karyokinesi

Karyokinesis, sometimes referred to as karyokinesis or karyokinesi in some linguistic contexts, is the process by which a eukaryotic cell’s nucleus divides to form two nuclei. It is a central component of cell division and is typically followed by cytokinesis, the division of the cytoplasm. Karyokinesis occurs during mitosis, producing two genetically identical nuclei, and during meiosis, where two successive divisions yield four haploid nuclei.

There are two structural modes of karyokinesis: open and closed. In open karyokinesis, the nuclear envelope breaks down during prophase to allow spindle microtubules access to the chromosomes. In closed karyokinesis, the envelope remains intact, and the spindle interacts with chromosomes through nuclear pores or openings. Open karyokinesis is common in animals and many plants, while closed karyokinesis occurs in several fungi and some protists.

The process typically involves chromosome condensation, formation of the spindle apparatus, and attachment of chromosomes to microtubules via kinetochores. During metaphase, chromosomes align at the metaphase plate; in anaphase, sister chromatids separate and move toward opposite poles (anaphase A and B processes); telophase re-forms the nuclei around the separated chromosome sets, and the chromatids de-condense as the nucleus is restored. Cytokinesis then partitions the cytoplasm, yielding distinct daughter cells.

Regulation of karyokinesis is tightly coordinated with the cell cycle, particularly at the G2/M checkpoint. Key regulators include cyclin-dependent kinases and various checkpoint proteins ensuring DNA integrity before chromosome segregation. Abnormal karyokinesis can lead to aneuploidy or polyploidy, with implications for development and disease.