Overview
This section outlines the key historical experiments that led to the discovery that DNA is the genetic material, describes transformation, and explains Chargaff's rules.
Discovery of Nucleic Acids
- Friedrich Miescher isolated phosphate-rich chemicals (later called DNA) from white blood cell nuclei in the 1860s.
- Miescher named these chemicals "nuclein" because they came from cell nuclei.
Griffith's Transformation Experiment
- Frederick Griffith, in 1928, discovered bacterial transformation using Streptococcus pneumoniae.
- Griffith worked with R (non-pathogenic) and S (pathogenic with capsule) bacterial strains.
- Injecting live S strain killed mice; live R strain or heat-killed S strain did not.
- Mice died when injected with live R strain mixed with heat-killed S strain; live S strain was recovered from dead mice.
- Griffith concluded a "transforming principle" converted R strain into the deadly S strain.
Identification of DNA as Genetic Material
- Avery, MacLeod, and McCarty isolated DNA, RNA, and proteins from S strain to test as the transforming principle.
- Only destruction of DNA prevented transformation, indicating DNA was responsible for heredity.
- Many scientists still doubted DNA carried genetic information due to its apparent simplicity.
Hershey and Chase Experiment
- In 1952, Hershey and Chase used bacteriophage viruses tagged with radioactive labels to distinguish DNA from protein.
- Radioactive phosphorus (32P) labeled DNA, and sulfur (35S) labeled protein.
- Only DNA (not protein) entered infected bacteria and directed viral reproduction, proving DNA is genetic material.
Chargaff’s Rules
- Erwin Chargaff found that DNA composition varies by species but is consistent within individuals or tissues.
- The amount of adenine (A) equals thymine (T), and guanine (G) equals cytosine (C) (A = T, G = C).
- Chargaff’s rules provided essential clues for the DNA double helix model.
Key Terms & Definitions
- Transformation — Uptake of external DNA by a cell, changing its traits.
- Nuclein — Name given by Miescher to DNA-like material found in nuclei.
- Transforming Principle — Substance (later identified as DNA) responsible for changing R strain to S strain in Griffith’s experiment.
- Bacteriophage — Virus that infects bacteria.
- Chargaff's Rules — In DNA, A = T and G = C; base composition varies between species.
Action Items / Next Steps
- Review the details of Griffith, Avery–MacLeod–McCarty, and Hershey–Chase experiments.
- Be able to explain Chargaff’s rules and why they matter for DNA structure.