History
In the early 20th century, scientists have established that genes lie on chromosomes, which are composed of both DNA and proteins. However, at the time, biologists and chemists believed that proteins are responsible for carrying genetic information because they are much more complex than DNA. After all, proteins are made up of 20 different amino acids and exhibit a wide array of 3-D structure, whereas DNA only consists of 4 nucleic acids and little was known about its structure.
In 1928, Fredrick Griffith discovered that when dead pathogenic cells are mixed with live non-pathogenic cells, some of the non-pathogenic cells became pathogenic. In addition, the descendants of these cells are also pathogenic, implying that the pathogenic trait has been inherited. Thus, Griffith concluded that something from the dead pathogenic cells entered the non-pathogenic cells. However, he was unable to determine what this absorbed material was. Later, Oswald Avery and his colleagues discovered that the material was DNA. Today, the process of assimilating DNA from the cell's surroundings is called transformation. Nevertheless, the scientific community still did not fully accept the genetic agent as DNA.
In 1947, Erwin Chargaff discovered that DNA nucleotide composition varies from species to species, which supports the idea that DNA has molecular diversity among species. In addition, he also found that regardless of the species, the amount of adenine bases is always equal to the amount of thymine bases, and the same is true for guanine bases and cytosine bases.
By the early 1950's, the scientific community was beginning to accept DNA as the genetic material. To provide them with additional evidence, Alfred Hershey and Martha Chase conducted an experiment in which they labelled viral proteins with radioactive sulphur and viral DNA with radioactive phosphorus. (Note that sulphur is not found in DNA and phosphorus is not found in protein.) When the separate samples of viruses were allowed to infect bacteria by injecting their genetic material into them, they found radioactivity in the cells which were exposed to phages labelled with radioactive phosphorus in their DNA. On the other hand, the cells that were exposed to phages with radioactive sulphur in their protein did not show radioactivity. From this, Hershey and Chase concluded that DNA is the genetic material. Faced with overwhelming evidence, the scientific community gradually accepted DNA, and not protein, as the genetic material.
In 1928, Fredrick Griffith discovered that when dead pathogenic cells are mixed with live non-pathogenic cells, some of the non-pathogenic cells became pathogenic. In addition, the descendants of these cells are also pathogenic, implying that the pathogenic trait has been inherited. Thus, Griffith concluded that something from the dead pathogenic cells entered the non-pathogenic cells. However, he was unable to determine what this absorbed material was. Later, Oswald Avery and his colleagues discovered that the material was DNA. Today, the process of assimilating DNA from the cell's surroundings is called transformation. Nevertheless, the scientific community still did not fully accept the genetic agent as DNA.
In 1947, Erwin Chargaff discovered that DNA nucleotide composition varies from species to species, which supports the idea that DNA has molecular diversity among species. In addition, he also found that regardless of the species, the amount of adenine bases is always equal to the amount of thymine bases, and the same is true for guanine bases and cytosine bases.
By the early 1950's, the scientific community was beginning to accept DNA as the genetic material. To provide them with additional evidence, Alfred Hershey and Martha Chase conducted an experiment in which they labelled viral proteins with radioactive sulphur and viral DNA with radioactive phosphorus. (Note that sulphur is not found in DNA and phosphorus is not found in protein.) When the separate samples of viruses were allowed to infect bacteria by injecting their genetic material into them, they found radioactivity in the cells which were exposed to phages labelled with radioactive phosphorus in their DNA. On the other hand, the cells that were exposed to phages with radioactive sulphur in their protein did not show radioactivity. From this, Hershey and Chase concluded that DNA is the genetic material. Faced with overwhelming evidence, the scientific community gradually accepted DNA, and not protein, as the genetic material.
Now the question was, how can DNA carry genetic information if it's only a combination of four nucleotides? To answer that question, scientists tried to determine the structure of DNA. Using X-ray crystallography, Rosalind Franklin was able to produce X-ray diffraction images of the DNA polymer. Two other scientists, James Watson and Francis Crick, saw her diffraction images (without obtaining prior consent) and deducted that DNA is helical in shape. In addition, after consultation with other scientists, they figured out that the hydrophobic nitrogenous bases of nucleotides are on the interior of the double-helix, away from water. They also noticed some interesting phenomena - the diameter of the double helix is always uniform at 2 nm, and it takes 10 layers of nucleotides for the helix to undergo one full revolution (one turn). Moreover, the length between the beginning of one revolution to the next is always 3.4 nm, and since there are 10 layers of bases, the distance between each is 0.34 nm. The uniform diameter is very important because it told Watson and Crick that purine (two-ringed) bases must bond with pyrimidine (one-ringed) bases in order to maintain constant diameter. On the other hand, if purines are paired with each other then the diameter would be too long, and if pyrimidines are paired together then the diameter would be too small. This explains Chargaff's earlier discoveries - A always bonds to T and G always bonds to C, which is why the number of adenine bases must equal to the number of thymine bases, and the number of guanine bases always equal to the number of cytosine bases. Although there are only four different nucleotides, their order can vary immensely, and different nucleotide orders causes variation among species and even individuals of the same species. Soon, the two scientists constructed a model of DNA and published their findings in 1953.
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