The chromosomes carry the genetic information for your entire body on massively long strands of DNA. It is the DAN whereby your cells recieve the instructions for how to make every single protein in your body, necessary for countless chemical processes, the immune system, cell signalling and so on. So the DNA is necessary to be present in each cell for the cell to function. When a somatic (non-germ cell) divides it must make a copy of the DNA so that both daughter cells contain one copy. The human cell normally has 23 pairs of chromosomes making 46 in total, so that you have a paternal copy of c'some 1 and a maternal copy of c'some 1, etc. al the way up to c'some 23. They then all replicate without the new halves breaking off, so that each of the 46 chromosomes that used to look like a long string of DNA, now looks like 2 long strings of DNA joined at the middle. The spindle then forms and the homologous c'somes line up so that when the spindle pulls them apart into two separate lots, the two halves of each c'some are sparated so that when mitosis finishes, each daughter cell has a copy of the maternal and paternal versions of each c'some, i.e. both should be an exact genetic copy of the original cell. In practise there is a slight error tjhat can occur in copying, so there is a DNA proofreading enzyme that is 99.9% efficient but even so single point mutations can occur (de novo mutations). A cell can normally cope well with these, so that if you took a cell from your tongue and a cells from your heart, the chromosomes would be *almost* entirely identical, no small feat considering size of the human genome. This process is mitosis, not meiosis which is what occurs in germ cels (sperm and ova). However, if a cell has the wrong quotient of c'some the results can be drastic. These are almost always meiosis based such as Down's syndrome where there are three copies of c'some 21, Turner's syndrome where there is only 1 X c'some and no Y c'some, and klinefelters syndrome where they have 2 X's and a Y c'some. Polyploidy, where there are more than 2 copies of each c'some is lethal. Your question referred to mitosis though, and this is much less written about. Since cells need a large variety of different proteins to be expressed jsut to function normally, and these proteins are coded for across all the different c'somes, without a particular c'some the cell would cease to function and would die. Down's syndrome can also occur in mitosis when the embryo is still developing - this is called chromosomal mosaicism. If one daughter cell received 3 copies of c'some 21 and the other recieved only 1 copy, the latter cell would die (it is said to be nullsomic). But the other cell with the extra c'some would continue to grow and divide as normal. So that the baby will have some proportion of cells normal and some proportion of cells with an extra c'some. People with Down's syndrome mosaicism can be very severely affected or very mildly affected depending on the proportion of normal cells to trisomic cells.