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Perhaps the most striking event in prophase is the condensation of the chromosomes into thread like structures that are visible under the microscope. Recall that DNA replication has already taken place by this point in the cell cycle. However, the duplicated strands of DNA are still physically connected. At the chromosomal level, the duplicated strands (called sister chromatids) are attached at the region called the centromere. The kinetochore is associated with each sister chromatid at the centromere.
During spindle formation, the microtubules in the cell disassemble and reassemble into the spindle, and the nucleus begins to break down. Once the nuclear envelope has disassembled, one end of each microtubule attaches to a kinetochore while the other end connects to one of two areas in the cell called the microtubule organizing centers (MTOC) The MTOC are located on opposite sides, or poles, of the cell. In some cells, a barrel shaped structure called a centriole is associated with each MTOC.

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