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Frederick Griffith: Experiments
A British scientist, Frederick Griffith, tried to figure out how bacteria made people sick by performing an experiment of injecting a mice with the disease-causing strain of bacteria. This resulted the mice to develop pneumonia and die. But when he injected the mice with harmless strain, the mice didn't get sick at all. To find out, he took heat-killed bateria and injected it into the mice, causing it to survive, which proved that the reason for pneumonia was not from bacteria. -
Frederick Griffith: Transformation
In Griffith's next experiment, he mixed heat-killed bacteria with live, harmless ones and injected the mixture into the mice. It was thought that the mice stayed neutral but instead, the mice developed pneumonia and died. Examing the lungs of the mice, he found that it was filled with diease-causing bacteria rather than harmless ones. Somehow, thw heat-killed bacteria had passed their disease-causing ability to the harmless strain. Griffith called this transformation. -
Frederick Griffith: The Hypothesis
Frederick Griffith hypthesized that when the live, harmless bacteria and the heat-killed bacteria were mixed, some factor was transferred from the heat-killed cells into the live ones. That factor, which was thought to be gene, must contain information that could transform a harmless bacteria into a disease-causing bacteria. -
Period: to
DNA scienists
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Oswald Avery and DNA
A Canadian biologist, Oswald Avery, repeated Griffith's work, in order to determine which molecule in the heat-killed bacteria was most important for transformation. Avery and his colleagues, at the Rockfeller Institute in NY, made an extract from the heat-killed bacteria and treated it with enzymes that destroyed proteins, lipids, carbohdrates, nucleic acid, RNA, and other molecules. Transformation occured but the molecules were not responsible for it since they were destroyed. -
Oswald Avery and DNA
Avery and other scientists repeated the experiment, but using enzymes that would break down DNA. When they destroyed the nucleic acid DNA in the extract, transformation did not take place. The one possible conclusion for this was that the DNA was the transforming factor. -
The Hershey and Chase Experiement: Bacteriophages
American scientists, Alfred Hershey and Martha Chase, collaborated in studying viruses, nonliving particles smaller than cell that can infect living organisms. Bacteriophage is a one kind of virus that infects bacteria and is composed of a DNA or RNA core and a protein coat. When it enters a bacterium, the virus attaches to the surface of the cell and injects its genetic info into it. As the viral genes produce many new bacteriophages, they gradually destroy the bacterium. -
The Hershey and Chase Experiment: Radioactive Markers
Hershey and Chase reasoned that if they find the part of the virus that entered the infected cell, they could figure out if the genes were made of protein or DNA. So, they grew viruses in cultures containg radioactive isotopes of phosphorous-32 and sulfur-35, which was helpful since proteins have no phosphorous and the DNA has no sulfur. The radioactive substances could be used as markers. If the sulfur was found in bacteria, then the viruses' protein had been injected into the bacteria. -
The Hershey and Chase Experiment: Radioactive Markers 2
It phosophorous was found in the bacteria, then it was the DNA that had been injected.The 2 scientists mixed the marked viruses wih bacteria and waited a few minutes for the viruses to inject the genetic material. THen, they sepaated the viruses from the bacteria and tested the bacteria for radioactivity. The results were that naerly all the radioactivity was from phosophorous-32, the marker found in DNA. -
Rosalind Franklin: X-Ray Evidence
A British scientist, Rosalind Franklin, began to study DNA. Using a technique called X-ray diffraction, she got inofrmation about the structure of the DNA molecule. She aimed powerful X-ray beam at concentrted DNA samples and recorded the scattering pattern of the X-rays on the film. This is important because it shows how strads of DNA are twisted aroun each other. -
Erwin Chargaff: Chargaff's Rules
An American biologist, Erwin Chargaff, disovered that the perentages og guanine and cytosine bases were almost equal in any sample of DNA. This also goes for adenine and thymine. The observation was that [A]=[T] and [G]=[C], which is known as the Chargaff's rules. -
Francis Crick and James Watson: The Double Helix
Francis Crick,a British physicist, and James Watson, an American biologist, were trying to understand the structure of DNA by building 3-dimensional models of the molecule using cardboard and wire. They twisted and stretched it in various ways but came to conclusions. Using Franklin's X-ray pattern and weeks with Watson and Crick had built a strctural model that explained how DNA carried info and how it coud be copied. -
Francis Cick and James Watson: The Double Helix
A double helix looks like a twisted ladder or a spiral staircase. They realized that the double helix accounted for many features in Franklin's X-ray pattern but did not explain what forces held the 2 strands together. They then found out that hydrogen bonds could form between certain ntrogenous bases and prvide enough force to hold the strands together.
