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MCAT Content / Mitosis / Mitotic Structures

Mitotic structures

Topic: Mitosis

Mitotic structures such as the spindle apparatus and motor proteins orchestrate the movement of chromosomes during mitosis.

Mitotic structures such as spindle fibers and motor proteins assist in the process and mechanism that separate chromosomes during mitosis. To accurately separate the chromosomes, there is a need for a condense, small, compact object, and a way to move these objects around the cell. Thus, microtubules are essential structures that accomplish this significant step.

Microtubules are rope-like tubes made of protein in cells. They form the mitotic spindle, which is responsible for the capture of chromosomes and align them at the center during prometaphase. 

Centrioles, asters, spindles

Mitotic structures responsible for pulling apart the sister chromatids are centrioles, asters, and spindles. These structures are a component of the spindle apparatus, which also includes motor proteins and chromosomes.

Centrioles are the main centers that help in the formation of microtubule fiber, which makes up the spindles. Without centrioles, there are no spindles that assist the movement of the chromosome.

Asters are star-shaped radial arrays that form around each pair of centrioles. Aster guides the chromosome to ensure that each daughter cell has the right complement chromosome. It helps to organize and position spindle apparatus during mitosis and also determines the site of cleavage furrow that splits the dividing cell in half during cytokinesis.

Spindles are microtubule fibers that arrange and move chromosomes. It is necessary to equally divide the chromosomes in a parental cell into two daughter cells during mitosis. Spindle fibers are also called the mitotic spindle.

Chromatids, centromeres, kinetochores

In the absence of centromere, a kinetochore will not assemble on the replicated chromatids. And that chromosome will fail to segregate during mitosis.

The centromere is a region of highly specialized chromatin. Without it, the cells cannot divide properly, and the overall process of mitosis fails. Its primary function is to provide the foundation for the assembly of the kinetochore, which is a patch of protein structure essential to proper chromosomal segregation. 

Kinetochores appear as platelike structures composed of several layers. During anaphase, several microtubules appear to insert into the kinetochore which situates on the side of the chromosome facing the spindle pole to which the chromosome attaches. The purpose of the kinetochore is to pull the chromatids apart. Kinetochores also help during cell division by making sure that each new cell has one chromatid from each pair.

In addition to their kinetochore-related function, centromeres perform another essential role in mitosis by serving as the sites of sister chromatid attachment. A chromatid is one of two strands that form when a chromosome replicates. Sister chromatids join together. For accurate mitoses, sister chromatids must remain connected until the spindle checkpoint permits.

Nuclear membrane breakdown and reorganization

The nuclear membrane or envelope provides a selective barrier between the nuclear interior and the cytoplasm and constitutes a central component of the intracellular structure. It needs to breakdownthe nuclear membrane at the beginning of mitosis, or during the prometaphase stage, to allow the mitotic spindle fibers to access the chromosomes inside and mix with the nuclear content in the cytosol. At the end of mitosis or during telophase, nuclear envelope reorganizesaround decondensed chromatin. Then, the nuclear compartment reestablishes in each of the newly formed daughter cells.

Mechanisms of chromosome movement

During anaphase, the sister chromatids separate at the centromere. Each chromatid is pulled rapidly toward the centrosome by the spindle fibers. Meanwhile, changes in microtubule length(provide the mechanism for chromosome movement. In the first part of anaphase, the kinetochore microtubules shorten and draw the chromosomes toward the spindle poles. In the second part, the astral microtubules, which anchor to the cell membrane, pull the poles further apart.

A schematic shows a dividing cell. The middle of the cell is slightly indented; it is pinching apart to form two separate, daughter cells. The chromosomes and important parts of the spindle apparatus are labeled.

Practice Questions

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MCAT Official Prep (AAMC)

Key Points

• Centrioles, asters, and spindles are mitotic structures responsible for pulling apart the sister chromatids.

• Chromatids, centromeres, and kinetochores are 

• For most eukaryotes, the nuclear membrane breaks down at the beginning of mitosis (prometaphase) and reorganizes around each of the two newly formed daughter cells at the end (telophase)

• Microtubules cause the chromosome movement; during anaphase, chromatids move apart by the spindle fibers.

Key Terms

microtubules:  small tubes that form part of the cytoskeleton and provide structure and shape to eukaryotic cells

motor proteins: molecular motors that aids in the formation of the spindle apparatus and the separation of the chromosomes during mitosis

spindle fibers:  filaments that are chiefly involved in moving and segregating the chromosomes

centrioles: the main centers that help in the formation of microtubule fiber

asters: star-shaped radial arrays that form around each pair of centrioles

cytokinesis: the physical process of cell division, which divides the cytoplasm

centromere: the specialized DNA sequence of a chromosome that links a pair of sister chromatids

kinetochore: a complex of proteins associated with the centromere of a chromosome during cell division, to which the microtubules of the spindle attach

nuclear membrane: the membrane around the nucleus

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