NEET Botany Molecular Basis of Inheritance Questions Solved


(a) Write the conclusion drawn by Griffith at the end of his experiment with  Streptococcus pneumoniae.

(b) How did O.Avery, C.MacLeod and M.McCarty prove that DNA was the genetic material? Explain.

(a) Transformation was first studied by Frederick Griffith in 1928. He conducted experiments with streptococcus pneumoniae, the bacterium responsible for pneumonia.

1. He observed two strains of this bacterium, some produce shiny colonies (S) while others produce rough colonies (R). Strain bacteria have a mucous (polysaccharide) coat, while R-strain does not have.

2. The S- strain bacteria are virulent while the R-strain bacteria are non-virulent.

3. When live S-type bacteria were injected into mice, they died from pneumonia.

S-strain  inject into mice mice die

4. Mice injected with R-strains, mice survived.

R-strain inject into mice mice live

5.Heat killed S-strain bacteria were injected into mice, did not kill them.

  S-strain (heat killed) inject into mice mice live

6. S-strain (heat killed) were mixed with R-Strains (live) and then injected into mice, the mice died of pneumonia.

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

7.He isolated living S bacterium from the dead mice. He concluded that the R-strain bacteria had somehow been transformed by the heal killed S-strain bacteria.Some transforming principle transferred from the heat killed S-strain bacteria. It changed the R-bacteria into S-bacteria , so R-bacteria were able to produce a smooth polysaccharide coat and become virulent.

(b) The three scientists were : Oswald Avery, Colin Macleod, Maclyn McCasty.

(c) They concluded that DNA is a hereditary material.

 They purified bio-chemicals (DNA,RNA, proteins etc.) from the heat killed S-cells.

These fractions were inserted individually alongwith live R-cells. DNA was able to transform R-cells into S-cells.

Also protein digesting enzymes and RNA digesting enzymes did not affect transformation, so transforming substance was not RNA or protein. Digestion with DNA which inhibits transformation suggested DNA causes transformation.

Difficulty Level:

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