BIOL 112 Lecture Notes - Lecture 12: Frederick Griffith, Erwin Chargaff, Martha Chase
16 May 2018
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BIOL 112- Lecture 15- DNA, Discovery and Replication
History of DNA
-!At the start of the century, it was known that chromosomes contained genetic
material
-!2 things required to determine the nature of genetic material: method of
isolating cell components and an assay for genetic materialness
-!Assay: can measure a substance (starch) or an abstract phenomenon (memory)
Friedrich Meischer:
He figured out a way to isolate the nuclei from cells (i.e. isolate cell components) and
discovered that it primarily consists of nuclein, which turned out to be DNA
He was the first to purify DNA (grind up an organism, extract lipids and proteins with
organic solvent, i.e. phenol, precipitate with ethanol)
Frederick Griffith: Transforming principle
Experimented using mice and 2 different strains of pneumococcus bacteria (causes
pneumonia)
-!R “rough” form: harmless, not encapsulated
-!S “smooth” form: harmful, encapsulated
Found that when dead S form and live R form were injected into mice, they died
-!R form were taking up genetic material from S form and thus surviving
Discovered the process of transformation: direct uptake and expression of exogenous
genetic material (DNA) from surroundings)
Assay became the ability to transform R bugs into S bugs based on Frederick Griffith’s
experiment
-!Difficult to find and assay, since ideal situation is to take the candidate’s genetic
material, give it to an existing organism and show that that
organism’s traits have changed
Avery, Macloed, McCarthy
Transforming principle: substance that transformed bugs
They took the bacteria, added purified lipid, carbohydrate,
protein and DNA to see if it would convert R to S
-!Conclusion: only DNA was able
Alfred Hershey and Martha Chase (1952)
They used the T2 virus (bacteriophage) which infects a bacterial
host (consists of a protein coat and DNA)
Tagged viral proteins with a radioisotope of sulfur and DNA with a
radioisotope of phosphorus
-!Only DNA has phosphorus
-!Only proteins have sulfur
-!In one experiment T-phage with protein tagged 35S was allowed to infect
bacteria
Entire mix was thrown into a blender, shearing protein coat off bacteria
-!Idea was that whatever got injected into the bacteria would be the DNA and
would stay behind
Little 32P left in the supernatant (what was shredded off), more of it in the pellet !
phosphate and thus DNA entered the bacteria
Erwin Chargaff (1949)
People knew DNA was a polymer with 4 deoxy nucleotides triphosphates (dNTPs)
-!Deoxyadenosine, deoxy guanosine, deoxycytidine, thymidine
It was clear that each sugar has a polarity based on whether the 3’ OH group of 5’
phosphate group is sticking out of the sugar-phosphate background
Rules:
1)!A=T and G=C proportion wise
2)!The ratio A&T/G&C varied between organisms
Crystallography:
o!DNA consists of two strands, twisted around each other in a double helix
shape
o!Phosphates are probably on the outside of the helix
o!The strands run antiparallel to each other
o!Shape of DNA discovered by Watson & Crick
o!The proposed bases formed H-bonds with each other such that A-T, and
C-G only.
"!Reverse compliments
Watson and Crick proposed the double-helix structure of DNA
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Document Summary
Biol 112- lecture 15- dna, discovery and replication. At the start of the century, it was known that chromosomes contained genetic material. 2 things required to determine the nature of genetic material: method of isolating cell components and an assay for genetic materialness. Assay: can measure a substance (starch) or an abstract phenomenon (memory) He figured out a way to isolate the nuclei from cells (i. e. isolate cell components) and discovered that it primarily consists of nuclein, which turned out to be dna. He was the first to purify dna (grind up an organism, extract lipids and proteins with organic solvent, i. e. phenol, precipitate with ethanol) Experimented using mice and 2 different strains of pneumococcus bacteria (causes pneumonia) Found that when dead s form and live r form were injected into mice, they died. R form were taking up genetic material from s form and thus surviving.
