Griffith’s experiments showed that hereditary material can pass from one bacterial cell to another.

Frederick Griffith - 1928

In 1928 Griffith was studying a bacterium called Streptococcus pneumoniae. He was trying to develop a vaccine against a virulent strain of the bacterium.

Virulent – disease causing

Vaccine – a substance that is prepared from killed or weakened microorganisms & introduced into the body to protect it from future infection by that microorganism.

Griffith's experiment

S bacteria is virulent because it has a capsule. The capsule protects it from attacks by the human immune system. It is able to survive long enough in the human body to replicate and cause disease.

R bacteria is NOT virulent because it lacks a capsule. Without the capsule it is left defenseless against the human immune system. Therefore it is destroyed in the human body before it is able to cause disease.

Griffith's experiment

Injected mouse with live R bacteria (w/o capsule).DID NOT kill mice

Injected mouse with live S bacteria (w/ capsule).Killed mice

Injected mouse with heat killed S bacteria.DID NOT kill mice

Injected mouse with heat killed S bacteria mixed with live R bacteria. Killed mice

Griffith's experiment

Griffith's experiment

The R bacteria took up the DNA found in the heat killed S bacteria and became virulent!

The transfer of genetic material from one cell to another cell or from one organism to another organism is called transformation.

Conclusion

Discovered that Adenine / Thymine and Cytosine / Guanine occurred in equal percentages in DNA.

For Example: If you had 22 Adenine and 5 Cytosine then you would have 22 Thymine and 5 Guanine.

Led to base pairing discovery

Chargaff - 1950

• Discovered that Adenine / Thymine and Cytosine / Guanine occurred in equal percentages in DNA.

• For Example: If you had 22 Adenine and 5 Cytosine then you would have 22 Thymine and 5 Guanine.

• Led to base pairing discovery

Chargaff - 1950

Hershey and Chase confirmed that DNA, and not protein, is the hereditary material.

Hershey & Chase -1952

In 1952, two researchers, Martha Chase and Alfred Hershey, set out to test whether DNA or protein was the hereditary material viruses transfer when viruses enter bacteria.

Bacteriophage - is a virus that infects bacteria.

Hershey & Chase experiment

Grew bacteriophage (with radioactive sulfur ) in dish of e.coli (sulfur is only found in proteins)

Grew bacteriophage (with radioactive phosphorous ) in dish of e.coli (phosphorous is only found in DNA)

They traced the radioactive elements that had entered the e.coli …to see which one was injected into the cell.

The DNA of the virus and not the proteins was what was in the e.coli causing it to produce more viral DNA.

Hershey & Chase experiment

The Hershey-Chase Experiment

Hershey and Chase confirmed that DNA, and not protein, is the hereditary material.

Conclusion

Watson and Crick used the information gathered by others to determine one of the most important

discoveries in human history – the Double Helix.

Watson and Crick - 1953

Until Hershey and Chase’s experiment, most people believed that protein was the hereditary material (because protein was involved in basically everything to do with cells and because it was believed DNA’s structure was too simple to encode the secret to life).

After the Hershey and Chase experiment in 1952 proved DNA was in fact the hereditary material, the race was on to discover it’s structure to gain a better understanding of it.

Watson and Crick discovered DNA shape (double helix) in 1953 using information gained from other scientists, mainly Wilkins / Franklin.

Watson and Crick (Wilkins / Franklin) - 1953

Crick actually studied mostly proteins and Watson studied neither before 1952.

Watson and Crick went to lectures from other scientists concerning DNA and compiled information they gained from them.

Watson “borrowed” X-rays done by Wilkins / Franklin showing a vague picture DNA’s shape (you could tell that it was the same thickness all the way through).

Watson, Crick, and Wilkins received a Nobel prize from it while Franklin (who did most of the work) died from cancer due to her exposure to X-rays.

Watson and Crick – continued