DNA

1. Griffith Experiment The genetic material in cells, was not clear to scientists for many years. Remember Chromosomes consist of proteins. OR

Scientists were trying to figure which one out of DNA or protein played the role of the genetic material. . . ? This led to several experiments, two of which are very important.

Griffith Transformation Experiment He was trying to find a vaccine against Streptococcus pneumonia (bacteria that caused pneumonia) Isolated two different strains of bacteria Rough surface appearance (R); no disease/harmless (no coating) Smooth surface appearance (S); disease causing/pathogenic (coating)

Set up four experiments Live R injected=mice lived Live S injected=mice died Dead S injected=mice lived Live R + dead S=mice died Why?  Griffith thought that the killed virulent bacteria had passed on a characteristic to the non-virulent one to make it virulent.  He thought that this characteristic was in the inheritance molecule.  This passing on of the inheritance molecule was what he called transformation.

Griffith found from this experiment that even though he had killed the S cells, he hadn’t destroyed their hereditary material, which was the one part that caused the disease! When some more experiments had been done, it had been discovered that the harmless R cells, had used the information from the hereditary material of the dead S cells and became harmful; this he called, hereditary transformation.

Transformation Organism transformed by gaining genetic material from another organism The dead genes of the smooth strand, transfered their DNA to the rough strand. The rough strand thus gained the instructions to produce a smooth and deadly coat. Since the genes that were transfered were dead, it is evident that the rough strand only became deadly because of the DNA it gained.

Oswald Avery 1944 Continued work of Griffith and found that extracts of killed bacteria could transform healthy bacteria cells Showed the heritable material in extracts was DNA (stores and transmits genetic information)

Hershey & Chase Experiment

What they know (write this down) viruses have DNA inside a protein coat/shell called a capsid Viruses which infect bacterium is a bacteriophage because the host bacterium cell is killed as the new viruses leave the bacterial cell. To do this, the virus must inject its genetic code into the host cell.

What is a bacteriophage? A virus that infects bacteria.

With its six legs, the bacteriophage attaches to the surface of the much larger bacteria Escherichia coli (E. coli).

Once attached, the bacteriophage injects DNA into the bacterium Once attached, the bacteriophage injects DNA into the bacterium. The DNA instructs the bacterium to produce masses of new viruses. So many are produced, that the E. coli bursts.

The Big Question? (write this down) Bacteriophages consist of both protein and DNA. 4. So, did proteins, DNA, or both contain the viral genetic information?

Procedure 5. They used E. coli as the experimental bacteria and a virus called T2. Next, they injected sulfur into the protein coat and phosphorus into the DNA core to make them radioactive allowing scientist to track down if the radioactive sulfur protein coating or phosphorus was passed on or not.

Results 6.The DNA, not the protein, is its genetic code.

Conclusion 7.People finally acknowledged DNA as the genetic material!!

Erwin Chargaff 1949 Studied composition of DNA The amount of adenine always equals the amount of thymine and the amount of cytosine always equals the amount of guanine. A=T C=G

Rosalind Franklin X-ray diffraction images of DNA fibers Position of atoms can be calculated from images

Franklin's data and her unpublished conclusions at a routine seminar, her work was provided - without Randall's knowledge - to her competitors at Cambridge University, Watson and Crick. The scientists used her data and that of other scientists to build their ultimately correct and detailed description of DNA's structure in 1953. Franklin was not bitter, but pleased, and set out to publish a corroborating report of the Watson-Crick model. Her career was eventually cut short by illness. It is a tremendous shame that Franklin did not receive due credit for her essential role in this discovery, either during her lifetime or after her untimely death at age 37 due to cancer.

James Watson & Francis Crick 1953 Put together model for DNA structure; double helix (two strands of DNA bonded together wound in a helix) Sugar-phosphate backbone (from the outside of the molecule) Nitrogenous base form the middle of the molecule purines bond with pyrimidines Won Nobel prize for their discovery!

Answer the Questions Explain what is meant by the term transformation. Why are Griffiths experiments on S. pneumococci considered to be so important? What's a bacteriophage How does it infect bacteria? Describe the procedure by which Hershey & Chase demonstrated that DNA is the genetic material.