On Day 15 there are only 7 protein-coding genes, and then we hit the centromere of Chromosome 1 – the place where the two sister chromatids meet, the middle of the classic “X” shape of microscope pictures of metaphase chromosomes. The centromere is where sister chromatids are grabbed by the cell during cell division.
You’ll notice that the sequence at the centromere is mostly displayed lowercase – made up of long noncoding sequences called satellite DNA, repeated over and over. This sequence at the centromeres may not actually be used by the cell to locate the centromere. It has been proposed that the location of the centromere is remembered epigenetically when the cell divides, and across generations.
The centromere is also where we switch over from the short (p) arm to the long (q) arm.
Click here to see the centromere of Chromosome 1.