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In eukaryotic organisms—humans included—genetic material is stored in chromosomes inside the nucleus. Cells cycle through growth and division. During interphase the cell duplicates its DNA. The next major event is mitosis, followed by cytokinesis. Prokaryotes, such as bacteria, employ a simpler fission process.
Chromosomes consist of double‑stranded DNA wrapped around histone proteins. During interphase, chromatin decondenses, allowing replication enzymes to synthesize an exact copy of each chromosome. The resulting sister chromatids are connected at a centromere, forming the classic X‑shaped chromosome. When the chromatids condense, interphase ends and mitosis begins.
Prophase marks the onset of mitosis: the nuclear envelope dissolves, and microtubules sprout from centrosomes that migrate to opposite poles. As prophase transitions to metaphase, microtubules attach to kinetochores on each chromatid. The chromosomes line up along the metaphase plate in the cell’s center. Humans possess 23 pairs of chromosomes—one from each parent.
During anaphase, centromeres split, releasing the sister chromatids. The microtubules then pull each chromatid toward opposite poles, temporarily giving the cell 92 chromosomes before division. In telophase, nuclear membranes reform around the two sets of chromosomes, which begin to relax from their condensed state.
Following telophase, the cell physically divides. A contractile ring forms at the former metaphase plate, pinching the membrane inward to create a cleavage furrow. As the furrow deepens and the membrane supplies new material, two daughter cells emerge, each containing a complete set of chromosomes. These cells enter interphase, and the cycle repeats.
Prokaryotic cells bypass mitosis and cytokinesis, instead dividing by binary fission, a single, streamlined process.
