The Discovery of DNA

Friedrich Miescher• Began working with white blood

cells in 1869.

• White blood cells are a major component of pus in infections. As a result, Miescher collected a lot of pus from bandages at a local hospital.

• From the nuclei, Miescher isolated a substance known as “nuclein”

• Added a weak alkaline solution to the white blood cells– when he did the cells decomposed and caused the nuclei to move out of the solution.

• After chemical analysis, nuclein was later renamed as DNA.

Frederick Griffith • In the 1920s he was trying to

develop a vaccine for pneumonia.

• He took two strands of pneumococcus bacteria (deadly strain [III-S] and non-deadly strain [II-R]) and injected them into healthy mice.

• Firstly, he found that when injected with the deadly strain (III-S) the mice died, and when injected with the non-deadly strain (II-R), the mice survived.

• Began to wonder if a strain of bacteria could transform into another strain.

Frederick Griffith • Secondly, he heated up the

deadly strain (III-S) to kill the bacteria and when injected into the mice, they did not die.

• Lastly, he co-injected the heated deadly strain (III-S) with the non-deadly strain (II-R) and when injected into the mice, they died.

• He concluded that some “principle” transformed the non-deadly strain (II-R) into a deadly strain (III-S).

• He called this the “transforming principle”

“Transforming Principle”Rough strain: II-R (non-deadly strain) Smooth strain: III-S (deadly strain)

First Experiment

Second Experiment

Third Experiment

Joachim Hammerling..

Cap

Stalk

Foot

• In the 1930s, began experimenting with Acetabularia, a form of green algae.

• He found that when you removed the cap, it grew back, but if you removed the foot, it would not grow back.

• This showed that the instructions for making the cap were in the foot (which was where the nucleus was).

• He concluded that the nucleus contained DNA, and was responsible for development.

Oswald Avery, Maclyn McCarty and Colin McLeod (1944)

• Was interested in the work that Frederick Griffith did.

• What was the “principle” that transformed the non-deadly strain (II-R) into a deadly strain (III-S).

• They discovered that it was DNA, not protein that was responsible for transformation.

Erwin Chargaff• In 1949, he began experimenting

with DNA. • He isolated DNA from different

organisms and measured the levels of adenine, guanine, thymine and cytosine. These were his results: DNA

SourceAdenin

e Thymin

eGuanin

eCytosin

e Calf

Thymus 1.7 1.6 1.2 1.0

Beef Spleen

1.6 1.5 1.3 1.0

Yeast 1.8 1.9 1.0 1.0Tubercl

e Bacillus

1.1 1.0 2.6 2.4

Chargaff’s Rule • Levene’s tetranucleotide theory

stated that adenine, guanine, cytosine, and thymine would equal each other.

• However, Chargaff concluded that the nucleotides must be arranged in a way that adenine and thymine are equal and that cytosine and guanine are equal (A=T/C=G). DNA

SourceAdenin

e Thymin

e Guanin

e Cytosin

e Calf

Thymus1.7 1.6 1.2 1.0

Beef Spleen

1.6 1.5 1.3 1.0

Yeast 1.8 1.9 1.0 1.0

Tubercle Bacillus

1.1 1.0 2.6 2.4

DNA Structure Was Discovered • Through the work of

Miescher, Griffith, Hammerling, Avery, McCarty, McLeod and Chargaff, the structure of the nucleotide was discovered.

• The structure of the nucleotide included: pentose sugar attached to a phosphate group at carbon 5 and a nitrogenous base at carbon 1.

Alfred Hershey and Martha Chase

• Conducted an experiment in 1952 on bacteriophage (viruses that attack and infect bacteria).

• Using radioactive isotopes, Hershey and Chase traced the movement of DNA and protein during phage infection.

• They showed that DNA, not protein entered the bacterial cell during phage reproduction and that only DNA was passed on to phage offspring.

• This concluded that DNA, not protein carries the genetic material of the cell.

Rosalind Franklin • Began researching the

structure of DNA in the early 1950s.

• Franklin used X-ray diffraction (a technique used to examine the shape and structure of a molecule) to take a picture of a DNA molecule.

• The picture showed the double helical structure of DNA. However, the picture shows the DNA molecule from the end, not the side.

James Watson, Francis Crick and Maurice Wilkins (1953)

• Wilkins passed on the photo that Franklin had taken to Watson and Crick.

• Using Franklin’s photo and Chargaff’s rule, Watson and Crick were able to explain and prove the double helical structure of DNA.

• Using Chargaff’s data, they were able to come up with the “complementary base pairing” in DNA where a purine (A/G) always binds with a pyrimidine (T/C).

James Watson, Francis Crick and Maurice Wilkins (1953)

• They proposed that the helix in DNA turned clockwise and was a right handed helix.

• They concluded that the helix would make a complete turn every 10 nucleotides, which is a distance of 3.4nm, so the space between nucleotides is 0.34nm. • Also they discovered that DNA strands run anti-parallel to each other (one strand runs in a 5’ to 3’ direction and the other strand runs 3’ to 5’.)

James Watson, Francis Crick and Maurice Wilkins (1953)

• For all their research, Watson, Crick and Wilkins won the Noble Peace Prize in 1962.

• Rosalind Franklin was never given any credit for her discovery, and died in 1958.