Lecture 29 Genes are DNA Notes

Lecture 29 Genes are DNA Notes - Lecture 29 DNA is the...

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Lecture 29 DNA is the genetic material Campbell 6 th Ed. Pp. 287-292; 303-304 + Fig. 17.1 7 th Ed. Pp. 293-298; 309-310 + Fig. 17.2 8 th Ed. pp 305-310; 325-327 + Fig. 17.2 What it the molecular basis of inheritance? Morgan's experiments showed that genes (identified as units of genetic information) are on chromosomes. Chromosomes contain DNA and protein. Which carries genetic information? Proteins look like good candidates because of their structural diversity. We'll talk about an experiment that provided evidence that DNA was the genetic material. This was by Griffith, in 1928. (You don't have to remember the names and dates of these experiments, but you need to know how they were done and what they showed.) Griffith used 2 strains of Streptococcus pneumoniae , a bacterium that causes pneumonia. One, the S (smooth) strain, is pathogenic (causes disease) because it is surrounded by a capsule that allows it to avoid being killed by the immune system of the organism it infects. The other, the R (rough) strain, has no capsule. It is harmless because the infected organism quickly kills it. Griffith killed S cells by heat. He showed that these were no longer pathogenic. However, when he mixed heat-killed S cells and live R cells, and treated mice with the mixture, the mice died of the disease. In addition, he could isolated live S cells from the dead mice. This showed that genetic information (basically, the information on how to make a capsule) was passed from the dead S cells to the live R cells. Some molecule in the dead S cells was not destroyed by heating, and could be passed on to R cells to convert them to S cells. Avery, in 1944, used this system to determine what molecule in the S cells was responsible. He showed that just by mixing dead S cells and live R cells in a test tube, he could get live S cells. Genetic material could be passed directly between the cells. He then got rid of lipids and carbohydrates from the dead S cells before mixing them with the R cells. These simple molecules seemed very unlikely to be the genetic material. As expected, he could still isolate live S cells from the mixture. He then divided the dead S cells (with no lipid or carbohydrate) in 3 tubes. He added enzymes that could degrade (destroy by breaking down to component parts) either DNA, RNA, or proteins. After this treatment, the first tube would contain only RNA and protein, the second only DNA and protein, and the third only DNA and RNA. He then mixed these with live R cells and asked whether he could get production of live S cells. Here were the results of mixing each tube with live R cells: Tube with RNA + protein: NO live S cells Tube with DNA + protein: live S cells Tube with DNA + RNA: live S cells. This showed that the “gene” that was transferred from dead S cells to live R cells that gave them the ability to make a capsule was made out of DNA. Even after the S cell was killed, this gene could be passed to an R cell, allowing this cell to make a capsule and turning it into an S cell.
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