History Of DNA Notes The Search and Discovery of Genetic Material

Search For Genetic Material Scientists prior to 1940s believed proteins were the genetic material Early discoveries – Levene (1919): discovered nucleic acids – Griffith (1928): discovers transformation – Avery (1944): discovers that DNA is the transforming agent.

Levene: 1919 Major Discovery: – Nucleotides contain: Sugar, phosphate and one of four nitrogen containing bases – Thought the bases were present in equal amounts (which is incorrect) Levene, however, thought this was too simple to store genetic code and his work was largely discredited at the time.

Griffith’s Experiment: 1928 Major Discover: There is a “transformation principle” that can transfer genetic information

Avery 1944 Using Griffith’s to isolate what the “transforming principle” Isolated Cells of the infectious strain – Used enzymes to digest and remove: Outer sugar coating All proteins All RNA – Left with only DNA: was still able to transform harmless strain. Major Discovery: DNA carries the genetic information!

Discovering DNA’s Structure Key People and Places

Hershey-Chase Experiment: 1952 Major Discovery: – Confirm DNA carries information and protein.

Chargaff: 1949 Looking at possible differences in DNA among different species. Major Discovery – “Chargaff’s Rule”: amount of adenine (A) is usually similar to the amount of thymine (T) amount of guanine (G) usually approximates the amount of cytosine (C).

Discuss:

The RACE begins In the 1940s/early 1950s, what did scientists know: – about genetic information? – What do you think they might be “racing” to figure out? Answer? They knew the make up of DNA and of its genetic importance, but they now were racing to discover the STRUCTURE

The Contenders Pauling Watson and Crick Franklin and Wilkins

Rosalind Franklin: 1952 At this time, women were not highly regarded in the scientific world. Franklin worked with many men, like Wilkins who saw her as an “assistant” Major Discovery: – She used x-ray crystallography to take pictures of DNA at different angles

Pauling 1953 Linus Pauling was at the forefront of chemistry – very interested in protein – “wrote the book” on chemical bonds Major Discovery: Knew the three components of nucleotides; Pauling believed the structure of DNA to be an alpha helix INCORRECT

Watson and Crick:1953 Eccentric scientists who worked together to put information for multiple sources together: – Levene: nucleotides contain 3 parts – Griffith/Avery/Hershey-Chase: DNA carries genetic code – Chargaff: A + T and C + G – Franklin: helix structure from x-ray photos

Watson and Crick’s Major Discovery

TIME LINE Hershey- Chase AveryLevene Griffiths Franklin Chargaff Pauling Watson- Crick