where does it have started?
141 years ago in 1869 a Swiss biochemist's research lead to discover the nuclein. He was Friedrich Miescher,detected a phosphorus-containing substance from white blood cells at the University of Tubiangen, Germany.
In 1930 Joachim Hammerling discovered the secret of location of genetic material,he demonstrated that its genetic information is contained in the nucleus. In his experiment he removed the cap of acetabularia, inthe result of next generation cap has grown, in other step he removed foot but in next generation there was no foot.
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Transforming principle
Later in 1928 Frederich Griffith was studying two strains of streptococcus pneumonia. Thorough a series of experimenst on mice observed when heated virulent strain(s-strain) was mixed with nonvirulent strain(r-strain) and was injected to mice the result is the same as injecting live virulent strain to mice, therefore he observed chemical component from the virulent S cells had somehow transformed the R cells into the more virulent S form, and also the heated s-strain released many substances which contains: Rna, Dna, Lipid, Protein, Carbohydrates
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But what is this agent which is transforming to next generation?!
Oswald avery and his colleagous, Colin MacLeod and Maclyn McCarty
Oswald Canadian-born U.S. bacteriologist, it was 1944 when he reported the result of their experiment.
By Griffith's experment they work on the same bacteria, Avery and his coworkers purified and tested different enzymes, eliminating all except DNA as the transforming, or genetic, material.
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Erwin Chargaff study on DNA in many different organisms, the result revealed that the proportion of Adenine in a DNA molecule is equal with to that Thymine, and the proportion of Guanine is equal to that of Cytosine.
Also his discovery showed that the total amount of purines equals in the total amount of prymidines These two rules that helped lead to the discovery of the double helix structure of DNA.In the human DNA, for example the four bases are present in these percentages: A=30.9% and T=29.4%; G=19.9% and C=19.8%
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In 1952 Alfred Hershey and Martha Chase made a big contribution in proving DNA role as the hereditary molecule.
They used a bacteriophage which sits on bacterial cells and injects material to reproduce itself. By using radioactive indicators Hershey and Chase were able to label the parts of the phage. The phage had a protein coat around its head, as well as viral DNA within it. In one experiment they marked the DNA with this radioactive chemical. They then discovered that after the phage infected the cell, the radioactive chemical was within the cell indicating the DNA was transferred inside. The second experiment they performed they marked the protein coat on the phage. Once infection was completely, the radioactive chemical was still on the phage indicating the protein is not used in the infection and reproduction process.
In 1953 it was Rosalind Franklin who was working at King's College in london with Maurice Wilkinsonon DNA structure.She was working on x-ray crystallography with her student Raymond Gosling, discovered that there were two forms of DNA at when wet, the DNA fibre became long and thin, when it was dried it became short and fat. She discovered important basic facts about DNA structure, She discovered that the sugar-phosphate backbone of DNA lies on the outside of the molecule, not the inside as was previously thought. The only part that she could not discovered from her data was how the bases paired on the inside of helix, which James Watson and Francis Crick had the answer.
James Watson and Francis Crick
who put the last piece of DNA Structure together from a variety of sources including Franklin's results,Rosalind Franklin was friendly with both James Watson and Francis Crick, and communicated regularly with them until her life and career were cut short by cancer in April of 1958, at the age of 37.
she never knew that Watson and Crick had gotten access to her results.
Maurice Wilkins
He studied the orientation of purines and pyrimidines in tobacco mosaic virus and in nucleic acids, by measuring the ultraviolet dichroism of oriented specimens, and he studied, with the visible-light polarizing microscope, the arrangement of virus particles in crystals of TMV and measured dry mass in cells with interference microscopes. He then began X-ray diffraction studies of DNA and sperm heads.
Meselson and Stahl
In their experiment they presented DNA replication is semiconservative (one-half of each new molecule of DNA is old; one-half new.
They grew E.coli is a medium using ammonium ions (NH4+) as the source of nitrogen for DNA (as well as protein) synthesis. 14N is the common isotope of nitrogen, but they could also use ammonium ions that were enriched for a rare heavy isotope of nitrogen, 15N.
After growing E. coli for several generations in a medium containing 15NH4+, they found that the DNA of the cells was heavier than normal because of the 15N atoms in it.
The difference could be detected by extracting DNA from the E. coli cells and spinning it in an ultracentrifuge. The density of the DNA determines where it accumulates in the tube.
Then they transferred more living cells that had been growing in 15NH4+ to a medium containing ordinary ammonium ions (14NH4+) and allowed them to divide just once.The DNA in this new generation of cells was exactly intermediate in density between that of the previous generation and the normal.
This, in itself, is not surprising. It tells us no more than that half the nitrogen atoms in the new DNA are 14N and half are 15N.
http://www.youtube.com/watch?v=OzSIGxKWqoo
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