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In the 1920s, Frederick Griffith conducted an experiment in which he mixed the dead cells of a bacterial strain that can cause pneumonia with live cells of a bacterial strain that cannot. When he cultured the live cells, some of the daughter colonies proved able to cause pneumonia. Which of the following processes of bacterial DNA transfer does this experiment demonstrate?

1. transformation
2. transduction
3. transposition
4. conjugation
5. vertical gene transfer

Answer :

In the 1920s, Frederick Griffith conducted a groundbreaking experiment involving bacterial strains to understand how certain traits are transferred among bacteria. This experiment is a classic study in the field of genetics and demonstrates the process known as transformation (option 1).

Explanation:

  1. What Griffith Did:

    • Griffith used two strains of the bacterium Streptococcus pneumoniae: a virulent strain that could cause pneumonia and a non-virulent strain that could not.
    • He killed the virulent bacteria by heat and mixed these dead bacteria with live non-virulent bacteria.

  2. Observations:

    • Unexpectedly, when these mixtures were cultured, some of the live non-virulent bacteria transformed and acquired the virulence from the dead bacteria.
    • The transformed bacteria showed virulence, meaning they gained the ability to cause pneumonia.

  3. Conclusion:

    • Griffith concluded that some "transforming principle" from the dead virulent bacteria was taken up by the live non-virulent bacteria, resulting in the latter’s transformation into a virulent form.
    • This "transforming principle" was later identified as DNA, confirming the role of DNA in heredity.

Importance:

  • This experiment demonstrated that DNA can be transferred from one organism to another, altering its genotype and phenotype.
  • It laid the foundational understanding that DNA is the genetic material responsible for heredity.

Hence, the process demonstrated in Griffith's experiment is transformation, which involves the uptake and incorporation of external DNA by a cell, particularly seen here with the bacteria. This insight was crucial for future discoveries in molecular biology and genetics.

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