Final answer:
The process leading to the brain cell and kidney cell's unique structures and functions, despite the same DNA, is differentiation, which dictates what portions of DNA are read and expressed.
Step-by-step explanation:
The unique structures and functions of a brain cell and a kidney cell, despite having the identical set of genetic information, can be attributed to the process of differentiation. Differentiation is analogous to actors in a film who all read from the same script but only perform their specific parts. Similarly, every cell has the same DNA, but a cell 'reads' only the segments necessary for its unique function.
Differentiation involves changes in size, shape, metabolic activity, and function as cells specialize. Transcription factors play a key role in determining which genes are expressed, guiding the specialization of cells into cell's unique types according to the body's needs. This specialization results from the cells' ability to express only certain genes from the entire genome, much like selecting lines from a script to read and act out.