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The Mitotic Spindle
The mitotic spindle—or spindle apparatus—is a eukaryotic, cytoskeletal structure made up of long protein fibers called microtubules. Formed during cel…
The mitotic spindle separates sister chromatids and moves them to opposite sides of the cell during anaphase of mitosis.
Fundamental structures of the mitotic spindle are hollow cylinders called microtubules. Two sets of microtubules are arranged on opposite ends, or poles, of the mitotic spindle.
Each microtubule has a minus end and a plus end. The minus ends of the microtubules meet at the center of the spindle poles. The plus ends extend outward from the poles.
There are different types of microtubules with distinct positions and roles in the mitotic spindle.
Kinetochore microtubules bind chromosomes to the spindle pole by attaching at their plus ends to kinetochores. Kinetochores are large protein complexes assembled at the chromatid centromere, a specialized DNA sequence that links sister chromatids.
The arrangement of interpolar microtubules resembles a pair of clasped hands. The plus end of an interpolar microtubule overlaps with the plus end of another interpolar microtubule extending from the opposite pole. Motor proteins associate with interpolar microtubules to direct spindle assembly.
Astral microtubules anchor the spindle in the cell. These microtubules collectively resemble a starburst, with each positive end projecting outward from the spindle pole to the cell cortex.
In most animal cells, the microtubules are organized around an organelle called a centrosome. One centrosome is present at each spindle pole.
A centrosome consists of two centrioles surrounded by a shapeless mass of proteins called the pericentriolar matrix. The centrosome produces, organizes, and anchors microtubules in the cell.
Two families of motor proteins are integral to the construction and operation of the mitotic spindle: kinesin-related proteins and dynein. Typically, kinesin-related proteins move toward the plus ends of microtubules and dynein moves towards the minus ends.
The intrinsic polarity of the mitotic spindle and its microtubules facilitates the mitotic segregation of chromosomes, preparing the cell for division.
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Q1: What is the mitotic spindle and what role does it play in cell division?
The mitotic spindle is a dynamic structure composed of microtubules and associated proteins that forms during cell division. It organizes and separates chromosomes to opposite poles of the cell, ensuring each daughter cell receives an identical set of genetic material. The spindle is essential for proper chromosome segregation during mitosis and cytokinesis.
Q2: How do microtubules form the structure of the spindle apparatus?
Microtubules polymerize from tubulin proteins and extend from centrosomes located at opposite poles of the cell. These microtubules organize into three functional groups: kinetochore microtubules that attach to chromosomes, polar microtubules that overlap at the cell's center, and astral microtubules that extend toward the cell membrane, collectively forming the spindle apparatus.
Q3: What happens to the mitotic spindle during metaphase?
During metaphase, spindle microtubules attach to kinetochores on chromosomes, pulling them to the cell's equatorial plane. The chromosomes align at the metaphase plate, a position midway between the two spindle poles. This alignment ensures that when sister chromatids separate, each daughter cell will receive one copy of every chromosome.
Q4: How does the spindle separate sister chromatids during anaphase?
During anaphase, kinetochore microtubules shorten, pulling sister chromatids toward opposite spindle poles. Simultaneously, polar microtubules lengthen, pushing the spindle poles apart and increasing cell length. This coordinated action ensures sister chromatids move to opposite ends of the cell, preparing them for cytokinesis and the formation of two genetically identical daughter cells.
Q5: What is the role of centrioles in spindle formation?
Centrioles are cylindrical structures located at the centrosomes that serve as organizing centers for microtubule nucleation. During cell division, the centrosome containing centrioles duplicates, and the two centrosomes move to opposite poles of the cell. From these poles, centrioles help nucleate and organize the microtubules that form the mitotic spindle.
Q6: Why is proper spindle function critical for maintaining chromosome number in daughter cells?
The spindle ensures accurate chromosome segregation by attaching to and moving each chromosome to the correct pole. If spindle function is disrupted, chromosomes may not separate properly, leading to aneuploidy—an abnormal number of chromosomes in daughter cells. This can cause cell death or, in some cases, contribute to cancer development.
Q7: How does the spindle disassemble after chromosome separation?
Following anaphase, as chromosomes reach the spindle poles, microtubules begin to depolymerize and the spindle structure breaks down. The cell enters telophase, where nuclear envelopes reform around the separated chromosome sets. Spindle disassembly is regulated by cellular signals that coordinate with cytokinesis to complete cell division.