Mitosis is the seperation of DNA. There are 4 stages of mitosis that you need to be aware of for A level Biology: Prophase, Metahase, Anaphase and Telophase. (I like to remember this order as P.Mat)
During prophase the nuclear envelope is broken down and the chromosomes condense.
During metaphase the chromosomes line u along the equatorial plane (along the middle line of the cell)
During anaphase the chromosomes are pulled to the poles of the cell (either side of the cell) by spindle fibres which attatch to the chromosomes at the centromere
During telophase new neuclear envelops form around the two sets of chromosomes.
This concludes mitosis; after which cytokinesis (the seperation of the cell into 2 cells) occurs.