Final answer:
The greatest concentration of radioactive deoxythymidine triphosphate (dTTP) in growing bacterial cells would be in the nucleoid region where DNA replication occurs.
Step-by-step explanation:
If radioactive deoxythymidine triphosphate (dTTP) is added to a culture of rapidly growing bacterial cells, the greatest concentration of radioactivity would be expected to be found in the nucleoid (D). This is because in bacterial cells, which lack a nucleus, the genetic material including DNA is localized in the nucleoid region. The addition of radioactive dTTP would be incorporated into the DNA during replication, which primarily takes place in this central part of the cell where the bacterial chromosome is found. Since thymidine triphosphate is a precursor to DNA synthesis, it will not be incorporated into RNA or proteins, and thus you would not expect to find the radioactive label in the ribosomes (E) or in the endoplasmic reticulum (C).
Rapidly growing bacterial cells are actively synthesizing DNA and are therefore actively incorporating dTTP into their DNA molecules. Since DNA synthesis occurs in the cytoplasm of bacteria, this is where the greatest concentration of radioactivity would be found. The other options, such as the nucleus, endoplasmic reticulum, nucleoid, and ribosomes, are not directly involved in DNA synthesis.