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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

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what 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
    • 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 capsule cause disease
Does the capsule cause disease?

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.

transformation
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
What is the transforming factor?

“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

what is the transforming factor1
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 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

Protein had “greater heterogeneity and specificity of function”

Little was know about nucleic acids at the time

evidence dna is genetic material
EVIDENCE DNA IS GENETIC MATERIAL

Alfred Hershey and Martha Chase

Worked with a simple system: E. coli bacteria and bacteriophages – viruses that infect bacteria

evidence dna is genetic material1
EVIDENCE DNA IS GENETIC MATERIAL

bacteria

VIRUSES are not cells. Consist of nucleic acid wrapped in a protein coat. Must infect a cell in order to reproduce.

evidence dna is genetic material2
EVIDENCE DNA IS GENETIC MATERIAL

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

evidence dna is genetic material3
EVIDENCE DNA IS GENETIC MATERIAL

Radioactive DNA entered the cell but radioactive protein stayed outside.

Conclusion?

what does dna look like
What does DNA look like?

Watson and Crick

Rosalind Franklin

how is dna replicated
HOW IS DNA REPLICATED?

and …

CHASE

Meselson and Stahl

Use radioactive isotopes to track old and new strands of DNA

Use centrifugation to separate DNA molecules by mass

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