What is an Aster in Cell Division?

Instructor: Stephanie Gorski

Steph has a PhD in Entomology and teaches college biology and ecology.

In this lesson, we'll discuss asters and their role in mitosis and meiosis. We'll talk about what asters are made of, and how asters help your cell nucleus to divide.

You are Made of Star Stuff!

'We are a way for the universe to know itself. Some part of our being knows this is where we came from. We long to return. And we can, because the cosmos is also within us. We're made of star stuff.' - Carl Sagan

Has anyone ever told you that you're made of star stuff? You are! Usually, when people say that, they're referring to the elements that make up your body. Carbon, nitrogen, and oxygen, for example, were formed in ancient stars over 4.5 billion years ago.

But your very own cells make intricate star-shaped structures called asters. Asters are structures animal cells use during mitosis or meiosis, and the word aster comes from the Latin word for star.

What are Asters?

Asters are considered part of the cytoskeleton, the structural component of the cell. Asters are made of microtubules, the largest of the three main cytoskeleton components (microfilaments, intermediate filaments, and microtubules). Microtubules are made of a protein called tubulin.

Before asters come centrosomes. Centrosomes are cytoskeleton structures consisting of two bundles of microtubules called centrioles, stuck to one another at a right angle. Centrosomes normally stay near the cell's nuclear envelope. They become important right before the cell's nucleus begins to divide, in processes called mitosis or meiosis. Mitosis and meiosis are how the cell produces two daughter nuclei, an important step in cell division. Mitosis is what most of your cells do; for example, if you cut your hand, your skin cells will undergo mitosis and then divide to close the wound. Meiosis is a special kind of nuclear division that takes place in your sex cells (eggs or sperm).

Right before mitosis or meiosis begins, the centrosome will divide into two daughter centrosomes, and each centrosome will migrate to the opposite side of the nucleus.

As mitosis or meiosis begins, centrosomes create two new structures: spindle fibers, which connect the sister centrosomes to each other, and asters, which radiate out from the centrosome. The asters then provide support and guidance to the chromosomes, ensuring that chromosomes end up in the right place as mitosis occurs.

During mitosis and meiosis, asters help chromosomes line up in the center of the nucleus. How these chromosomes are arranged depends on whether we are talking about mitosis or meiosis, but the important thing to know is that asters then pull one half of this genetic information to each side of the nucleus. Then, the nucleus itself will divide in half, leaving two daughter nuclei.

You can see the asters helping the chromosomes line up
You can see the asters helping the chromosomes line up

It's very important that chromosomes end up in the right place, because each daughter nucleus should end up with exactly half the genetic material after cell division. And the genetic information the daughter nuclei ends up with should be the same; you don't want all your eye genes going to one daughter nucleus and all your hand genes going to the other one! So mitosis and meiosis need to be precise. On the rare occasions that chromosomes end up in the wrong place, the daughter nucleus will contain the wrong amount of information. This will likely result in a daughter cell that is nonviable. Occasionally, it will result in a disorder such as Down's syndrome (where a person has an extra copy of Chromosome 21), or Jacobs syndrome or Klinefelter syndrome (where a person has an extra copy of a sex chromosome).

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