DNA
People have always wondered How do traits get passed from one generation to the next?
When did we discover that DNA existed? 1869- Johann Freidrich Meischer Found in the nucleus To simple for heredity No one was really sure what DNA did
Early 1900 s Other scientists discover the nucleotide components Sugar, nucleotide, nitrogenous bases
Fredrick Griffith - 1928 Transformation experiment What was he trying to do? What did he discover? How was his discovery important?
Oswald Avery- 1944 Avery, MacLeod, McCarty What were they trying to do? What did they discover? How was his discovery important and how did people react?
Alfred Hershey and Martha Chase-1944 What were they trying to do? What was their experiment like? What did they discover? How was this discovery important?
Now we know lots more The nature of science Science builds on itself Leads to more questions Technology allows for more discovery
Once people knew DNA was the genetic material it became a race to discover its structure Linus Pauling Maurice Wilkins and Rosalind Franklin James Watson and Francis Crick
Why has the discovery of DNA structure been so important?
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)
He added the rough strain And the mouse lived!
He added the smooth strain And the mouse died
He added the smooth strain that had been killed by heat And the mouse lived!
He added smooth strain killed by heat with rough strain And the mouse died. What happened?
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 Griffith thought that the killed virulent bacteria had passed on a molecule of inheritance to the nonvirulent bacteria.
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!
Transformation An organism is transformed by gaining genetic material from another organism How was the rough bacteria transformed?
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) People did not believe his results
The Hershey-Chase Experiment
What is a bacteriophage? A virus that infects bacteria.
How do viruses work? The bacteriophage attaches to the surface of the bacteria Escherichia coli (E. coli).
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.
What did Hershey and Chase know about viruses? 1. Viruses are made of DNA and protein 2. Bacteriophages infect bacteria and the cell dies when the new viruses leave 3. The virus injects its genetic material into the host cell in order to take it over
4. The purpose of their experiment Bacteriophages can be used to determine what the heritable information of the cell is because they are made of proteins and DNA
5. The experimental procedure Radioactive sulfur was added to the proteins. Radioactive phosphorous was added to the DNA
Their final results 6.Conclusion: DNA is the heritable material, not protein People finally acknowledge that DNA is 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 was provided without her knowledge to other researchers named Watson and Crick. These scientists were also working on discovering the structure of DNA. These scientists used her data and that of other scientists to build their ultimately correct and detailed description of DNA's structure in 1953.
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 1. Explain what is meant by the term transformation. Why are Griffiths experiments on S. pneumococci considered to be so important? 2. What's a bacteriophage How does it infect bacteria? 3. Describe the procedure by which Hershey & Chase demonstrated that DNA is the genetic material.