DNA Replication, Repair and Cellular Reproduction

The Eukaryotic Cell

You must be able to narrate the steps of mitosis reasonably well going into the MCAT. The exam often poses questions on the events associated with different stages. Look out for the subtleties and possible confusions.

In prophase, the chromosomes condense, appearing as two identical chromatids connected at the centromere. The DNA has already been duplicated, so the chromosome at this stage contains two identical copies, two copies within each chromosome (not two chromosomes, but two chromatids). After the chromosomes split, later in metaphase, the individual copies can then separately be known as chromosomes.

One favorite set of MCAT facts to remember is that the nucleolus disappears during prophase, and that the end of prophase is marked by the breakdown of the nuclear envelope, which allows the microtubules of the centioles at opposite ends of the cell to reach the centromeres of the chromosomes.