Aster cell biology
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| Mitotic metaphase (upper) and anaphase (lower) in Drosophila tissue culture cells are depicted in this image. Cell Image Library/David Sharp, Dong Zhang, Gregory Rogers, and Daniel Buster |
Asters in animal cells are radial microtubule arrays. During mitosis, these star-shaped structures form around each pair of centrioles. Asters aid in the manipulation of chromosomes during cell division to ensure that each daughter cell has the correct chromosome complement. They are made up of astral microtubules, which are formed from cylindrical microtubules known as centrioles. Centrioles are found within the centrosome, an organelle that forms the spindle poles and is located near the cell nucleus.
Asters and Cell Division
Asters are required for mitosis and meiosis to occur. They are part of the spindle apparatus, along with spindle fibers, motor proteins, and chromosomes. During cell division, asters aid in the organization and positioning of the spindle apparatus. They also determine the location of the cleavage furrow, which is responsible for splitting the dividing cell in half during cytokinesis. Asters form during the cell cycle around the centriole pairs located at each cell pole. Each centrosome generates microtubules known as polar fibers, which lengthen and elongate the cell. During cell division, other spindle fibers attach to and move chromosomes.
During the early stages of mitosis in an animal cell, an aster is a cellular structure shaped like a star, consisting of a centrosome and its associated microtubules. Plants do not form asters during mitosis. Astral rays, which are made up of microtubules, radiate from the centrosphere and resemble a cloud. Astral rays are one type of centrosome microtubule; others include kinetochore microtubules and polar microtubules.
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| The organization of a typical mitotic spindle found in animal cells is depicted in this diagram. The kinetochore is a multiprotein complex that connects chromosomes to microtubules called kinetochores. Polar microtubules interdigitate at the spindle midzone and use motor proteins to push the spindle poles apart. Spindle poles are held to the cell membrane by astral microtubules. The microtubule organizing center initiates the polymerization of microtubules. |
Cell division occurs in five stages during mitosis: prophase, prometaphase, metaphase, anaphase, and telophase. Two aster-covered centrosomes migrate to opposite sides of the nucleus during prophase in preparation for mitotic spindle formation. Prometaphase is characterized by nuclear envelope fragmentation and the formation of mitotic spindles.
During metaphase, the kinetochore microtubules that extend from each centrosome connect to the chromosome centromeres. The kinetochore microtubules then pull the sister chromatids apart into individual chromosomes and pull them towards the centrosomes, which are located at opposite ends of the cell, during anaphase. This allows the cell to divide properly, with each daughter cell containing complete chromosome replicas. The orientation of the asters determines the plane of division upon which the cell divides into some cells.
Astral microtubules
Astral microtubules are a type of microtubule that exists only during and immediately before mitosis. They are defined as any centrosome-derived microtubule that does not connect to a kinetochore. Astral microtubules form in the actin skeleton and communicate with the cell cortex to aid in spindle orientation. They are arranged radially around the centrosomes. This population of microtubules has the highest turnover rate of any population.
Aster microtubules function
Astral microtubules are aided in their function by dyneins specific to this function. The light chains (static portion) of these dyneins are attached to the cell membrane, while the globular parts (dynamic portion) are attached to the microtubules. The globular chains try to move towards the centrosome, but because they are bound to the cell membrane, they pull the centrosomes towards the membrane, assisting cytokinesis.
Astral microtubules are not required for mitosis progression, but they are required to ensure the process's fidelity. The function of astral microtubules can be broadly defined as cell geometry determination. They are absolutely necessary for proper positioning and orientation of the mitotic spindle apparatus and thus play a role in determining the cell division site based on cell geometry and polarity.
The integrity of the centrosome is required for the maintenance of astral microtubules. It also requires several microtubule-associated proteins, including EB1 and adenomatous polyposis coli (APC).
Growth of Microtubules
The two microscopic processes that cause aster growth are polymerization and nucleation. Centrosomes will nucleate (form a nucleus) and anchor to the microtubules at the negative ends of the aster. Polymerization of the aster will occur towards the outer end at the positive end. Cortical Dyenein, a motor protein, moves along the cell's microtubules and is important in the growth and inhibition of aster microtubules. A barrier-attached dyenein can both inhibit and stimulate growth.
Asters in Mitosis
- Asters in prophase: asters first appear in prophase. They form around each pair of centrioles. Asters organize spindle fibers (polar fibers) that extend from the cell poles and fibers that attach to chromosomes at their kinetochores (kinetochores in cell division).
- During metaphase, spindle fibers transport chromosomes to the cell's center. The equal forces of the spindle fibers pushing on the chromosome centromeres keep the chromosomes in place at the metaphase plate. Polar fibers extending from the poles interlock like folded hands' fingers.
- Spindle fibers in cell division: during anaphase, duplicated chromosomes (sister chromatids) separate and are pulled to opposite ends of the cell. This separation occurs as spindle fibers shorten, dragging attached chromatids with them.
- Spindle fibers break down during telophase, and separated chromosomes are enveloped within their own nuclear envelope.
- Cytokinesis is the final stage of cell division. Cytokinesis is the process by which the cytoplasm of a dividing cell divides into two new daughter cells. A contractile ring of microfilaments forms a cleavage furrow in animal cells, pinching the cell in half. The asters determine the position of the cleavage furrow.
How do Asters Induce Cleavage Furrow Formation?
what are asters in mitosis?
During the early stages of mitosis in an animal cell, an aster is a cellular structure shaped like a star, consisting of a centrosome and its associated microtubules. Plants do not form asters during mitosis. Astral rays, which are made up of microtubules, radiate from the centrosphere and resemble a cloud.
what is the role of asters in cell division?
In animal cells, asters are radial microtubule arrays. During mitosis, these star-shaped structures form around each pair of centrioles. Asters aid in the manipulation of chromosomes during cell division to ensure that each daughter cell has the correct chromosome complement.
where are asters found?
Flowering plants are known as "asters" and are members of more than a dozen genera in the Sunflower family's tribe Astereae. Asters of all kinds can be found in North America, Northern and Central Eurasia, and South America, with a few exceptions.