Which cell structures incorporate radioactively labeled uracil in dividing mammalian cells?

The correct answer is that ribosomes incorporate radioactively labeled uracil in dividing mammalian cells. Uracil is a nucleotide base found in RNA and is involved in the synthesis of mRNA during transcription. When cells are dividing, they are actively synthesizing RNA to produce proteins, and ribosomes play a crucial role in translating this mRNA into proteins.

In the process of transcription, uracil is incorporated into the newly synthesized RNA strand as it pairs with adenine on the DNA template. Since ribosomes are the sites of protein synthesis and rely on mRNA, they will be associated with this newly synthesized RNA that contains uracil. Thus, when dividing mammalian cells are exposed to radioactively labeled uracil, it will be incorporated into the RNA molecules and will be seen in the ribosomes where these RNA molecules are being utilized for protein synthesis.

Other cell structures like the nucleus, mitochondria, and endoplasmic reticulum, while they play important roles in cellular function, do not specifically incorporate uracil in the same manner during the division process. The nucleus is where RNA is synthesized, but it does not directly incorporate uracil as it is more about the process of transcription. Mitochondria are primarily