- What is the relationship between DNA & proteins?
- DNA contains the information a cell needs to make a protein
- Why are proteins important?
- Proteins make up the structural components of cells and direct most of their chemical activity
- Key concept: DNA was identified as the genetic material through a series of experiments
- Bacteriophage
- DNA, gene, enzyme
- Fredrick Griffith - 1928
- helped determine that genes are composed of DNA
- Experimented with Streptococcus pneumonia (bacteria) in mice
- two strains used
- S -- because it looked smooth and caused pneumonia
- R -- because it looked rough and did NOT cause pneumonia
- Griffith injected mice with heat-killed S strains and they did NOT get pneumonia
- Griffith injected a mixture of heat-killed S and healthy R and the mice got pneumonia
- Type R (nonvirulent) -- survived
- Type S (virulent) -- died
- Heat-killed S -- survived
- Heat killed S + type R -- died
- Griffith’s conclusion:
- Griffith reasoned that a material must have been transferred from the heat-killed S strain to the R strain
- called this material the “transforming principle”
- what evidence suggests that there was a transforming principle?
- 1944 Oswald Avery discovered that this transforming principle was DNA and it was the carrier of genes
- tried to figure out if it was DNA or protein
- process involved purifying S bacteria extract
- used different tests to determine whether transforming principle was DNA or protein
- results:
- qualitative - chemical tests showed no protein was present but revealed DNA was present
- Chemical analysis -- proportions of elements match those found in DNA
- Enzyme tests -- used enzymes known to break down proteins -- since extract still transformed -- not protein
- When added enzymes that broke down DNA -- no transformation
- Hershey Chase procedure
- protein contains sulfur but very little phosphorus
- DNA contains phosphorus but no sulfur
- Used radioactive tags on phosphorus for one bacteriophage culture and tags on sulfur for other culture
- Exp. 1 -- bacteria infected with phages tagged with sulfur in their protein
- separated bacteria from parts of phages that were still outside -- no sulfur in bacteria
- Exp. 2 -- repeated with phages tagged with phosphorus in DNA -- phosphorus inside bacteria
- Phage consists of only DNA surrounded by a protein coat
- allowed the question, “Is the genetic material DNA or protein?” to be answered
- Hershey Chase results
- Phages injected their DNA into bacteria
- Protein did not enter the bacteria
- mix radioactive tags with DNA and protein, found that the protein was the shell and the DNA was injected into cells
- James Watson and Francis Crick…1953
- Rosalind Franklin