7.5
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 are the main structural components of the mitotic spindle?
The mitotic spindle is composed of hollow protein cylinders called microtubules arranged in two sets at opposite poles. Each microtubule has a minus end meeting at the spindle pole center and a plus end extending outward. Three types of microtubules—kinetochore, interpolar, and astral—work together to organize and move chromosomes during cell division.
Q2: How do kinetochore microtubules attach chromosomes to the spindle?
Kinetochore microtubules bind chromosomes by attaching their plus ends to kinetochores, which are large protein complexes assembled at the centromere of each chromatid. This attachment links sister chromatids to opposite spindle poles, positioning them for separation during anaphase of mitosis and ensuring proper chromosome segregation.
Q3: What role do motor proteins play in spindle assembly and function?
Two families of motor proteins—kinesin-related proteins and dynein—are integral to spindle construction. Kinesin-related proteins typically move toward plus ends of microtubules, while dynein moves toward minus ends. These motors facilitate spindle assembly, chromosome segregation, and the coordinated movement necessary for proper cell division.
Q4: How do interpolar microtubules contribute to spindle organization?
Interpolar microtubules extend from opposite spindle poles with their plus ends overlapping in the middle, resembling clasped hands. Motor proteins associate with these microtubules to direct spindle assembly and maintain the bipolar structure essential for separating sister chromatids to opposite cell ends during anaphase.
Q5: What is the function of astral microtubules in cell division?
Astral microtubules anchor the spindle apparatus within the cell, projecting outward from each spindle pole to the cell cortex in a starburst pattern. These microtubules help position the spindle and ensure proper chromosome segregation by stabilizing spindle pole locations during mitosis and anaphase.
Q6: How do centrosomes organize the mitotic spindle in animal cells?
Most animal cells contain two centrosomes, one at each spindle pole. Each centrosome consists of two centrioles surrounded by the pericentriolar matrix and produces, organizes, and anchors microtubules. This organization creates the bipolar spindle structure necessary for proper chromosome segregation and cell division.
Q7: Why is the polarity of microtubules important for chromosome segregation?
The intrinsic polarity of microtubules—with distinct minus and plus ends—enables motor proteins to move directionally along them. This directional movement, combined with the bipolar spindle arrangement, ensures sister chromatids are pulled to opposite cell poles during anaphase, facilitating accurate chromosome segregation.