Discovery of DNA as Genetic Material

Griffith’s Experiment (1928)

• Studied pneumonia bacteria in mice.

• Result: Harmless bacteria could be "transformed" into harmful bacteria.

• Conclusion: Some "transforming principle" carries genetic information.

Avery-MacLeod-McCarty Experiment (1944)

• Used enzymes to determine the "transforming principle."

• Result: DNA (not protein) is responsible for transformation.

• Conclusion: DNA is the genetic material.

Hershey-Chase Experiment (1952)

• Used bacteriophages (viruses that infect bacteria).

• Labeled DNA with phosphorus-32 and protein with sulfur-35.

• Result: Only DNA entered bacterial cells.

• Conclusion: DNA, not protein, is the genetic material.

Components of DNA

• DNA is a nucleic acid made of nucleotides.

Each nucleotide consists of:

• A five-carbon sugar (deoxyribose).

• A phosphate group.

• A nitrogenous base (Adenine, Thymine, Cytosine, Guanine).

Base-Pairing Rules (Chargaff’s Rule)

• A pairs with T (Adenine → Thymine) → 2 hydrogen bonds

• C pairs with G (Cytosine → Guanine) → 3 hydrogen bonds

• Chargaff’s Findings: A = T, C = G, but A + T ≠ C + G

DNA’s Double Helix Structure

Discovered by Watson & Crick (1953), using Rosalind Franklin’s X-ray diffraction data (Photo 51).

DNA’s Double Helix Structure Key Features:

• Two strands, anti-parallel (run in opposite directions: 5’ → 3’ and 3’ → 5’).

• Held together by hydrogen bonds between complementary base pairs.

• Backbone = sugar & phosphate; Rungs = nitrogen bases.

Semiconservative Replication (Meselson & Stahl Experiment, 1958)

• Each new DNA molecule has one old strand and one new strand.

Steps of DNA Replication

1. Unzipping the DNA

   • Helicase breaks hydrogen bonds → DNA strands separate.

2. Starting Replication

   • Primase adds RNA primers to guide DNA polymerase.

3. Building New DNA Strands

   • DNA polymerase adds nucleotides only in the 5’ → 3’ direction.

   • Leading Strand: Built continuously.

   • Lagging Strand: Built in short fragments (Okazaki fragments), later joined by ligase.

4. Proofreading & Finishing

   • DNA polymerase proofreads & corrects errors.

   • Ligase seals gaps in the lagging strand.

Helicase

Unzips the DNA strands

Primase

Adds RNA primers to guide DNA polymerase

DNA Polymerase

Adds nucleotides to build the new DNA strand

Ligase

Joins Okazaki fragments on the lagging strand

Proofreading Enzymes

Detect and fix errors during replication