History Of DNA Notes

The Search and Discovery ofGenetic 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

Discuss: Why do you think

this occurs?

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.

Discuss: What do you think this discovery meant?

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).

If the amount of:A is the same as TG is the same as CWhat would you assume about DNA structure?

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

Discuss this with

your tablemates

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

Discuss: why might this be important?

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 whoworked 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

Discuss: Why might sharing data

be important in

science?

Watson and Crick’s Major Discovery

TIME LINE

1919 1928 1944 1949 1952 1952 1953 1953

Hershey-Chase

AveryLevene

Griffiths 

FranklinChargaff

Pauling

Watson-Crick

With you group:

complete the timeline with the

people and their

major discoveries