Spindle Fibers

Spindle Fibers Mitosis
This is a fluorescence micrograph of a cell during metaphase of mitosis. During metaphase, chromosomes (green) line up along the center of the cell and spindle fibers (purple) grow from their poles to centromeres (yellow) at the center of each chromosome.

DR PAUL ANDREWS, UNIVERSITY OF DUNDEE/Science Photo Library/Getty Images

Spindle fibers are aggregates of microtubules that move chromosomes during cell division. Microtubules are protein filaments that resemble hollow rods. Spindle fibers are found in eukaryotic cells and are a component of the cytoskeleton as well as cilia and flagella.

Spindle fibers are part of a spindle apparatus that moves chromosomes during mitosis and meiosis to ensure even chromosome distribution between daughter cells. The spindle apparatus of a cell is comprised of spindle fibers, motor proteins, chromosomes, and, in some animal cells, microtubule arrays called asters. Spindle fibers are produced in the centrosome from cylindrical microtubules called centrioles.

Spindle Fibers and Chromosome Movement

Spindle fiber and cell movement occur when microtubules and motor proteins interact. Motor proteins, which are powered by ATP, are specialized proteins that actively move microtubules. Motor proteins such as dyneins and kinesins move along microtubules whose fibers either lengthen or shorten. The disassembly and reassembly of microtubules produces the movement needed for chromosome movement and cell division to occur.

Spindle fibers move chromosomes during cell division by attaching to chromosome arms and centromeres. A centromere is the specific region of a chromosome where duplicates are linked. Identical, joined copies of a single chromosome are known as sister chromatids. The centromere is also where protein complexes called kinetochores are found.

Kinetochores generate fibers that attach sister chromatids to spindle fibers. Kinetochore fibers and spindle polar fibers work together to separate chromosomes during mitosis and meiosis. Spindle fibers that don't contact chromosomes during cell division extend from one cell pole to the other. These fibers overlap and push cell poles away from one another in preparation for cytokinesis.

Spindle Fibers in Mitosis

Spindle fibers are highly active during mitosis. They migrate throughout the cell and direct chromosomes to go where they need to go. Spindle fibers function similarly in meiosis, where four daughter cells are formed instead of two, by pulling homologous chromosomes apart after they have been duplicated to prepare for division.

Prophase: Spindle fibers form at opposite poles of the cell. In animal cells, a mitotic spindle appears as asters that surround each centriole pair. The cell becomes elongated as spindle fibers stretch from each pole. Sister chromatids attach to spindle fibers at their kinetochores.

Metaphase: Spindle fibers called polar fibers extend from cell poles toward the midpoint of the cell known as the metaphase plate. Chromosomes are held to the metaphase plate by the force of spindle fibers pushing on their centromeres.

Anaphase: Spindle fibers shorten and pull sister chromatids toward spindle poles. Separated sister chromatids move toward opposite cell poles. Spindle fibers not connected to chromatids lengthen and elongate the cell to make room for the cell to separate.

Telophase: Spindle fibers disperse as the chromosomes are separated and become housed within two new nuclei.

Cytokinesis: Two daughter cells are formed, each with the correct number of chromosomes because spindle fibers ensured this. The cytoplasm divides and the distinct daughter cells fully separate.