Hi,
Here are the answers:
1) Rosalind Franklin and Maurice Wilkens
<u><em>Found DNA to be a helix structure because of their x-ray crystallography </em></u>
During 1953, Rosalind Franklin and Maurice Wilkens attempted to explore the structure of DNA by firing the x-rays on the fibres of DNA. X-rays were scattered when they hit the DNA and later detected on the photographic film. The image on photographic film indicated that DNA had a helical structure. This served as a basis for further analysis of DNA structure by coming scientists, most importantly Watson and Crick.
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2) Alfred Hershey and Martha Chase:
<u><em>Conclusively proved DNA is the hereditary molecule with their work with bacteriophages. </em></u>
Harshey and Chase, did experimentation on a bacteriophage called T2. During their experiment, they infected some bacteria E-coli with T2. They used radioactively labelled S and P atoms in the protein coat and DNA of virus respectively. When the virus infected bacteriophage, they found that the radioactively labelled P atoms were found in bacteriophage but S were not found. Now, as we know that S- Sulphur atoms become part of protein coat of virus and when virus infects bacteria, protein remains outside. Only DNA containing P-Phosphorus is injected in bacteria. They, proved that hereditary molecule was DNA and not proteins. Their experimentation was a great breakthrough in the field of Biology and it paved the paths for better understanding of DNA by coming generation of scientists.
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3) Avery, McLoed and McCarty:
<u><em>Determined Griffith's transforming factor was DNA </em></u>
Avery, McLoed and McCarty proceeded the experimentation of a scientist Griffith. Griffith used two strains of bacteria <em>Steptococcus pneumonia</em> for his experimentation i.e S strain and R strain. S strain bacteria had capsule of polysaccharide around them and they produced smooth colonies while R strain did not have and produce rough colonies. Griffith killed S strain bacteria and injected in mice, but mice stayed alive. This depicted that polysaccharide coat was not responsible for the death of mice.
He then injected the mice with live S strains along with some dead R strains. The mice died of pneumonia. So Griffith thought that something was transferred from dead R strains into the S strains due to which they became infectious.
In 1944, Avery, McLoed and McCarty, continued his experimentation and proved that the molecule that was transferred from R strain to S stain was DNA and DNA is basically the transforming factor.
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4) James Watson and Frances Crick:
<u><em>Determined DNA to be double helix structure </em></u>
In 1953, using the work of Rosalind and Franklin, two scientists James Watson and Frances Crick got successful in creating a perfect three dimensional structure of a DNA molecule. They described their model as double helix that is composed of backbones of sugar and phosphate molecules that are held together through bonding between nitrogenous bases. They also proposed that there exists hydrogen bonding between nitrogenous bases, purines and pyrimidines.
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5) Erwin Chargaff:
<u><em>Determined base pair ruling and that percentages of bases is species specific. </em></u>
Erwin Chargaff studied the DNA of different living organisms and gave out a rule called Chargaff's rule. His rule also contributed and helped in the understanding and preparation of DNA structure by Watson and Crick. According to his rule, In the DNA of any cell at any time, the amount of guanine units is equal to the amount of cytosine units, and the amount of adenine units is equal to the amount of thymine units. This gave an important clue about the complementary bonding of bases i.e Adenine always pairs with Thymine and Guanine always pairs with Cytosine.
Hope it helps! :)