Promoter, Operator, Repressor
• Semi-conservative
• Genetic code
• Operon
• Polymorphism
• VNTRs

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🧩 19. High-Yield Topics for NEET & Boards

✅ Hershey & Chase Experiment
✅ DNA Packaging
✅ Meselson–Stahl Experiment
✅ Central Dogma
✅ Transcription & Translation
✅ Genetic Code (Properties + Exceptions)
✅ Lac Operon
✅ DNA Fingerprinting
✅ RNA World Hypothesis

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🧾 20. Summary (Quick Revision Sheet)

• DNA → genetic material in most organisms.
• Double helix: A–T (2 H bonds), G–C (3 H bonds).
• Replication: Semi-conservative, 5′→3′ direction.
• Transcription → RNA; Translation → Protein.
• Genetic code → Triplet, universal, degenerate.
• Lac operon → Gene regulation model.
• HGP → Mapped entire human genome.
• DNA fingerprinting → Based on VNTR polymorphism.

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