Fluorescence light micrograph of a set of human chromosomes from an unidentified cell. Chromosomes are visible with the light microscope only during cell division, either at mitosis or meiosis. During cell division they contract in length, becoming short thick rods as seen here. At mitosis these rods divide into two equal parts, ultimately forming two identical daughter nuclei. Human chromosomes consist of the DNA or the genetic material of the cell. Human cells contain a set of 46 chromosomes.