During interphase (not shown), the genetic material in the nucleus replicates. In prophase (first box), the nuclear membrane dissolves and the chromatin of the nucleus becomes distinct threads ("mitos" from the Greek for "thread"). These threads--chromosomes--condense and thicken. At prophase, the cell contains twice the normal number of chromosomes (46 pairs). During metaphase, the 46 pairs line up on the cell equator, with the centrioles at either end of the cell. From each centriole extends a fiber to the chromosomes. These spindles consist of microtubules and are responsible for the subsequent separation and movement of the the pairs. Each chromosome pair consists of a pair of identical chromatids, joined at a point called a centromere. In anaphase, the pairs separate and retract toward the ends of the cell along the spindle fibers, as if being pulled in. In telophase, the chromosomes disappear into chromatin, and the two masses of it are surrounded by new membranes. The spindles fibers dissolves and the cytoplasm divides along the equator, forming two new daughter cells.