acid (DNA) (which at this point consists of long thin strands called chromatin) is replicated. As each strand is replicated, it is linked to its duplicate by a structure known as a centromere (although in S. cerevisiae and similar organisms microtubules are nucleated by spindle pole bodies rather than by centromeres). When the S stage is complete, the cell enters a brief stage known as G2, when specialized enzymes correct any errors in the newly synthesized DNA, and proteins involved with the next phase, mitosis, are synthesized.
Mitosis itself occurs in four steps. In prophase the replicated, linked DNA strands slowly wrap around proteins that in turn coil and condense into the rodlike structures called chromatids. Two structures called centrioles, both located on one side of the nucleus, separate and move toward opposite poles of the cell. As the centrioles move apart, they begin to radiate thin, hollow, proteins called microtubules, which arrange themselves in the sh
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