The idea that the genetic material of organisms is DNA has its roots within the discovery of transformation by Frederick Griffith in 1928. The bacterium Streptococcus (formerly Pneumococcus) pneumonia kills mice by causing pneumonia. The virulence of the bacterium is decided by its capsular polysaccharide, which allows the bacterium to flee destruction by its host. Several sorts of S. pneumonia have different capsular polysaccharides, but all of them have a smooth “S” appearance. Each of the S types can produce variants that fail to supply the capsular polysaccharide and thus have a rough “R” surface (consisting of the fabric that was beneath the capsular polysaccharide). The R types are avirulent and don't kill the mice, because the absence of the polysaccharide capsule allows the animal’s system to destroy the bacteria. When S bacteria are killed by heat treatment, they will not harm the animal. however, shows that when heat-killed S bacteria and avirulent R bacteria are jointly injected into a mouse, it dies because of the results of the pneumonia infection. Virulent S bacteria are often recovered from the mouse’s blood.
In this experiment, the heat-killed S bacteria were of type III and therefore the live R bacteria had been derived from type II. The virulent bacteria recovered from the mixed infection had the graceful coat of type III. So, some properties of the dead IIIS bacteria can transform the live IIR bacteria in order that they create the capsular polysaccharide and become virulent. It shows the identification of the component of the dead bacteria liable for transformation. This was called the reworking principle. it had been purified during a cell-free system during which extracts from the dead IIIS bacteria were added to the live IIR bacteria before being plated on agar and assayed for transformation. Purification of the reworking principle in 1944 by Avery, MacLeod, and McCarty showed that it's DNA.
Having shown that DNA is that the genetic material of bacteria, the subsequent step was to demonstrate that DNA is that the genetic material during a quite different system. Phage T2 may be a virus that infects the bacterium Escherichia coli. When phage particles are added to bacteria, they attach to the surface, some material enters the cell, then approximately 20 minutes later each cell bursts open, or lyses, to release a large number of phage.
The results of an experiment conducted in 1952 by Alfred Hershey and Martha Chase during which bacteria were infected with T2 phages that had been radioactively labeled either in their DNA component (with phosphorus-32 [32P]) or in their protein component (with sulfur-35 [35S]). The infected bacteria were agitated during a blender and two fractions were separated by centrifugation. One fraction, containing the empty phage “ghosts that were released from the surface of the bacteria, consisted of protein and contained approximately 80% of the 35S label. the opposite fraction consisted of the infected bacteria themselves and contained approximately 70% of the 32P label. Previously, it had been shown that phage replication occurs intracellularly in order that the genetic material of the phage would need to enter the cell during infection.
Most of the 32P label was present within the fraction containing infected bacteria. The progeny phage particles produced by the infection contained approximately 30% of the first 32P label. The progeny received but 1% of the protein contained within the original phage population. This experiment directly showed that only the DNA of the parent phages enters the bacteria and becomes a part of the progeny phages, which is strictly the expected behavior of genetic material.
The phage possesses genetic material with properties analogous to those of cellular genomes: Its traits are faithfully expressed and are subject to equivalent rules that govern the inheritance of cellular traits. The case of T2 reinforces the overall conclusion that DNA is that the genetic material of the genome of a cell or an epidemic.
References :
LEWIN’S GENES XII by Krebs, Jocelyn E., author. | Goldstein, Elliott S., author. | Kilpatrick, Stephen T., author.
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