Spindle Fiber
Spindle fibers are dynamic protein structures made of microtubules that form the mitotic spindle apparatus during cell division, responsible for physically separating chromosomes to opposite poles of the cell. They originate from microtubule-organizing centers (centrosomes in animal cells) and attach to chromosomes at specialized protein complexes called kinetochores located on the centromere. Spindle fibers exert pulling forces by depolymerizing (shortening), generating the mechanical work needed to move chromosomes reliably during mitosis and meiosis.
Types of Spindle Fibers and Their Functions
| Type | Attachment Points | Function |
|---|---|---|
| Kinetochore microtubules | Centrosome to kinetochore | Pull chromosomes to poles |
| Polar microtubules | Centrosome to midzone (overlap) | Push poles apart, stabilize spindle |
| Astral microtubules | Centrosome to cell cortex | Position spindle within the cell |
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Related Terms
Centromere
The centromere is a specialized DNA sequence on a chromosome that serves as the attachment point for the kinetochore, the protein complex that connects chromosomes to spindle fibers during cell division. It holds sister chromatids together after DNA replication until they are separated during anaphase. The position of the centromere determines chromosome shape and is used in chromosome classification: chromosomes may be metacentric (central), submetacentric, acrocentric, or telocentric based on centromere location.
Metaphase
Metaphase is the stage of cell division in which chromosomes reach maximum condensation and align along the cell's equatorial plane, known as the metaphase plate, pulled by spindle fibers attached to each chromosome's centromere. This precise alignment ensures that when sister chromatids separate, each daughter cell receives exactly one copy of every chromosome. Metaphase is used in karyotyping because chromosomes are most visible and identifiable at this stage.
Anaphase
Anaphase is the stage of cell division during which sister chromatids are pulled apart by shortening spindle fibers and migrate toward opposite poles of the cell. In mitosis, cohesion proteins between sister chromatids are cleaved, allowing each chromatid (now called a chromosome) to move to a pole. Anaphase is the shortest phase of mitosis and is driven by the depolymerization of spindle microtubules and motor proteins such as dynein.
The word "spindle" refers to the tapered, fusiform shape of the apparatus, resembling a thread-spinning spindle; "fiber" refers to the elongated filamentous structure of microtubules.