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Molecular Basis of Inheritance — BIOLOGY STD 12 Science — Question

Maharashtra 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 MacLeod, McCarty and Avery establish the biochemical nature of the so called ‘genetic material’ identified by Griffith in his experiment.
✓

Answer

  1. The discovery of nuclein by Miescher and the proposition of principal of inheritance by Mendel were almost at the same time, i.e. $1869$ and $1866$, respectively. But the fact that DNA acts as a genetic material took a long time to be discovered and proven. By $1926$, the quest to determine the mechanism for genetic inheritance had reached the molecular level and gradually the question, what molecule acts as genetic material got answered.
Transforming Principle

Frederick Griffith in $1928$, carried out a series of experiments with Streptococcus pneumoniae (a bacterium that causes pneumonia). He observed that when these bacteria (Streptococcus pneumoniae) were grown on a culture plate, some of them produced smooth, shiny colonies (S-type), whereas the others produced rough colonies (R-type). This difference in appearance of colonies (smooth/rough) is due to the presence or absence of mucus (polysaccharide) coat on S-strains, but not on R-strains. In his experiments, he first infected two separate groups of mice. The mice that were infected with the S-strain died from pneumonia as S-strains are the virulent strains causing pneumonia. S-strain (virulent strain) → Injected into mice → Mice died. The mice that were infected with the R-strain did not develop pneumonia and they lived. R-strain (non-virulent strain) → Injected into mice → Mice lived.

In the next set of experiments, Griffith killed the bacteria by heating them. The mice that were injected with heat-killed S-strain bacteria did not die and lived. S-strain (heat killed) → Injected into mice → Mice lived. Whereas, on injecting a mixture of heat-killed S-strain and live R-strain bacteria, the mice died. Moreover, living S-bacteria was recovered from the dead mice. S-strain (heat killed) + R-strain (live) → Injected into mice + Mice died.

From all these observations Griffith concluded that the live R-strain bacteria, had been transformed by the heat-killed S-strain bacteria, i.e. some 'transforming principle' had transferred from the heat-killed S-strain, which helped the R-strain bacteria to synthesise a smooth polysaccharide coat and thus, become virulent. This must be due to the transfer of the genetic material. However, he was not able to define the biochemical nature of genetic material from his experiments.
  1. Griffith was not able to find the chemical nature of transforming agent. It was finally Hershey and Chase who proved that DNA is the genetic material. They prov ed it by using radio labelled elements, such as $^{35}S$ and $^{32}P$. Radioactivity was detected in the cell which was due to the presence of $32$ Pin DNA.

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