The process of mitosis, fundamental to eukaryotic cell replication, allows the nuclei of cells to divide and provides each daughter cell with a complete and identical set of chromosomes. The progression may take only a few minutes or may encompass several hours, largely depending upon plant or animal species and cell type. The digital image displayed below features fluorescently tagged dividing cells in which the mitotic spindle is green, the chromosomes are orange, and the peroxisomes are blue.

Mitotic Spindle Specimen: Fluorescently labeled cell culture Technique: Confocal Microscopy

Highly dynamic, the mitotic spindle is composed of microtubules and associated proteins, which work together to form the mechanical framework needed for cellular division. During prophase, the important mitotic apparatus begins to form outside the nucleus at opposite ends of a cell, stretching from pole to pole. In metaphase, the mitotic spindle attaches to the centromere of each chromosome, moving them through the mitotic process. The chromatids of each chromosome are then pulled apart in anaphase and the spindle fibers disperse in telophase.

Various classes of microtubules make up the mitotic spindle, each carrying out a specific function. The microtubules converge at the ends of the spindle, known as the spindle poles, and are occupied by centrosomes. The centrosomes appear to organize the microtubule groups and, within animal cells, each centrosome contains a pair of centrioles, which consist of a complex arrangement of microtubules. The true significance of centrioles in mitosis is obscure, however, since they are absent from plant cells that undergo cell division.