NL 8:30-2:30 Weather Plan?. How do we know DNA is the genetic material. What is the genetic material?. Fredrick Griffith How do certain bacteria cause pneumonia? 2 strains (types) of Streptococcus pneumonia Smooth – when grown in petri dish smooth edged colonies and shiny
DEAD S cells were mixed with live R cells.
Bacteria isolated from blood of dead mice were S strain and produced more S strain cells
Change was permanent and heritable – capsule did not cause disease.
Something in dead, disease-causing bacteria turned harmless bacteria into killers!
He called this process TRANSFORMATION
Some factor in dead pathogens turned harmless bacteria into pathogens!
“C (somatic) carbohydrate, nucleoproteins, free nucleic acids of both the yeast [RNA] and thymus [DNA] types, lipids and other cell constituents. Try to find in that complex mixture the active principle..!! Try to isolate and chemically identify the particular [transforming] substance....!Some job--full of heartaches and heartbreaks. But at last perhaps we have it....”
Oswald Avery and colleagues
Made extract of bacteria
Destroyed proteins, lipids, carbohydrates, and nucleic acids (RNA and DNA) one-by-one with enzymes
When DNA was destroyed by enzymes the cells did not become pathogenic
Many thought protein must be the genetic material.
Protein is more complicated – 20 amino acids
DNA is made of only 4 bases – adenine, guanine, cytosine and thymine
Protein had “greater heterogeneity and specificity of function”
Little was know about nucleic acids at the time
Alfred Hershey and Martha Chase
Worked with a simple system: E. coli bacteria and bacteriophages – viruses that infect bacteria
VIRUSES are not cells. Consist of nucleic acid wrapped in a protein coat. Must infect a cell in order to reproduce.
How could they determine whether DNA or protein from phage entered cell?
Protein has sulfur, DNA does not – grow phages in radioactive sulfur 35S
DNA has phosphorus, protein does not – grow phages in radioactive phosphorus 32P
Virus capsid – protein coat labeled with 35S
Virus core – DNA labeled with 32P
Infect the bacteria and see which one enters the cell
Radioactive DNA entered the cell but radioactive protein stayed outside.
Watson and Crick
Meselson and Stahl
Use radioactive isotopes to track old and new strands of DNA
Use centrifugation to separate DNA molecules by mass