How can the cells in a multicellular organism differ from each other when they all have identical DNA?

Question

How can the cells in a multicellular organism differ from each other when they all have identical DNA?

Answer 1

Different cells in a multicellular organism have become specialised in a process called differentiation. This involves turning certain genes off and certain genes on to produce the proteins and structures specific to that cell type. It is a question of different gene expression in different cell types. Eg. liver cells have "liver genes turned on", muscle cells have "muscle genes turned on".