Final answer:
In eukaryotic cells, cellular reproduction is initiated and concluded by growth factors and cyclins, respectively, which regulate the cell cycle and lead to cytokinesis.
Step-by-step explanation:
In eukaryotic cells, the two substances that signal the start and end of cellular reproduction are cyclins and growth factors. At the beginning of the cell cycle, growth factors signal the cell to prepare for division, while specific cyclins accumulate and activate cyclin-dependent kinases (CDKs) at different points in the cycle, driving the process forward. Eventually, other signaling pathways come into play to ensure that cells exit mitosis, concluding cellular reproduction with cytokinesis, where the cytoplasm and its contents are divided between the daughter cells.