The Discovery of DNA

ORIGINAL QUESTION: Which molecule contains genetic information?

Experiments Frederick Griffith (1928) - English A. Smooth bacteria → mouse = dies of pneumonia B. Rough bacteria → mouse = lives C. Heat-killed smooth bacteria → mouse = lives D. Heat-killed smooth bacteria + → mouse = dies of pneumonia Live rough bacteria

Fig. 12.2 p. 327

Fredrick Griffith

Griffith Conclusion: Some molecules changed the harmless rough bacteria into deadly smooth bacteria → transformation!

Oswald Avery (1944) - Canadian Question: What molecule was needed for transformation? Avery examined 4 macromolecules – destroying 1 at a time 1. Lipids 3. Proteins 2. Carbohydrates 4. Nucleic Acids (DNA)

Oswald Avery

Conclusion = Genes are made of DNA  Transformation was blocked ONLY when DNA was destroyed   Conclusion = Genes are made of DNA

Alfred Hershey & Martha Chase (1952) - American Bacteriophage = a virus that infects bacteria Made of protein and DNA Question: Which enters a bacterium during infection? They used “tracers” = radioactive isotopes 1. phosphorus-32 → part of DNA (not proteins) 2. sulfur-35 → part of proteins (not DNA)

Question: Which enters a bacterium during infection? They used “tracers” = radioactive isotopes phosphorus-32 → part of DNA (not proteins) sulfur-35 → part of proteins (not DNA)

Fig. 12.3

Hershey & Chase Procedure: A) Let bateriophages infect bacteria B) Analyzed bacteria for radioactivity C) Found phosphorus-32 sample made the bacteria radioactive Conclusion: Genetic material is DNA NOT protein

Martha Chase & Alfred Hershey

DNA Structure

Nucleic acids = chain of nucleotides (polymer) Nucleotides consist of : 1. 5-carbon sugar 2. phosphate group 3. nitrogenous base

Nucleotides Fig. 12.4

DNA: deoxyribonucleic acid is a nucleic acid  Made of 4 different nucleotides: 1. adenine 3. guanine 2. cytosine 4. thymine

Chargaff’s Rules - Erwin Chargaff (1950) - American In samples of DNA: The amounts of adenine (A) and thymine (T) were equal The amounts of cytosine (C) and guanine (G) were equal Chargaff’s Rule: C = G and T = A

Erwin Chargaff

Rosalind Franklin & Maurice Wilkins (1952) – British Used X-Ray diffraction to analyze DNA molecules These X-Ray patterns helped to solve the structure of DNA

Rosalind Franklin

Fig. 12.6

Maurice Wilkins

James Watson & Francis Crick (1953) – American & English Solved the mystery! DNA is made of 2 strands twisted in a “double helix” “Base pairs” are formed between nitrogenous bases (A, T, C, G) Hydrogen bonds formed between A and T Hydrogen bonds formed between C and G

Frances Crick James Watson

Watson & Crick Kind of like a twisted ladder! Rails of the ladder are strands of alternating deoxyribose and phophate One rail runs up, the other runs down The steps or rungs are the pairs of bases (C-G or T-A)

DNA Structure: Fig. 12.8

Nucleosomes Tiny particles that help to fold and package DNA so it can fit inside the nucleus If all of the DNA in one human cell were laid out end to end, it would be almost 2 meters long!

Nucleosomes: Fig. 12.9

DNA Replication - Semiconservative

QUESTION: How does DNA replicate?

Semiconservative Replication Enzymes separate the 2 strands New strands are synthesized This occurs during the “S” phase of the cell cycle

Fig. 12.10

Why must DNA be replicated? When a cell divides, each new cell needs a copy of the genetic material

Semiconservative Replication A DNA strand that looks like… A T A G C G C A T T A C T A T C G C G T A A T G

…can be untwisted, broken apart, and have new nucleotides paired with BOTH of the original sides, forming 2 new strands. After the DNA strand is replicated it will look like this: A T A G C G C A T T A C T A T C G C G T A A T G A T A G C G C A T T A C T A T C G C G T A A T G