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How do we know DNA is the genetic material 12-1 DNA.

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Presentation on theme: "How do we know DNA is the genetic material 12-1 DNA."— Presentation transcript:

1 How do we know DNA is the genetic material 12-1 DNA

2 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

3 ??????????

4 Does the disease causing smooth bacteria produce a poison? Killed pathogenic bacteria. Injected into mice. Mice survive. Is it a poison?

5 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!

6 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?

7 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

8 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?

9 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

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11 EVIDENCE DNA IS GENETIC MATERIAL Radioactive DNA entered the cell but radioactive protein stayed outside. Conclusion?

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