Spindle Fibers

Spindle Fibers Mitosis
This is a fluorescence micrograph of a cell during metaphase of mitosis. At metaphase, the chromosomes (green) line up along the center of the cell, and the spindle fibers (purple) grow from their poles to the centromeres (yellow), at the center of each chromosome. Credit: DR PAUL ANDREWS, UNIVERSITY OF DUNDEE/Science Photo Library/Getty Images

Definition: Spindle fibers are aggregates of microtubules that move chromosomes during cell division. Microtubules are protein filaments that resemble hollow rods. They are found in eukaryotic cells and are a component of the cytoskeleton, cilia and flagella. Spindle fibers are a part of the spindle apparatus, which moves chromosomes during mitosis and meiosis to ensure each daughter cell gets the correct number of chromosomes.

The spindle apparatus consists of spindle fibers, motor proteins, chromosomes, and in some cells, structures called asters. In animal cells, spindle fibers are produced from cylindrical microtubules called centrioles. Centrioles form asters and asters generate spindle fibers during the cell cycle.

Spindle Fibers and Chromosome Movement

Spindle fibers move chromosomes by attaching to chromosomes at their centromeres. The centromere is the region where duplicated chromosomes are joined. These joined identical copies of a single chromosome are known as sister chromatids. It is at the centromere that specialized protein complexes called kinetochores are found. Kinetochores generate kinetochore fibers, which attach sister chromatids to spindle fibers. Kinetochore fibers and spindle polar fibers work together to manipulate and separate chromosomes during mitosis and meiosis.

Spindle Fibers in Mitosis

  • During prophase of mitosis, spindle fibers form at opposite poles of the cell. In animal cells, the mitotic spindle initially appears as asters, which surround each centriole pair. The cell becomes elongated as spindle fibers extend from each cell pole. Sister chromatids attach to spindle fibers at their kinetochores.
  • During metaphase, spindle fibers called polar fibers extend from the cell poles toward the midpoint of the cell known as the metaphase plate. Chromosomes are held at the metaphase plate by the equal forces of the spindle fibers pushing on the centromeres of the chromosomes.
  • In anaphase, spindle fibers shorten and pull sister chromatids toward the spindle poles. Sister chromatids separate and move toward opposite cell poles. Spindle fibers not connected to chromatids lengthen and elongate the cell.
  • In telophase, the spindle fibers disperse as the chromosomes are separated become housed within distinct new nuclei.

At the end of mitosis and cytokinesis, two daughter cells are formed. Each with the correct number of chromosomes.