The molecular basis of inheritance

Miescher (1869) – Discovery of Nucleic Acid

• Used pus from war bandages (rich in white blood cells)

• Isolated nuclei → extracted material rich in phosphorus (P) and nitrogen (N)

• Named it “nuclein” (later shown to be DNA)

• Found:

  • P-rich acidic fraction → nucleic acid

  • N-rich fraction → protein

Components of Nucleic Acids

• Five-carbon sugar

• Nitrogenous base

• Phosphate group

Two types:

• DNA (Deoxyribose) – in chromosomes

• RNA (Ribose) – in nucleoplasm but not in chromosomes

Griffith's Experiment (1928)

• R strain (benign, no capsule)

• S strain (virulent, capsule)

• Heat-killed S + live R → mice died

• R transformed into S-type → "transforming principle"

• Identity unknown at the time

Avery, MacLeod & McCarty (1944)

• Purified the "transforming principle"

• Not protein (resistant to proteases)

• Not RNA (resistant to RNases)

• Not lipid or carbohydrate

• Reacted like DNA in chemical tests → DNA is the transforming principle

Hershey-Chase Experiment (1952)

• Used bacteriophages (viruses with protein coat and DNA core)

• Labelled:

  • Protein with ³⁵S → found in supernatant (outside cells)

  • DNA with ³²P → found in pellet (inside cells)

• Conclusion: DNA is the genetic material injected into cells

Chargaff’s Rules

• A = T, G = C → Purines pair with Pyrimidines

• Base composition varies by species (suggests genetic code function)

• Purines = A, G

• Pyrimidines = C, T

X-ray diffraction (Rosalind Franklin)

• DNA is a double helix

• Width = 2 nm

• One turn = 3.4 nm

• 0.34 nm between nucleotides → 10 bases per turn

Watson & Crick (1953)

• DNA is a double helix

• Purine + pyrimidine pairing fits X-ray data

  • A–T = 2 H-bonds

  • G–C = 3 H-bonds

• Each strand can serve as a template for replication → semi-conservative model

Meselson-Stahl Experiment

Proved semi-conservative replication

1. Bacteria grown in ¹⁵N medium

2. Switched to ¹⁴N

3. After one replication, intermediate density band seen → each DNA molecule has one old & one new strand

Helices

unwinds dna helix

SSBs (Single-Stranded Binding Proteins)

prevent strand reannealing

primase

Synthesizes RNA primers

DNA polymerase III

Adds DNA bases in 5’→3’ direction (main builder)

DNA polymerase I

Replaces RNA primers with DNA

DNA ligase

Seals gaps between Okazaki fragments

Replication details

• Requires:

  • Single-stranded DNA template

  • dNTPs (A, T, G, C)

  • Free 3’-OH (from RNA primer)

• Synthesized 5′ to 3′

Leading strand

continuous

Lagging strand

discontinuous → Okazaki fragments