chromosomes
To begin mitosis, the DNA in the nucleus wraps around proteins to form chromosomes . Each organism has a unique number of chromosomes. In human cells, our DNA is divided up into 23 pairs of chromosomes. Replicated DNA forms a chromosome made from two identical sister chromatids , forming an "X" shaped molecule Figure below . The two chromatids are held together on the chromosome by the centromere . The centromere is also where spindle fiber microtubules attach during mitosis. The spindles separate sister chromatids from each other.