Final answer:
A plaque is a clear spot on a bacterial lawn indicating where a bacteriophage has infected and lysed the host cell, which can be seen on agar plates as clearings. This phenomenon is crucial for virology studies, including the famous blender experiments that highlighted DNA as the genetic material.
Step-by-step explanation:
A plaque is observed as a clear area where phage-infected bacterial cells cultivated on agar have been disrupted. When bacteriophages (phages) infect bacterial cells, such as E. coli, on an agar plate, they replicate inside the cells and eventually cause the host cells to lyse or break open. This lysis results in clear spots, or plaques, on the bacterial lawn. Each plaque represents where a single virus began the infection process, and the area of lysis grows as the virus continues to infect neighboring cells. This is a critical technique in virology used for quantifying the number of virus particles in a sample and studying viral infection.
Phages are considered "inert" until they bind to bacterial cells. This interaction can be visualized using an electron microscope. An important experiment involving phages used blender separation to show that only the viral DNA, not the protein coat, entered the host cells during infection. This was demonstrated using isotopes 32P and 35S to label phage DNA and protein, respectively, leading to significant discoveries about the nature of genetic material.