Discovery of DNA as Genetic Material

Griffith's Experiment and the Concept of Transformation

In 1928, Frederick Griffith's experiments with bacteria revealed that some unknown genetic "factor" could control the traits of organisms. This led to the discovery of the process called transformation, where external DNA is taken up by a cell, resulting in a genotypic and phenotypic change.

• Transformation: The uptake of external DNA by a cell, leading to genetic change.

• Example: Griffith's experiment showed that non-pathogenic bacteria could become pathogenic when mixed with heat-killed pathogenic bacteria, indicating the transfer of genetic material.

Avery, McCarty, and MacLeod: Identifying the Transforming Substance

Later, Oswald Avery, Maclyn McCarty, and Colin MacLeod identified DNA as the transforming substance, providing strong evidence that DNA is the genetic material, not protein.

• Many scientists were initially skeptical, believing proteins were more likely candidates for genetic material due to their complexity.

Hershey-Chase Experiment: DNA is the Genetic Material

In 1952, Alfred Hershey and Martha Chase used bacteriophages (viruses that infect bacteria) to confirm that DNA, not protein, is the genetic material.

• Bacteriophage: A virus that infects bacteria, consisting of a protein coat and nucleic acid (DNA or RNA).

• Hershey and Chase labeled viral DNA with radioactive phosphorus and viral protein with radioactive sulfur. Only the DNA entered the bacterial cells, confirming DNA as the genetic material.

Key Practice Questions

• Which molecule entered the bacterial cell in the Hershey-Chase experiment? DNA

• What fact did Hershey and Chase use? DNA contains phosphorus, protein contains sulfur.

DNA Composition and Structure

Chargaff's Rules

Erwin Chargaff discovered two important rules about DNA composition:

• DNA base composition varies between species.

• In each species, the percentage of adenine (A) is roughly equal to thymine (T), and the percentage of guanine (G) is roughly equal to cytosine (C).

Summary: The percentages of A ≈ T and G ≈ C in DNA support the base-pairing rules and the double helix structure.

X-ray Diffraction and the Double Helix

Rosalind Franklin used X-ray diffraction to photograph DNA, producing "Photo 51," which provided crucial information about DNA's helical structure. James Watson and Francis Crick used this data to build their model of the DNA double helix.

• DNA is a double helix with two anti-parallel strands.

• Base-pairing occurs between A-T and G-C via hydrogen bonds.

Detailed DNA Structure

DNA consists of two strands of nucleotide monomers linked together:

• Each strand has a 5' end (free phosphate group) and a 3' end (free -OH group).

• The strands are anti-parallel, running in opposite directions.

• Nucleotides are joined by phosphodiester bonds between the phosphate group of one nucleotide and the sugar of the next.

Base Pairing

• Adenine (A) pairs with Thymine (T) via two hydrogen bonds.

• Guanine (G) pairs with Cytosine (C) via three hydrogen bonds.

Key Equations and Concepts

• Base Pairing Rule: $$ [A] = [T], \quad [G] = [C] $$

• Phosphodiester Bond Formation: The bond forms between the 3' -OH of one nucleotide and the 5' phosphate of the next.

Summary Table: DNA Structure Features

Applications and Examples

• Transformation: Used in biotechnology to introduce new genes into bacteria.

• Chargaff's Rules: Essential for understanding DNA replication and genetic coding.

• X-ray Diffraction: Still used today to determine the structures of biological macromolecules.

Additional info: These foundational experiments and discoveries established DNA as the molecule of heredity and clarified its structure, paving the way for modern molecular biology and genetics.