Chromatin is the genetic material (DNA) in a cell. It replicates during interphase, prior to cell division. Once replicated, chromatin organizes into visible chromosomes, two strands of DNA (sister chromatids) joined by a centromere. In mitosis, each sister chromatid is distributed to one of two daughter cells. In this way each daughter cell is identical to its parent.
These somatic cells are diploid. They have homologous pairs of chromosomes, one from each parent, that carry the same genetic traits. Gametes are produced by meiosis. They are haploid: each chromosome is unique. When the haploid gametes from a male and a female combine to form a zygote, the zygote is diploid and ready to produce the somatic cells of the new baby.
Prior to meiosis, chromatin is replicated. Meiosis then occurs in two stages. In meiosis I, the homologous chromosomes distribute to individual daughter cells. These daughter cells are haploid and each chromosome consists of two sister chromatids joined at a centromere. In miosis II, the sister chromatids split at the centromeres and distribute to daughter cells.
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