Friedrich Miescher (1869) Isolated nucleic acids from the nuclei of white blood cells

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2 Friedrich Miescher (1869) Isolated nucleic acids from the nuclei of white blood cells Collected pus from local hospital bandages After further examination he discovered a substance that he called Nuclein Nuclein was composed of a combination of Nucleic Acids and Protein Miescher discovered that this substance was present in EVERY type of cell Did not know what its purpose was

3 Walter Sutton, Suggested that genetic material of the cell is contained in chromosomes Theodor Boveri ( ) Developed the chromosomal theory of inheritance (a story for another day) which stated that chromosomes determined traits T.H. Morgan (1911) Showed that genes are linear arrays on chromosomes (lines of code, groups) and that these genes determined traits

4 Phoebus Levene (1910) American Biochemist was the first to propose the structure of the DNA (and RNA) nucleotide Showed that DNA and RNA were distinct nucleic acids composed of long chains of nucleotides The DNA nucleotide was composed of a Deoxyribose sugar linked to a Phosphate Group and 1 of 4 Nitrogenous bases. (Adenine, Guanine, Cytosine or Thymine) Proposed that the DNA molecule was a block like structure composed of one of each type of nucleotide. A Tetranucleotide Block Because of the structure, Levene belived that DNA lacked the intelligence to be the genetic unit and that inheritance must be based on the Protein

5 Fred Griffith (1928) Conducted experiments that showed that DNA is probably the genetic material (see p ) Griffith s Experiment used transforming factors in pneumococcus bacteria in mice Was able to turn regular strains (R Strain) of the bacteria into lethal strains (S Strain) by injecting R Strain mice with dead S Strain cells. The dead S Strain cells transformed the living R Strain cells into S Strain cells) Griffith was killed in World War II before he could confirm what the transforming factors were and publish his results Oswald Avery (1944) Completed Griffith s work and proposed that the transforming factors were DNA see p.570 (won the Nobel Prize)

6 Alfred Hershey, Clearly showed that DNA was the genetic material of the cell (not protein) Martha Chase (1953) Conducted the Blender Experiment Tagged bacteriophages with radioactive isotopes Placed the bacteriophages in a blender with non radioactive E. Coli bacteria Discovered that only the radioactively tagged viral DNA was transferred to the E. Coli. (see p ) Results showed that DNA was the transforming factor

7 Edwin Chargaff (1952) Believed that DNA was more than the repetitive tetranucleotide blocks proposed by Levine. Noticed that the number of adenine bases always equalled the number of thymine bases and that the number of cytosines always equalled the number or guanines. BUT the number of adenines and thymines DID NOT always equal the number of cytosines and guanines This became known as Chargaff s Ratios This disputed Levene s theory since there were not equal amounts of each nucleotide in the DNA molecule

8 Linus Pauling (1952) Biochemist that discovered the alpha helix and pioneered the technique of X Ray crystallography Proposed that the DNA molecule was a triple helix Pauling s structure was rejected because he forgot to consider the ( - ) oxygen ions (dang ions) and his structure did not fit with Chargaff s ratios

9 Rosalind Franklin, Franklin was the expert on X-ray crystallography Maurice Wilkins ( ) Her techniques produced two sets of X-ray images of DNA Used X ray crystallography to suggest that DNA was a helical structure BUT couldn t figure out how the nucleotides fit the structure in a stable pattern Wilkins gave one set of X ray images to another group of scientists Franklin died of cancer in 1953

10 James Watson Francis Crick (1953) Used Franklin and Wilkins X ray images and Pauling s technique to determine the structure of DNA Proposed the DNA structure was a double helix Found a base pairing method that used hydrogen bonds and covalent bonds that agreed with Chargaff s ratios Found that the two strands of DNA in the double helix were anti parallel (ran in opposite directions)