More from Chapters 12-13. 1928 – Frederick Griffith is trying to develop a vaccine for...

More from Chapters 12-13

1928 – Frederick Griffith is trying to develop a vaccine for Streptococcus pneumoniae, which causes pneumonia

Griffith was able to isolate 2 strains of the bacterium: one with a rough surface (R) and one with a smooth surface (S)

Griffith injected one group of mice with the smooth (virulent) strain and these mice died after a few days.

He then injected another group with the rough (nonvirulent) strain and these mice continued to be healthy.

Griffith took a heat-killed strain of the smooth bacteria and injected it into mice and observed that they did not die.

Griffith's fourth experiment was to inject heat treated, killed, smooth strain mixed with the rough strain.

He found that after a few days the mice died. The blood of the dead mice showed high levels of virulent pneumococcus.

Griffith theorized that some type of transformation takes place from the virulent to the non-virulent strain for it to synthesize a new polysaccharide coat.

Expanding on the work of Griffith, found that the agent responsible for genetic transferring is the nucleic acid DNA and not protein as most biochemists theorized at the time.

In 1944 Avery and his coworkers, McCarty and MacLeod, discovered the "transforming principle."

First they treated the bacteria with centafugation, which eliminates large cellular pieces. The result: bacteria still transformed

Added protease, which removes all proteins. The result: bacteria still transformed

Treated the bacteria with deoxyribonuclease, which eliminates all DNA. The result: no transformation in the bacteria

The trio concluded that DNA is the cause of transformation, where in this experiment virulence is inherited.

Noticed a pattern in the amounts of the four bases: adenine, guanine, cytosine, and thymine. 

Took samples of DNA of different cells and found that the amount of adenine was almost equal to the amount of thymine.

The amount of guanine was almost equal to the amount of cytosine. 

Thus you could say: A=T, and G=C.  This discovery later became Chargaff’s Rule. (Complimentary base pairing)

Decided to try to make a crystal of the DNA molecule. 

If they could get DNA to crystallize, then they could make an x-ray pattern, thus resulting in understanding how DNA works. 

They obtained an x-ray pattern.  The pattern appeared to contain

rungs, like those on a ladder between to strands that are side by side. 

It also showed by an “X” shape that DNA had a helix shape.

DNA RNAHas

deoxyriboseHas ribose

Has thymine Has uracilDouble Helix Single Strand

Has instructions to make proteins

Carries out protein making

instructions

http://upload.wikimedia.org/wikipedia/commons/thumb/6/6a/Griffith_experiment.svg/450px-Griffith_experiment.svg.png

http://library.thinkquest.org/20465/griffith.html http://library.thinkquest.org/20830/Textbook/

HistoryofDNAResearch.htm http://www.mun.ca/biology/scarr/CG_Chargaff.gif http://www.chemistry.msu.edu/Portraits/

images/franklinc.jpg http://genome.jgi-psf.org/images/dna.jpg http://www.nndb.com/people/979/000030889/

maurice-wilkins-MED.jpg