The historical experiments of Fredrick Griffith

Fredrick Griffith

He is one of the great scientists who bought revolution in genetics. He was the first one to give idea that the bacteria are capable of transmitting genetic information’s by the process known as transformation. To prove this, he did an experiment using a particular strain of mice and hence called Griffith experiment.

Griffith experiment

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In this experiment, the strain of bacteria used was streptococcus pneumoniae . This bacteria are of two types, called type- II and type- III, also called rough strain and smooth strain respectively.. The difference between these two strains are that, smooth strain have a protective covering of polysaccharide due to which the bacteria is not affected by host enzymes or immune system and hence is also called virulent type whereas II- R does not have any such coverings and are easily affected by the host immune system and is also called non- virulent type. Griffith performed step- wise experiment to prove this hypothesis, where he first injected III- S in mice and as expected the mice died. But when he injected II- R type in the mice, the mice survives and is not affected in any way. Now, when he heated the III- S and then injected in mice, the mice survived. It is because the virulent part is converted to non- virulent form due to heat and hence becomes ineffective in the mice. When he carried on with the final experiment, where he injected mice with heat killed III S type and living II R, to his surprise the mice died.

Therefore, this final result showed that though III S type was heat killed still the mice died because the information was transferred from heat killed III S type to the living II R, which became virulent and hence killed the mice.

Though at that time he didn’t knew the actual reason behind this but slowly when different scientist went on further research with this experiment they discovered the reason and hence the way for the transfer of information.

Avery

He was the scientist who further worked on the hypothesis of Griffith to find out the material or molecule that was involved in transfer of genetic information.

Avery

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For this, he took bacteria with type III S where type III represents the type of particular strain of bacteria that have smooth covering of polysaccharide sheath. Now type III S is heated, different enzymes like protease, RNase and DNase is added in different test-tube containing the heat killed S type. When the mixture with Protease is added to type II R strain, it gets transformed to virulent form having DNA and RNA. Likewise, when the mixture with RNase is added to type II R strain, it also gets transformed to virulent form having DNA and Protein. Finally, when mixture having DNase is added to type II R strain, only the R strain bacteria can be noticed that are non- virulent with protein and RNA in them. Therefore, this proves that DNA is responsible for transfer of genetic information.

Hersey and Chase

They were the first one to prove that the transfer of information from one bacterium to the other occurred due to the presence of DNA or it can be clearly said that DNA carried the genetic information.

Hersey and Chase

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In their experiment they used viruses named bacteriophage to infect bacteria. They have discovered that viruses have two main parts, the first one is DNA and the other is the protein coat inside which the DNA lies. They then used radioactive material to tag different part of the virus. In the first step of their experiment, they tagged the protein coat with sculpture- 35 so as to find out whether protein is the one that carries the genetic information. Father, they kept bacteria and the virus together for some time, then they used blender to make apart and finally centrifugation was done to separate the complete material. Then spinning was done with the help of spinning machine that resulted in settling of heavy material done the test-tube while the lighter material remained at the upper portion. When they carried on the test, they found that radioactive material i.e. protein was absent in the complete solution. This explained that protein was not injected into the bacteria. Now in the next part of the experiment, they tagged DNA with phosphorus- 32. Again the same process of grinding, centrifugation, spinning was used. They found out that DNA was present in the mixture.

Therefore, this experiment proves that DNA is the genetic material that infected the bacteria and not the protein.

Watson and Crick

Watson and Crick

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They were the first scientist who gave the exact model of DNA. Before, looking to the detailed model, they were already equipped with some of the basic information due to the different experiments carried on by different scientist prior to them. The information’s including:

1. DNA is a helix.

2. It is made of two strands.

3. The strands are anti- parallel to each other, one runs from 5’ direction to 3’ direction and other from 3’ to 5’ direction.

4. Finally, is the pairing that involves purines = pyrimidines

Therefore, with this information available, they worked on further with other chemicals so as to get the exact model . Finally, after many trials they were able to discover the accurate structure of the DNA molecule.

1. DNA is a double helical structure.

2. The two helix runs anti- parallel to each other.

3. They are made of sugar phosphate backbone that points on outside of the helix and nitrogenous bases those points towards the centre of the helix.

4. The two strands are hold together by the complementary pairing of the bases i.e. adenine always pairs with thymanine and guanine always pair with cytosine.

These are some of the experiments performed by some of the famous scientist in discovering the genetic material and its model.