In a culture of actively dividing mammalian cells, where would radioactively labeled uracil be incorporated?

Radioactively labeled uracil would be incorporated in the ribosomes, primarily because uracil is a nucleotide that is a key component of RNA. During the process of transcription, uracil replaces thymine in RNA synthesis, which means that it is directly involved in the formation of messenger RNA (mRNA).

In actively dividing mammalian cells, the synthesis of mRNA is crucial for producing the proteins necessary for cell growth and division. Once mRNA is synthesized in the nucleus, it is transported to the ribosomes, where it serves as a template for protein synthesis during translation. Thus, the ribosomes are the cellular structures where the incorporation of uracil labeled radioactively is most relevant, given their role in translating mRNA into proteins.

Other cellular components, such as the nucleus and the endoplasmic reticulum, play roles in different stages of the cellular processes, but for the incorporation of uracil specifically in the context of dynamic protein synthesis, the ribosomes are the focal point. Mitochondria have their own unique role tied to energy metabolism and also have a small amount of their own genetic material, but they do not directly involve uracil in the same way the ribosomes do for protein synthesis.