In his transformation experiments, what phenomenon did Griffith observe?

• A. Mixing a heat-killed pathogenic strain of bacteria with a living nonpathogenic strain converts the living cells into the pathogenic form.
• B. Mixing heat-killed nonpathogenic bacteria with a living pathogenic strain makes the living strain nonpathogenic.
• C. Infecting mice with nonpathogenic strains of bacteria makes them resistant to pathogenic strains.
• D. Mice infected with a pathogenic strain of bacteria can spread the infection to other mice.

Answer: (A)

Verified step by step guidance

1. Understand the context of Griffith's transformation experiments, which were pivotal in demonstrating the concept of bacterial transformation.
2. Recognize that Griffith worked with two strains of Streptococcus pneumoniae: a virulent (pathogenic) strain with a smooth capsule (S strain) and a non-virulent (nonpathogenic) strain without a capsule (R strain).
3. Griffith observed that when he mixed heat-killed S strain bacteria with live R strain bacteria and injected them into mice, the mice died. This indicated that the R strain had transformed into the pathogenic S strain.
4. The key phenomenon observed was that the genetic material from the heat-killed S strain was taken up by the live R strain, transforming it into the virulent form. This was one of the first demonstrations of horizontal gene transfer.
5. Reflect on the implications of Griffith's findings, which suggested that DNA could be transferred between bacteria, laying the groundwork for understanding genetic transformation and the role of DNA as the genetic material.