Before cell division (mitosis or meiosis) begins, a stage called interphase takes place where the DNA is copied or replicated to form duplicated chromosomes. These are two identical chromosomes (called sister chromatids) that are stuck together, side by side, to give an X-like shape. They’re held together by protein complexes called cohesins.
This isn’t the same as the homologous chromosomes, where we get one copy of chromosome 1 (for example) from the sperm and one from the egg. The two copies of chromosome 1 from the sperm and egg are homologous, meanly they’re nearly the same but could have different gene variants, which is how we get genetic diversity from our parents.
Homologous chromosomes are duplicated before cell division:
