When cells are preparing for division via mitosis (or meiosis) the DNA that is in the form of chromatin coils up tighter around proteins called histones and form chromosomes. At this point they are visible under a microscope as individual chromosomes. This will be 46 in a human somatic cell. It is important for chromosomes to form prior to cell division so that DNA can be replicated and then chromosomes separated evenly to make new cells.
Work Step by Step
See 3rd paragraph under 6.3 "The Nucleus: Information Central."