Identifying the Genetic Material

03/11/2010 § 1 Comment

Last class, we started our new unit about what’s inside DNA and how it hold the information that makes a cell’s proteins and ultimately commands a cell to do this and that. Our essay question this unit sounds like a difficult one: How does information produce meaning? I think that scientifically, this question relates all the way to the core of the unit: DNA. I will have to learn primarily what’s inside DNA and then how that material makes information and later, how this information means something to the cell. This might mean that we could possibly learn about how exactly RNA retrieves information and decodes the genetic information to tell the ribosomes inside the rest of the cell.

On Monday’s class, we learned about Frederick Griffith’s experiments in the 1920s. He used bacteria that caused pneumonia for this experiment. He took two different samples (strains). One was an S-strain, meaning that this type of virus had a smooth edge and had a capsule. The second strain was R-strain, R standing for rough, representing the virus’ rough edge. This strain did not have a capsule.

When Griffith injected the S-strain into his subjects (mice), the mouse would die. But when he injected the R-strain into a mouse, it would be fine and would continue to live. So at first, Griffith must have known that his S-strain samples were virulent, meaning that they caused disease, while his R-strain samples were avirulent and didn’t cause disease. Now, when Griffith heated the smooth strain under fire and killed the bacteria, and when he tested the heat-killed strain on the mice, the bacteria proved to be avirulent. However, when he mixed the heat-killed S-strain (avirulent) and the R-strain (also avirulent), the result proved to be fatal and the mouse died. So typically, Griffiths wondered what caused the change and named this change transformation.

To find out, more experiments were made using lots of different enzymes that took away either proteins, carbohydrates, RNA and DNA. With every stage, one by one, the other factors were taken away and still the R-strain would change into S-strain. (The samples were still R-strain substances and S-strain substances.) When the DNA material was finally taken away, there was no transformation, therefore it became known that the genetic material was DNA.

So when we speed through time a little bit, up to 1952 (according to the textbook), Alfred Hersey and Martha Chase prove this by using a substance of radioactive sulfur to follow the path of a virus’s proteins and another substance of radioactive phosphorus to follow the path of the virus’s DNA. During the first run, when Hersey and Chase followed the path of radioactive sulfur (protein), they noticed that the new viruses that grew in the bacteria were normal viruses, meaning protein didn’t have any effect on the type of virus or its information. However, when Hersey and Chase found that the new viruses that grew inside the bacteria was radioactive because of the DNA.

These experiments basically proved that DNA is the genetic material in cells and organisms, and although it took a lot of time, DNA is what holds information for a cell.

§ One Response to Identifying the Genetic Material

  • Dave Ferguson says:

    Kari,

    Well done. Good understanding of the experimental basis for identifying DNA as the genetic material.

    Mr. F.

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