Chromosome theory of inheritance
In 1928,Frederick Griffith found that heat-killed virulent (i.e.disease-causing) strains of a type of bacterium could convert the non-virulent form into the virulent strain. He called the substance which caused the change, the transforming principal. The next step was to isolate, purify and then analyse the nature of this transforming principle.
By 1944,experiments carried out by scientists Avery, MacLeod and McCarty led them to the conclusion that the transforming principal was deoxyribonucleic acid (DNA). Their proof :
-the purified extract, which had had all it's protein and fat removed, was far more active, per unit of weight, then the original mass of cells.
-tests indicated the presence of large amounts of DNA, but only a little ribo nucleic acid (RNA).
-the extract had almost the same percentage composition of carbon, hydrogen, nitrogen and phosphorus as that postulated of DNA.
-when the enzyme known to destroy RNA was added, there was no loss of activity. However, when the enzyme known to break down DNA was added, the extract lost its power to transform cells.
The scientists thus suggested that DNA, as the transforming dactor, stimulated a specific gene mutation in the non-capsulated cells. This mutation resulted in the bacteria becoming virulent. In 1952, a series of experiments by Hershey and Chase proved that DNA was the actual substance of inheritance.
Bacterial viruses or bacteriophages attack bacterial cells, killing the cells while replicating themselves. The T2phage that Hershey and Chase worked with consisted of a protein coat and a DNA core. Radioactive sulfur and phosphorus were used to tag the protein coat and DNA core respectively, since sulfur is found in protein and phosphorus in DNA
When the tagged T2phages were allowed to infect their host bacterium, only the radioactive phosphorus tags appeared in the new generation of phages. This meant that information was begin transmitted from one generation to the next through DNA
Chromosome theory of inheritance In 1928,Frederick Griffith found that heat-killed virulent (i.e.disease-causing) strains of a type of bacterium could convert the non-virulent form into the virulent strain. He called the substance which caused the change, the transforming principal. The next step was to isolate, purify and then analyse the nature of this transforming principle. By 1944,experiments carried out by scientists Avery, MacLeod and McCarty led them to the conclusion that the transforming principal was deoxyribonucleic acid (DNA). Their proof :-the purified extract, which had had all it's protein and fat removed, was far more active, per unit of weight, then the original mass of cells. -tests indicated the presence of large amounts of DNA, but only a little ribo nucleic acid (RNA).-the extract had almost the same percentage composition of carbon, hydrogen, nitrogen and phosphorus as that postulated of DNA.-when the enzyme known to destroy RNA was added, there was no loss of activity. However, when the enzyme known to break down DNA was added, the extract lost its power to transform cells. The scientists thus suggested that DNA, as the transforming dactor, stimulated a specific gene mutation in the non-capsulated cells. This mutation resulted in the bacteria becoming virulent. In 1952, a series of experiments by Hershey and Chase proved that DNA was the actual substance of inheritance. Bacterial viruses or bacteriophages attack bacterial cells, killing the cells while replicating themselves. The T2phage that Hershey and Chase worked with consisted of a protein coat and a DNA core. Radioactive sulfur and phosphorus were used to tag the protein coat and DNA core respectively, since sulfur is found in protein and phosphorus in DNA
When the tagged T2phages were allowed to infect their host bacterium, only the radioactive phosphorus tags appeared in the new generation of phages. This meant that information was begin transmitted from one generation to the next through DNA
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