USA Biology Olympiad (USABO) Practice Exam

If radioactive sulfur is used in the culture medium of bacteria that harbor bacteriophage viruses, where will it later appear?

1. Progeny bacteria

2. Cytoplasm of bacteria

3. Viral coats

4. Bacterial cell membranes

The correct answer is: Viral coats

Radioactive sulfur will appear in the viral coats because sulfur is a key component of amino acids, particularly cysteine and methionine, which are found in proteins. When bacteriophages infect bacteria, they introduce their genetic material into the host cells, where the host's cellular machinery uses the nucleic acids from the phages to produce new virus proteins, including the protein coats. Since radioactive sulfur is part of the culture medium, it gets incorporated into the proteins synthesized during viral assembly within the infected bacteria. Consequently, as the viruses are produced, the sulfur-labelled proteins become part of the phage coat, allowing them to be marked with radiation. Once the newly formed bacteriophages exit the bacterial cell, they carry the radioactive sulfur in their protein coats. The other options do not align with the biochemical processes at play. Progeny bacteria would not incorporate the radioactive sulfur because it is not involved in the synthesis of bacterial components; instead, it specifically integrates into proteins formed by the bacteriophage. Although some radioactive sulfur could theoretically end up in the cytoplasm or bacterial cell membranes, it primarily gets incorporated into the viral structures formed as a direct result of the bacteriophage activity and assembly within the bacterial host.