A - Evidence from bacterial transformation
B - Evidence from phage infection of bacterial cells
C - Evidence from Transduction
D - Evidence from DNA measurements A - Evidence from bacterial transformation
The first evidence that DNA is the genetic material came from the work of Frederick Griffith on the pneumonia causing bacterium Streptococcus peumoniae, He distinguished two strains of the pneumococcus: one caused pneumonia (virulent) and had smooth colony outlines on the agar and its cells are surrounded by a polysaccharide capsule, which he designated the S strain and a strain that did not cause pneumonia, had rough colony outlines and lack the capsule, which he designated as the R strain. Griffith (1928) injected the different strains of bacteria into mice; the S strain killed the mice; the R strain did not. He further noted that if heat killed S strain was injected into a mouse, it did not cause pneumonia. When he combined heat- killed S with Live R and injected the mixture into a mouse that the mouse developed pneumonia and died. Bacteria recovered from the mouse had a capsule and killed other mice when injected into them. ❖ Griffith concluded that a transforming principle in the killed pathogenic S strain transforms the R strain to the pathogenic S strain. In 1944, Oswald Avery, Colin MacLeod, and Maclyn McCarty revisited Griffith’s experiment and concluded the transforming factor was DNA. They fractionated the killed S strain into DNA, RNA, proteins, carbohydrates, lipids mixed each component with the R strain and injected the mixture into a mouse. Only live R strains mixed with DNA were able to transform nonpathogenic into pathogenic strain; DNA treated Dnase failed to transform the R strain into S strain. Avery and his coworkers concluded that DNA should be considered the favorite candidate as the genetic material.
B - Evidence from phage infection of bacterial cells
The breakthrough in the quest to determine the genetic material came from the work of Max Delbruck and Salvador Luria in the 1940s on the mode of virus infection of bacterial cells. The viruses that they worked with were types of the human intestines bacterium Escherichia coli known as the T2 phages. Delbruck and Luria found that these phages have a head and tail shape and consists of protein coats covering DNA in the head; the tail is linked to the head via a neck and a collar and ended a base plate and tail fibers.
In 1952, Alfred Hershey and Martha Chase conducted a series of experiments to determine whether protein or DNA was the genetic material. Hershey and Chase found that the radioactive 35S remained outside the cell while the radioactive 32P was found inside the cell, indicating that DNA was the physical material of genes. C - Evidence from Transduction
In 1952, Zinder and Lederberg performed a series of experiment on the typhoid bacterium Salmonella typhimurium; the results of which elucidated the phenomenon of transduction. They used two strains of the Salmonella; one is prototrophic for the amino acid methionine and auxotrophic for the amino acid thrionine that may be labeled meth+thr- and the other is prototrophic for the amino acid thrionine and auxotrophic for the amino acid methionine and may be labeled met-thr+. • In another experiment they used met+thr- strain that was infected by the lysogenic virus P22, centrifuged the culture to throw down the living cells and the supernated fluid was heated. When this fluid was mixed with met-thr+ cells prototrophic cells for methionine and thrionine were produced. • From these results Zinder and Lederberg suspected that a gene for prototrophy is transferred in the supernatant with the viral DNA. The P22 phage is a lysogenic phage that integrates its DNA, as a prophage, in the bacterial genome and remains in this temperate state.
D - Evidence from DNA measurements
1. Cells of the same organism contain the same amount of DNA. 2. Somatic cells contain twice as much DNA as gametic cells; this is compatible with the segregation of genes and chromosomes during meiosis. 3. The amount of DNA is congruent with the ploidy level i.e. number of sets of chromosomes in polyploid species. 4. DNA does not undergo turnover during cell metabolism; all other molecules are made and degraded in the cells. 1. DNA must carry information from parent cell to daughter cell and from one generation to the next generation. Therefore it must contain information for replicating itself and repairing its replication mistakes. 2. Genes in the DNA must be expressed as phenotypic traits; the process of gene expression is demonstrated by the control of DNA in protein synthesis. 3. The DNA is chemically stable, relatively unchanging. However, it must be capable of mutational change that could not be repaired. Without mutations there would be no process of evolution.