Experiments Discussed in Chapter 1

Some of the discussion in class that we had over these experiments was not that detailed or maybe you weren't taking notes because Dr.Robson wasn't writing the notes….so here is a little summary:

The Griffith Experiment
In 1928 Frederick Griffith showed that genetic information can be transferred from one bacterial cell to another. He did this using two strains of Streptococcus pneumoniae identified S (smooth) and R (rough). The S bacteria has a capsule surrounding it which protects it from defense mechanisms and makes it more virulent. The R bacteria does not have anything special (such as a capsule) to protect it, it is just simply a bacterium that doesn't cause pneumonia because without the capsule, the bacteria is broken down by an immune system (more vulnerable).

In this experiment Griffith injected mice with varying types of the two different strands, such as alive and dead. From this he concluded that the mice injected with a mixture of living R cells and heat-killed S cells contract the disease and often die. The bacteria cultured from these mice were similar to the S strain but they were killed with heat before injected into the mice. Therefore, the material of the dead S cells can be transferred to the living R cells that enables them to make cells with capsules (S cells).


Avery/MacLeod Experiment
Not until 1944 was the chemical substance responsible for transferring the genetic material from dead S cells to live R cells discovered. This experiment was performed by Oswald Avery, Colin MacLeod, and Maclyn McCarty. It showed the substance causing the change was DNA!!! They discovered ways to isolate pure DNA from cells and then were able to observe transformation. This experiment proved that DNA is the genetic material.


Hershey-Chase Experiment
Alfred Hershey and Martha Chase uncovered important findings also, in 1952. They studied the cells of E. coli after infection by the virus T2 (bacteriophage/phage). The used the information they already knew, that the T2 phage attached by the tip of its tail to the bacterial cell wall, then the phage material entered the cell, multiplication occurred, and then lysis of the bacterial cell followed. Radioactive isotopes allowed them to discover that the T2 phage transfers most of its DNA, but little of its proteins (the radioactive agent attacks proteins but not DNA) to the host cell. This led them to discover that the genetic material in a T2 phage is DNA.


According to our text and many scientists, the experiments of Avery, MacLeod, and McCarty, Hershey, Chase, and Griffith, are classic demonstrations that genes consist of DNA.

There aren't currently any exceptions to the generalization that DNA is the genetic material in all cellular organisms and many viruses, in several types of viruses the genetic material has RNA, such as HIV and AIDS, making them very difficult to treat because the process from RNA—>DNA is very error prone, therefore many mutations occur, which makes is hard for us to treat since it is constantly changing.

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