1. Section 12-1: Identifying The Substance of Genes
Chapter 12
Section 12-1: Identifying The Substance of Genes

2. How do genes work?
First step was to identify what molecule or molecules go into making a gene

3. Identifying the Molecule of Heredity
Scientists had to discover the chemical nature of the gene
Identifying the molecule that carries genetic information would help us understand how genes control the inherited characteristics of living things
Began in 1928 with British scientist Frederick Griffith, who was trying to figure out how certain types of bacteria produce pneumonia
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4. Griffith’s Experiments
Griffith isolated two different strains of the same bacterial species - both strains grew very well in culture, but only one of the strains caused pneumonia
The disease-causing bacteria (S strain) grew into smooth colonies on culture plates, whereas the harmless bacteria (R strain) produced colonies with rough edges
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5. Griffith’s Experiments
Griffith injected mice with disease-causing bacteria  mice developed pneumonia and died
Injected mice with harmless bacteria  mice stayed healthy
Perhaps the S-strain bacteria produced a toxin that made the mice sick?
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6. Griffith’s Experiments
Griffith took a culture of the S strain, heated the cells to kill them, and then injected the heat-killed bacteria into laboratory mice
Mice survived - cause of pneumonia was not a toxin from these disease-causing bacteria
He mixed the heat-killed, S-strain bacteria with live, harmless bacteria from the R strain and injected the mixture into mice
Mice developed pneumonia, and many died
Lungs of these mice were filled with the disease-causing bacteria
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7. Transformation
Some chemical factor transferred from the heat-killed cells of the S strain into the live cells of the R strain changed harmless bacteria into disease-causing bacteria
Griffith called this process transformation, because one type of bacteria had been changed permanently into another
Ability to cause disease was inherited by the offspring of the transformed bacteria  concluded that the transforming factor had to be a gene
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8. Summary of Griffith’s Experiments
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9. The Molecular Cause of Transformation
Group of scientists at the Rockefeller Institute in New York, led by the Canadian biologist Oswald Avery, researched which molecule in the heat-killed bacteria was most important for transformation
Avery and his team treated heat-killed bacteria with enzymes that destroyed proteins, lipids, carbohydrates, and some other molecules, including the nucleic acid RNA
Transformation still occurred
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10. The Molecular Cause of Transformation
Repeated the experiment using enzymes that would break down DNA
When they destroyed the DNA in the mixture, transformation did not occur
Therefore, DNA was the transforming factor
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11. Bacteriophages
When a bacteriophage enters a bacterium, it attaches to the surface of the bacterial cell and injects its genetic information into it
Viral genes produce many new bacteriophages, which gradually destroy the bacterium
When the cell splits open, hundreds of new viruses burst out
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12. Bacterial Viruses
Alfred Hershey and Martha Chase performed the most important of the experiments relating to Avery’s discovery using viruses—nonliving particles that can infect living cells
The kind of virus that infects bacteria is known as a bacteriophage, which means “bacteria eater”
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13. The Hershey-Chase Experiment
They studied a bacteriophage that is composed of a DNA core and a protein coat
Wanted to determine which part of the virus entered the bacterial cell – attempting to prove or disprove Avery
Hershey and Chase grew viruses in cultures containing radioactive isotopes of phosphorus-32 (P-32) and sulfur-35 (S-35)
Proteins contain almost no phosphorus and DNA contains no sulfur
Radioactive isotopes used as markers – could trace which molecule entered cell
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14. The Hershey-Chase Experiment
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15. The Hershey-Chase Experiment
Mixed the marked viruses with bacterial cells
Tested the bacteria for radioactivity
Bacterial radioactivity was from phosphorus P-32 , the marker found in DNA
Concluded that the genetic material of the bacteriophage was DNA, not protein
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16. The Role of DNA
Storing information: Genes control patterns of development – each cell has instructions for that organism written in its DNA
Copying information: Before a cell divides, it makes a complete copy of its genes – once DNA structure was figured out, its copying method became clear
Transmitting information: Each daughter cell must get a complete set of genes during cell division, and sorting of genes is important in meiosis – nothing can be lost without consequences
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