Final answer:
Rough pneumococci developing granules in the presence of DNA from dead smooth pneumococci is an example of bacterial transformation, where external DNA changes the recipient cell's traits.
Step-by-step explanation:
The observation that rough pneumococci can develop granules after being grown in the presence of DNA from dead smooth pneumococci is a classic example of bacterial transformation. Frederick Griffith's experiment in the 1920s demonstrated this phenomenon when he worked with two strains of Streptococcus pneumoniae, the bacterium that causes pneumonia.
The R strain, which forms rough colonies because it lacks a polysaccharide capsule, took up DNA from the heat-killed S strain, which had a capsule and formed smooth colonies. This uptake of external DNA changed not only the morphology but also the physiology of the R strain, enabling it to produce a capsule and appear smooth, similar to the S strain.
This transformation is significant because it illustrates a mechanism of horizontal gene transfer wherein a bacterium can acquire new genetic traits from its environment, which in this case led to a non-pathogenic strain gaining the virulence factors of a pathogenic one. The incorporation of the genetic material from the dead S strain allowed the R strain to produce a polysaccharide capsule, which is a virulence factor that helps bacteria evade the immune system.