Molecular Basis of Inheritance

'DNA is the MOLECULE OF LIFE — it stores the blueprint for EVERY living organism, from bacteria to blue whales.'

1. Chapter Overview

This chapter explores the MOLECULAR mechanisms underlying inheritance. Topics include: DNA as GENETIC MATERIAL (Griffith, Avery-MacLeod-McCarty, Hershey-Chase experiments), the STRUCTURE OF DNA (Watson-Crick model), DNA REPLICATION (semiconservative replication, Meselson-Stahl experiment), TRANSCRIPTION (RNA synthesis), the GENETIC CODE (codon table, degenerate code), TRANSLATION (protein synthesis), GENE REGULATION (Lac operon model), HUMAN GENOME PROJECT, and DNA FINGERPRINTING.

2. The Search for Genetic Material

Griffith's Transformation Experiment (1928)

• Bacteria: S strain (VIRULENT — smooth, with capsule) and R strain (NON-VIRULENT — rough, no capsule).
• Key result: Heat-killed S strain + Live R strain → LIVE S strain (mice died). 'Something from dead S bacteria TRANSFORMED the R bacteria into S. That "something" was DNA.'

Avery, MacLeod, McCarty (1944)

• Showed that the TRANSFORMING PRINCIPLE was DNA — by systematically destroying proteins, RNA, and DNA. Only when DNA was destroyed, transformation did NOT occur.

Hershey-Chase Experiment (1952)

• Bacteriophage T₂: Virus that infects bacteria. Contains ONLY DNA and protein.
• Method: Labelled phage proteins with ³⁵S (sulfur) and DNA with ³²P (phosphorus). Infected E. coli.
• Result: ³²P (DNA) entered bacterial cells and was transmitted to progeny phages. ³⁵S (protein) stayed OUTSIDE.
• 'Conclusion: DNA — NOT protein — is the GENETIC MATERIAL.'

3. Structure of DNA (Watson-Crick Model, 1953)

Key Features

• Double helix: TWO polynucleotide chains wound around each other.
• Antiparallel: One chain runs 5'→3', the other 3'→5'.
• Base pairing: A=T (TWO hydrogen bonds). G≡C (THREE hydrogen bonds).
• Sugar-phosphate backbone: On the OUTSIDE. Bases on the INSIDE.
• Diameter: 2 nm. Distance between base pairs: 0.34 nm. One full turn: 3.4 nm (10 base pairs).
• 'The double helix structure EXPLAINS both replication and information storage.'

Packaging of DNA

• Prokaryotes: Circular DNA — LOOPED and supercoiled (nucleoid-associated proteins).
• Eukaryotes: Linear DNA wound around HISTONE PROTEINS — forming NUCLEOSOMES (octamer of histones H2A, H2B, H3, H4 + 146 bp of DNA). Chromatin → Chromosomes.

4. DNA Replication

Semiconservative Replication

• 'Each daughter DNA molecule has ONE parental strand and ONE newly SYNTHESISED strand.'
• Meselson-Stahl experiment (1958) : Grew E. coli in ¹⁵N (heavy) medium, then transferred to ¹⁴N (light) medium. After one generation: DNA had INTERMEDIATE density (one ¹⁵N + one ¹⁴N) — confirming SEMICONSERVATIVE replication.

Enzymes and Process

1. Helicase: UNWINDS the DNA double helix.
2. DNA primase: Adds RNA PRIMER (short RNA sequence).
3. DNA polymerase III: Adds NEW nucleotides in 5'→3' direction.
   • LEADING STRAND: CONTINUOUS synthesis (ONE primer needed).
   • LAGGING STRAND: DISCONTINUOUS — OKAZAKI FRAGMENTS (many primers needed).
4. DNA polymerase I: Removes RNA primers and fills gaps.
5. DNA ligase: JOINS Okazaki fragments.

5. Transcription (DNA → RNA)

• RNA polymerase: Binds to the PROMOTER region, unwinds DNA, and synthesises RNA in 5'→3' direction.
• Prokaryotes: ONE RNA polymerase synthesises all types of RNA.
• Eukaryotes: THREE RNA polymerases — RNA Pol I (rRNA), Pol II (mRNA), Pol III (tRNA).
• Processing of pre-mRNA (eukaryotes): 5' CAPPING (methylguanosine), 3' POLYADENYLATION (poly-A tail), SPLICING (removal of INTRONS, joining of EXONS).
• 'The discovery of splicing WON the Nobel Prize — it showed that genes are NOT continuous in eukaryotes.'

6. The Genetic Code

Characteristics

1. TRIPLET: Three nucleotides code for ONE amino acid.
2. DEGENERATE: Multiple codons can code for the same amino acid (except Methionine and Tryptophan).
3. UNIVERSAL: Same code in ALL living organisms (nearly).
4. NON-OVERLAPPING: Read sequentially from START codon (AUG = Methionine).
5. NON-AMBIGUOUS: Each codon specifies ONLY ONE amino acid.
6. STOP codons: UAA, UAG, UGA — signal termination of translation.

Key finding (Nirenberg & Matthaei, 1961) : 'The first codon (UUU = Phenylalanine) was DECODED using synthetic RNA.'

7. Translation (RNA → Protein)

Steps

1. ACTIVATION: Amino acid + ATP → aminoacyl-tRNA (by aminoacyl-tRNA synthetase).
2. INITIATION: Small ribosomal subunit + mRNA + Initiator tRNA (Met) → Large subunit binds.
3. ELONGATION: tRNA brings amino acid to A site → Peptide bond forms → Ribosome moves (translocation).
4. TERMINATION: STOP codon reached → Release factor binds → Polypeptide RELEASED.

• Polyribosomes: MULTIPLE ribosomes translating the SAME mRNA simultaneously — EFFICIENT protein production.

8. Gene Regulation — Lac Operon (Jacob-Monod Model)

• 'The Lac operon is a MODEL for understanding how genes are TURNED ON and OFF.'
• Components: Promoter (P), Operator (O), Structural genes (lacZ, lacY, lacA).
• Regulation:
  • NO lactose: Repressor protein BINDS to operator → BLOCKS RNA polymerase → Genes OFF.
  • Lactose present: Allolactose (INDUCER) binds to repressor → Repressor RELEASES from operator → RNA polymerase TRANScribes → Genes ON.
• 'The lac operon is an INDUCIBLE system — the genes are OFF until INDUCED by the presence of lactose.'

9. Human Genome Project (HGP)

• Goal: Sequence the ENTIRE human genome (~3 billion base pairs).
• Completed: 2003 (13 years, ~$3 billion).
• Key findings:
  • Humans have ~20,000-25,000 protein-coding genes (FEWER than expected).

  • 98% of DNA is NON-CODING ('JUNK DNA' — much of it is regulatory).

  • Human genomes are 99.9% IDENTICAL — only 0.1% accounts for individual variation.
• Methods: Expressed Sequence Tags (ESTs), Shotgun sequencing, and Bioinformatics.

10. DNA Fingerprinting

• Principle: EXCEPT for identical twins, EVERY individual has UNIQUE satellite DNA sequences (Variable Number Tandem Repeats — VNTRs).
• Steps: (1) DNA isolation. (2) Digestion with restriction enzymes. (3) Gel electrophoresis. (4) Southern blotting. (5) Hybridisation with radioactive probe. (6) Autoradiography.
• Applications: Forensic science, paternity testing, immigration disputes, wildlife conservation, and historical investigations.

11. Common Mistakes

1. DNA replication is SEMICONSERVATIVE: Each daughter molecule has ONE old and ONE new strand — NOT one whole daughter being old and the other new (that would be CONSERVATIVE).
2. RNA primer is synthesised by PRIMASE: NOT by DNA polymerase. Primase is an RNA polymerase.
3. Genetic code is read 5'→3': The codon in mRNA is read from the 5' end to the 3' end.
4. All RNA is NOT mRNA: There are THREE main types: mRNA (messenger), tRNA (transfer), and rRNA (ribosomal).

12. CBSE Exam Focus

1. DNA as genetic material — Griffith, Avery, Hershey-Chase experiments
2. Watson-Crick model — structure, base pairing, antiparallel nature
3. DNA replication — Meselson-Stahl, enzymes, leading and lagging strands
4. Transcription and genetic code — codons, degeneracy, start and stop codons
5. Translation — ribosome, tRNA, amino acid activation
6. Lac operon — components, induction, repression
7. Human Genome Project and DNA fingerprinting

13. Self-Test

Q1: Why was the Hershey-Chase experiment decisive in proving DNA as genetic material? A1: Hershey and Chase used RADIOACTIVE LABELLING — ³⁵S (protein) and ³²P (DNA). Only ³²P (DNA) entered bacterial cells and was inherited by progeny phages. This CONCLUSIVELY showed that DNA — not protein — is the genetic material.

Q2: What is semiconservative replication? Describe the Meselson-Stahl experiment. A2: Semiconservative replication: each daughter DNA has one parental strand and one newly synthesised strand. Meselson-Stahl grew E. coli in ¹⁵N, transferred to ¹⁴N. After one generation, DNA was HYBRID (¹⁵N-¹⁴N) — proving semiconservative replication.

Q3: How many amino acids would a polypeptide with 150 codons have? A3: 150 codons → 150 amino acids (assuming AUG is the start and there is a stop codon). Excluding stop: 149 amino acids. 'Each codon codes for ONE amino acid.'

Q4: What is the function of the Lac operon repressor? A4: The Lac repressor BINDS to the OPERATOR region in the ABSENCE of lactose — PHYSICALLY blocking RNA polymerase from transcribing the structural genes (lacZ, lacY, lacA). When lactose (allolactose) binds to the repressor, it CHANGES SHAPE and RELEASES from the operator — allowing transcription.

Q5: What is the significance of VNTRs in DNA fingerprinting? A5: VNTRs (Variable Number Tandem Repeats) are UNIQUE to each individual (except identical twins). The NUMBER of repeats at specific loci varies between people — producing a DISTINCT banding pattern after electrophoresis and hybridisation. Used for identification.

14. Conclusion

The molecular basis of inheritance is the HEART of modern biology:

• DNA: 'The DOUBLE HELIX — elegant, simple, and capable of storing ENORMOUS amounts of information.'
• CENTRAL DOGMA: 'DNA → RNA → Protein. The flow of genetic information is ONE-WAY in most organisms.'
• REGULATION: 'Genes are NOT always ON — the Lac operon shows how bacteria ADAPT to changing environments.'
• 'From the discovery of the double helix to the sequencing of the human genome — molecular biology has transformed our understanding of LIFE itself.'