Final answer:
The differing proteomes of liver cells, mammary cells, and skin cells, despite having the same genome, is explained by cell differentiation, where each cell type expresses only a specific subset of genes relevant to its function.
Step-by-step explanation:
The observation that liver cells, mammary cells, and skin cells all contain the same genome, yet have drastically different proteomes, is best explained by the phenomenon of cell differentiation. Cell differentiation is the process by which cells become specialized as the body develops. Despite all somatic cells containing the same set of genes, or same genome, each cell type only expresses a subset of these genes at any given time. This selective gene expression is directed by different transcription factor proteins that bind to DNA, promoting or hindering the transcription of specific genes. As a result, each type of cell 'reads' only the portions of DNA that are relevant to its own function, much like actors in a movie reading only their own part of the script. This process leads to cells with distinct morphological and physiological characteristics tailored to their specific roles in the organism.