During prophase, a cell prepares itself for cell division by condensing its genetic material into separate chromosomes. The chromatin fibers, which were previously relaxed and loose, coil tightly into organized chromosomes. Each chromosome consists of two single chromatids, which are jointed at a central portion called the centromere. The nuclear envelope dissolves in late prophase, allowing the genetic material to flow more loosely throughout the cytoplasm.
In addition to readying the genetic material for cell division, the cell also undergoes several other changes during prophase. The mitotic spindles are assembled from proteins and microtubules in the cytoplasm. In the case of an animal cell, these mitotic spindles are referred to as asters, and they are organized around each centriole pair.
In late prophase, the nuclear envelope breaks down, and the polar tubules attach themselves to the chromosomes at a region called the kinetochore. This attachment gives the cell a means by which to move and manipulate the chromosome pairs as mitosis progresses. The polar tubules, which are a specialized type of microtubules, are connected to one of the cell's poles on one end and to the chromosome at the kinetechore on the other end. Prophase ends, and the next stage of mitosis, metaphase, begins when the polar tubules direct the chromosomes towards the equator of the cell.