DNA THE GENETIC MATERIAL Discovery of Genetic Material

DNA: THE GENETIC MATERIAL

Discovery of Genetic Material � Griffith: 1928 � Studied streptococcus pneumoniae, which causes pneumonia � Injected smooth and rough strains of the virus into mouse. � Live smooth strain kills mouse, dead smooth strain does not kill mouse, rough strain does not kill mouse, dead smooth strain mixed with live rough strain kills mouse. � Mixed bacteria taken from dead mouse showed live smooth bacteria This suggests that the disease causing factor was passed from smooth to rough.

Cont. � Avery: 1944 � Isolated protein, DNA, and lipids from killed smooth cells and exposed them to rough cells. � Found that when rough cells were exposed to smooth cells, they became smooth. � Suggested that DNA was responsible and was the genetic material

Hershey and Chase � Hershey and Chase: 1952 � Provided definitive evidence that DNA is the genetic material. � Used a bacteriophage (a virus that attacks bacteria) The virus is not a cell and can’t replicate on its own, it injects it’s genetic material into a host cell. � Isolated the two parts of the bacteriophage (protein and DNA) and found that the DNA was injected into the host cell to replicate the virus.

Hershey and Chase � Radioactive Labeling � They labeled a set of bacteriophages with radioactive phosphorous (no phosphorous in protein, only DNA) so DNA would be radioactive. � They labeled another set of bacteriophages with radioactive sulfur (sulfur is in protein but not DNA) � They then injected the radioactive bacteriophages into the bacteria. Only the bacteriophages injected with phosphorous (radioactivity on DNA) transferred this radioactivity to the bacteria.

DNA Structure � Nucleotides: P. A. Levine discovered that Adenine, Guanine, Cytosine, and Thymine were the nitrogenous “bases” that made up DNA. � Adenine and Guanine are double ringed bases. � Cytosine and Thymine are pyrimidine Single ring) bases

Chargaff � Chargaff: Found that the amount of guanine equaled the amount of cytosine, and the amount of adenine equaled the amount of thymine. This gave us Chargaff’s Rule: �A pairs with T, G pairs with C.

Double Helix � � Now, the structure had to be determined. X-Ray Diffraction � Rosalind Franklin shot X-Rays at DNA and took a picture.

Watson and Crick � Using Franklin’s and Chargaff’s data as well as mathematics (yay math!), Watson and Crick built a model of the double helix that fit the available evidence. � Two outside strands made of dioxyribose (sugar) and phosphate � C and G paired together by three hydrogen bonds � A and T paired together by two hydrogen bonds

Orientation � The phosphate backbone strands of DNA run in opposite directions of each other. One runs 5’ to 3’, the other 3’ to 5’.