Presentation on theme: "How do we know DNA is the genetic material 12-1 DNA."— Presentation transcript:
How do we know DNA is the genetic material 12-1 DNA
What is the genetic material? Fredrick Griffith How do certain bacteria cause pneumonia? Two strains (types) of bacteria –Smooth – when grown in petri dish smooth edged colonies and shiny –Rough – when grown in petri dish rough edged colonies and not shiny SMOOTH COLONIES ARE PATHOGENIC – disease causing ROUGH COLONIES ARE USUALLY DESTROYED BY HOST IMMUNE SYSTME
Does the disease causing smooth bacteria produce a poison? Killed pathogenic bacteria. Injected into mice. Mice survive. Is it a poison?
TRANSFORMATION 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!
What is the transforming factor? 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 Conclusion?
Many people remained unconvinced that DNA was the genetic material! 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
EVIDENCE DNA IS GENETIC MATERIAL Alfred Hershey and Martha Chase Worked with a simple system: E. coli bacteria and bacteriophages – viruses that infect bacteria Can anyone tell me about the life cycle of viruses?
EVIDENCE DNA IS GENETIC MATERIAL They had to find a way to label DNA and protein so they could follow the molecules in the virus and the cell Protein has sulfur, DNA does not – make protein with radioactive sulfur 35 S DNA has phosphorus, protein does not – make DNA with radioactive phosphorus 32 P Virus capsid – protein coat labeled with 35 S Virus core – DNA labeled with 32 P Infect the bacteria and see which one enters the cell
EVIDENCE DNA IS GENETIC MATERIAL Radioactive DNA entered the cell but radioactive protein stayed outside. Conclusion?