Molecular Basis of Inheritance

Key Concepts

The DNA (Deoxyribonucleic Acid)

DNA is a long polymer of deoxyribonucleotides.

• Structure of Polynucleotide Chain:
  • Nucleotide: Composed of a nitrogenous base (Purines: A, G; Pyrimidines: C, T/U), a pentose sugar (Deoxyribose/Ribose), and a phosphate group.
  • Linkages: N-glycosidic linkage (base to sugar), Phosphoester linkage (phosphate to sugar), and 3'-5' phosphodiester linkage (joining nucleotides).
• Double Helix Model (Watson and Crick, 1953):
  • Two polynucleotide chains with anti-parallel polarity (5'→3' and 3'→5').
  • Bases pair via H-bonds: A=T (2 bonds), G≡C (3 bonds).
  • Right-handed coiling; pitch = 3.4 nm; 10 bp per turn.
• Packaging of DNA:
  • Prokaryotes: DNA held by positive proteins in a 'nucleoid'.
  • Eukaryotes: DNA wraps around a histone octamer (rich in lysine and arginine) to form a nucleosome (200 bp).
  • Chromatin: 'Beads-on-string' structure. Euchromatin (loose, active) and Heterochromatin (dense, inactive).

The Search for Genetic Material

• Transforming Principle (Griffith, 1928): Observed transformation of R-strain Streptococcus pneumoniae into virulent S-strain when mixed with heat-killed S-strain.
• Biochemical Characterisation (Avery, MacLeod, McCarty): Proved DNA is the transforming substance.
• Hershey-Chase Experiment (1952): Unequivocal proof using radioactive isotopes (32P for DNA, 35S for protein) in bacteriophages; DNA entered the bacteria, not protein.

RNA World

• RNA was the first genetic material.
• Functions as a messenger, adapter, and catalyst (ribozymes).
• DNA evolved from RNA for better stability.

DNA Replication

• Semiconservative Mode: Proposed by Watson and Crick; experimentally proven by Meselson and Stahl (1958) using 15N isotope in E. coli.
• Enzymes:
  • DNA Polymerase: Synthesizes new strands in 5'→3' direction.
  • Helicase: Unwinds DNA.
  • DNA Ligase: Joins Okazaki fragments on the lagging strand.
• Replication Fork: Site of active replication. One strand is continuous (leading), the other discontinuous (lagging).

Transcription

Process of copying genetic information from DNA to RNA.

• Transcription Unit: Promoter, Structural gene, and Terminator.
• Strands: Template strand (3'→5') and Coding strand (5'→3').
• Processing (Eukaryotes): Splicing (removing introns), Capping (5'-end), and Tailing (3'-end).

Genetic Code

• Triplet Codon: 64 codons total; 61 code for amino acids, 3 are stop codons (UAA, UAG, UGA).
• Features: Degenerate (many codons for one amino acid), Universal, Non-overlapping, and AUG (dual role: Methionine and Initiator).

Translation

Process of protein synthesis.

• Charging of tRNA: Aminoacylation using ATP.
• Ribosome: The cellular factory for protein synthesis.
• Steps: Initiation (AUG recognized), Elongation (peptide bond formation), and Termination (release factor).

Regulation of Gene Expression

• Lac Operon (Jacob and Monod):
  • Regulatory Gene (i): Codes for repressor.
  • Structural Genes: z (β-gal), y (permease), a (transacetylase).
  • Inducer: Lactose (inactivates the repressor).
  • Negative Regulation: Repressor protein prevents transcription.

Human Genome Project (HGP)

• Launched in 1990, completed in 2003.
• Goals: Sequence 3 billion bp, identify ~20,000-25,000 genes, and address ELSI.
• Findings: Genome has 3164.7 million bp; < 2% codes for proteins; SNP (1.4 million locations).

DNA Fingerprinting

• Technique to identify variations at the DNA level using Polymorphism.
• Uses VNTR (Variable Number of Tandem Repeats) as probes.
• Steps: Isolation, Digestion, Electrophoresis, Blotting, Hybridization, and Autoradiography.