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Molecular Basis of Inheritance — BIOLOGY STD 12 Science — Question

Gujarat BoardEnglish MediumSTD 12 ScienceBIOLOGYMolecular Basis of Inheritance5 Marks
Question
  1. Explain with the help of Griffith's experiment, how the search for genetic material was conducted and what was the conclusion drawn.
  2. How did Mac Leod, Mc Carty and Avery establish the Biochemical nature of the socalled. "genetic material' identified by Griffith in his experiments?
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Answer

  1. Frederick Griffith (1928) performed the experiments on Bacterial transformation with Streptococcus pneumoniae, the bacterium that causes pneumonia.
  1. He observed two strains of this bacterium, one forming smooth shiny colonies with capsule (S-type) and the other forming rough colonies without capsule (R-type).
  2. The S-type cells are virulent while the R-type cells are non-virulent.
  3. When live S-type cells were injected into the mice, they suffered from pneumonia and died.
S strain → Injected into mice → Mice died.
  1. When live R-type cells were injected into the mice, the disease did not appear and the mice survived.
R strain → Injected into mice → Mice lived
  1. When heat-killed S-type cells were injected, the disease did not appear.
S strain → Injected into mice → Mice lived (Heat-killed)
  1. When heat-killed S-type cells were mixed with live R-type cells and injected into the mice, the mice died of pneumonia and live S-type cells were isolated from the body of the mice.
S strain (Heat-killed) + R strain (live)→ Injected into mice → Mice died.
  1. He concluded that the R-strain had somehow been transformed by the heat-killed S-strain bacteria into virulent S-strain, which must be due to the transfer of genetic material, the transforming principle.
  1. ​​​​​​​
  1. Avery, MacLeod and McCarty purified biochemicals like proteins, DNA and RNA from the heat-killed S-cells.
  2. When these fractions were added individually to the culture of live R-cells, DNA was able to cause transformation of R-cells into S-cells.
  3. They also found that protein-digesting enzymes and RNA-digesting enzymes did not affect transformation, indicating that transforming substance is not a protein or RNA.
  4. Digestion with DNase did inhibit transformation; this suggests that the DNA caused transformation.

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