Final answer:
Cells differentiate as a fertilized egg divides because they are totipotent stem cells with the potential to become any type needed for organismal development, through a process of selective gene expression.
Step-by-step explanation:
As a fertilized egg, known as a zygote, divides through a process known as cleavage cell divisions, the resulting cells differentiate because they are the ultimate stem cells, initially described as totipotent. These totipotent stem cells have the potential to specialize into any type of cell required for the organism's development. This differentiation arises as cells undergo changes in size, shape, metabolic activity, and function to become specialized, a process driven by unique genetic expression. Although all cells contain the same DNA, each cell type only transcribes the portion of the DNA relevant to its own function.
Differentiation and Stem Cells
Cell differentiation is crucial for the development of a multicellular embryo from a single zygote, allowing for the specialized structures and functions of the many different cell types. Stem cells, due to their capacity for unlimited division and potential to differentiate under specific conditions, become the specialized cells that form the various tissues and organs, each using unique genetic instructions encoded in the DNA.
Why Cells Divide
Cell division is essential for growth, maintenance, and repair of tissues in multicellular organisms. From the initial zygote to a complex organism, billions of cell divisions are required. The regulation of this process is critical to proper development, and differentiation is a tightly controlled outcome of cell division, leading to the variety of cell types in an adult.