The answer is To become specialized, cells need to control the production of proteins coded for in their DNA.
All genetic information in a cell's DNA is not fully expressed at all times. On the contrary, in different cell lines, different DNA segments are expressed at different times. Some genes are active in unspecialized cells and later deactivate, some are activated only in specialized cells, for example. These are facultative genes which are active when needed, unlike constitutive genes which are expressed continually. Therefore, to become specialized, cells need to control the production of proteins coded for in their DNA. DNA segments, however, remain in cell and are not destroyed. Also, during the mitosis, all genes are passed, not only required genes.
