prophase

Prophase is the first stage of cell division in both mitosis and meiosis. During prophase, chromatin condenses into visible chromosomes; each chromosome has two sister chromatids held at the centromere. The nucleolus disappears as ribosomal RNA synthesis declines. Centrosomes move to opposite poles, and the mitotic spindle begins to assemble from microtubules and other proteins, organizing the cytoskeletal framework for chromosome movement.

In mitosis, early prophase features continued chromosome condensation while the nuclear envelope remains intact. As prophase progresses, spindle fibers attach to kinetochores at centromeres, and the centrosomes separate. The nuclear envelope then disassembles, allowing spindle microtubules to interact with chromosomes, signaling the transition to prometaphase.

In meiosis, prophase I is distinct: homologous chromosomes pair in synapsis, mediated by the synaptonemal complex, and exchange genetic material through crossing over during pachytene. This pairing persists into diplotene/diakinesis as chiasmata hold homologs together. Prophase II in many organisms resembles mitotic prophase, with condensed chromosomes and a reformed spindle preparing for the second division. The end of prophase demarcates the move to prometaphase or metaphase, depending on the division stage.