about mitosis.......緊急......................................

ya....it is true that the genetic material of all cell types in an individual is the same(except that somatic cells are diploid while germ cell are haploid), but actually, we know that the cell would make different kinds of proteins for their specialized function from the process gene expression(synthesis of cellular protein from gene through gene transcription and translation). And the underlying reason for different cells can have differnet function is that differnet cells have different regulation on the gene expression.
different kinds of genes is expressed in differnet cell to make different proteins to fulfill the cells specific function.

its because a process called differentiation.( specialization)
during this process.....cell being specify to do the particular thing...particular function, particular cell.