RPC Biology Lesson 21: Discovery of DNA

Gregor Mendel

• Cross-bred peas in experimental garden

• Found predictable patterns and discovered laws

• Some unknown “factor” at work

Frederick Griffith

• Rough strain (nonvirulent)→ morse lives

• Smooth strain (virulent → mouse dies

• Heat-killed smooth strain → mouse lives

• Rough-strain & heat killed smooth strain → mouse dies

• Dead bacteria could somehow transform live bacteria

• "Transforming principle”

Friedrich Miescher: Protein or Nucleic Acids?

• Isolated chemicals from the nucleus

• Protein known to be the factor that does almost everything

• Proteins don’t change (except in poisoning or disease)

Oswald Avery: Protein or DNA?

• Experiments similar to Griffith

• Used enzymes to remove proteins from bacterial cells

• “Transformation” didn’t occur

Hershey-Chase Experiment

• Tagged sulfur in bacteriophage protein

• Tagged phosphorous in bacteriophage DNA

• Proved bacteriophage DNA enters bacteria; protein does not

DNA

• Made of nucleotides

• AGCT

• Purines: Adenine, Guanine

• Pyrimidines: Cytosine, Thymine

Rosalind Franklin

• Pioneered x-ray crystallography

• “Photo 51”

• Maurice Wilkins showed Photo 51 to James Watson

Erwin Chargaff

• Each species has different proportions of Adenine, Guanine, Cytosine, and Thymine in its DNA

• Every organism has equal amounts of purines and pyrimidines

• Every organism has equal amounts of adenine and thymine

• Every organism has equal amounts of guanine and cytosine

Chargaff’s Rules

• Adenine — Thymine

• Guanine — Cytosine

Most Important Concepts

• DNA is a double helix with 4 nucleotide bases and a sugar-phosphate backbone

• Nucleotides fit together according to Chargaff’s Rules: A-T G-C

• Mendel uncovered rules for “factor”; Hershey-Chase proved this “factor” was DNA

• Crick, Watson, and Wilkins discovered the DNA structure