Class 12th Biology 2012 Set3 Delhi Board Paper Solution

Question 30

Name the scientists who proved experimentally that DNA is the genetic material. Describe their experiment.
OR
(a) List the three different allelic forms of gene I in humans. Explain the different phenotypic expressions, controlled by these three forms.
(b) A woman with blood group A marries a man with blood group O. Discuss the possibilities of the inheritance of the blood groups in the folllowing starting with yes or no for each:
(i) They produce children with blood group A only.
(ii) They produce children some with O blood group and some with A blood group.

Answer

The two scientists that proove DNA is the genetic material are:

(i) Griffith Transformation Experiment

(a) In 1928, Frederick Griffith, in a series of experiments with Streptococcus pneumoniae (bacterium responsible for pneumonia), witnessed a miraculous transformation in the bacteria.

(b) When Streptococcus pneumoniae (pneumococcus) bacteria are grown on a culture plate, some produce smooth shiny colonies (S) while others produce rough colonies (R). This is because the S strain bacteria have a mucous (polysaccharide) coat, while R strain does not. Mice infected with the S strain (virulent) die from pneumonia infection but mice infected with the R strain do not develop pneumonia.

S strain ---> Inject into mice ----> Mice die

R strain ----> Inject into mice ----> Mice live

Griffith was able to kill bacteria by heating them. He observed that heatkilled S strain bacteria injected into mice did not kill them.

S strain (heat-killed) ----> Inject into mice ----> Mice live

When he injected a mixture of heat-killed S and live R bacteria, the mice died.

S strain (heat-killed) + R strain (live) ----> Inject into mice ----> Mice die

Moreover, he recovered living S bacteria from the dead mice.

(c) He concluded that the R strain bacteria had somehow been transformed by the heat-killed S strain bacteria. Some ‘transforming principle’, transferred from the heat-killed S strain, had enabled the R strain to synthesise a smooth polysaccharide coat and become virulent. This must be due to the transfer of the genetic material.

(d) Prior to the work of Oswald Avery, Colin MacLeod and Maclyn McCarty (1933-44), the genetic material was thought to be a protein. They worked to determine the biochemical nature of ‘transforming principle’ in Griffith’s experiment.

(e) They purified biochemicals (proteins, DNA, RNA, etc.) from the heat-killed S cells to see which ones could transform live R cells into S cells. They discovered that DNA alone from S bacteria caused R bacteria to become transformed.

(f) They also discovered that protein-digesting enzymes (proteases) and RNAdigesting enzymes (RNases) did not affect transformation, so the transforming substance was not a protein or RNA. Digestion with DNase did inhibit transformation, suggesting that the DNA caused the transformation. They concluded that DNA is the hereditary material.

(ii) Hershey and Chase Experiment

(a) DNA is the genetic material came from the experiments of Alfred Hershey and Martha Chase (1952). They worked with viruses that infect bacteria called bacteriophages.

(b) The bacteriophage attaches to the bacteria and its genetic material then enters the bacterial cell. The bacterial cell treats the viral genetic material as if it was its own and subsequently manufactures more virus particles. Hershey and Chase worked to discover whether it was protein or DNA from the viruses that entered the bacteria.

(c) They grew some viruses on a medium that contained radioactive phosphorus and some others on medium that contained radioactive sulfur. Viruses grown in the presence of radioactive phosphorus contained radioactive DNA but not radioactive protein because DNA contains phosphorus but protein does not. Similarly, viruses grown on radioactive sulfur contained radioactive protein but not radioactive DNA because DNA does not contain sulfur.

(d) Radioactive phages were allowed to attach to E. coli bacteria. Then, as the infection proceeded, the viral coats were removed from the bacteria by agitating them in a blender. The virus particles were separated from the bacteria by spinning them in a centrifuge.

(e) Bacteria which was infected with viruses that had radioactive DNA were radioactive, indicating that DNA was the material that passed from the virus to the bacteria. Bacteria that were infected with viruses that had radioactive proteins were not radioactive. This indicates that proteins did not enter the bacteria from the viruses. DNA is therefore the genetic material that is passed from virus to bacteria.

genetic material