Final answer:
Cells differ from one another due to specialization and differentiation, where each cell type expresses only certain genes relevant to its function despite all of them having the same DNA. This allows for a variety of cell types with unique functions necessary for an organism's survival.
Step-by-step explanation:
All cells are not the same because they undergo specialization and differentiation. Just as actors read only their parts from a movie script, every cell in a multicellular organism contains the same DNA but only "reads" the part that is necessary for its unique function. This is the fundamental concept behind why a muscle cell, a liver cell, and a skin cell look and act differently, despite having an identical genetic blueprint. This specialized gene expression allows cells with the same DNA to become different cell types, enabling the diverse functions necessary for organismal development and survival.
For example, epithelial cells provide protection, bone cells support structure, immune system cells fight pathogens, and red blood cells transport oxygen. These variations among cells are essential for the growth, development, and maintenance of the body. Even unicellular organisms exhibit gene regulation in response to environmental changes, highlighting the importance of gene expression in cell function and diversity.