Radioactive tracers in bacterial cell biology An Escherichia coli (E. coli) culture is grown for 48 hours in medium containing radioactive sulfur (for example, ^35S in sulfate or labeled methionine/cysteine). After this incubation, in which cellular macromolecules would you expect the radioactive label to be predominantly located, and why?

Introduction / Context:
Radioisotopes are powerful tracers for following the biosynthesis of macromolecules in microbiology and biochemistry. Sulfur radioisotopes such as ^35S are especially informative because cellular macromolecules differ in their sulfur content. Understanding where a tracer will be incorporated helps in experiment design and interpretation.

Given Data / Assumptions:

• E. coli is grown in medium containing radioactive sulfur for 48 hours.
• Macromolecular classes of interest include DNA, RNA, proteins (enzymes), and lipids.
• DNA and RNA contain phosphorus-rich backbones; proteins may contain sulfur-bearing amino acids.

Concept / Approach:
Cellular sulfur resides primarily in the amino acids methionine and cysteine. When ^35S-labeled sulfate, methionine, or cysteine is available, cells synthesize proteins incorporating the label at positions where these amino acids occur. In contrast, nucleic acids (DNA and RNA) have backbones of sugar and phosphate and do not contain sulfur; they are tracked with ^32P or ^3H labels. Most membrane phospholipids also lack sulfur.

Step-by-Step Solution:
Identify sulfur-containing macromolecules: proteins via methionine and cysteine.Recognize that DNA/RNA contain phosphorus, not sulfur.Note that typical phospholipids are phosphorus-containing, sulfur-free.Conclude ^35S will accumulate in proteinaceous enzymes synthesized during growth.

Verification / Alternative check:
Classic Hershey–Chase experiments exploited exactly this distinction: ^35S labeled protein coats of phage while ^32P labeled nucleic acids. The same biochemical logic applies in bacteria.

Why Other Options Are Wrong:

• DNA: lacks sulfur; labeled with phosphorus isotopes, not sulfur.
• RNA: likewise lacks sulfur.
• All of these: incorrect because not all listed classes contain sulfur.
• Membrane phospholipids: predominantly sulfur-free in bacteria.

Common Pitfalls:
Assuming any macromolecule will incorporate any isotope; the chemical element must be present in the molecular structure to retain the label.

Final Answer:
Enzymes (proteins)