Centrosomes are organelles which serve as the main microtubule organizing centers for animal cells.
Centrosomes are made of from arrangement of two barrel-shaped clusters of microtubules, called “centrioles,” and a complex of proteins that help additional microtubules to form.
These proteins allow the centrosomes to start and stop the formation of microtubule proteins. This allows them to control the formation of mitotic spindle fibers and other structures that play important roles in cellular development.
Centrosomes assist with several important functions, including:
- Organizing changes to the shape of the cell membrane that allow the membrane to “pinch” in two during cell division.
- Ensuring that chromosomes are properly distributed to daughter cells by creating and shortening mitotic spindle fibers.
- Overseeing other important changes to cell membrane shape, such as those seen in phagocytosis.
In animal cells, centrosomes are treated very much the same way as DNA.
Each daughter cell gets one centrosome from the parent cell during cell division. The centrosome is then copied during the cell cycle, so that the cell can give one to each daughter cell when it divides.
During cell division, when chromosomes are lined up and then pulled toward opposite ends of the cell, it is the centrosomes that are responsible.
The centrosomes, which migrate to opposite “poles” of the cell as the cell prepares for division, direct the mitotic spindle fibers. These spindle fibers pull the sister chromatids apart and ensure that one copy of each chromosome ends up in each daughter cell.
The graphic below shows a cell midway through telophase of cell division. You can see that its DNA has already been pulled by mitotic spindle fibers to opposite sides of the parent cell, and that the cytoskeleton is now beginning to “pinch” the cell in two.
In the absence of centrosomes, some animal cells can still complete this assortment of DNA, but the process is less reliable. A few animal species can develop normally without centrosomes, but in most species, cells may begin dividing incorrectly or stop dividing at all if centrosomes are destroyed.
Mutations that harm centrosome function are associated with rates of cancer in some species, which is consistent with failures to correctly sort DNA. Biologists think that some cases of cancer are caused in by errors in copying and distribution of chromosomes.
Centrosomes are not necessary in plant and fungi cells, because these cells do not change the shape of their cell membranes during cell division. These cells have stuff, inflexible cell walls which prevent them from changing their membrane shape to “pinch” in two during mitosis.
Function of Centrosomes
Centrosomes are sometimes referred to as the “MTOC,” or “microtubule organizing center” of the cell.
They serve to direct the movements of microtubules and other cytoskeletal structures and proteins, ultimately allowing large changes to the shapes of animal cell membranes.
Animal cells are unique among cell types because they are highly flexible, giving animals their soft tissues and highly versatile bodies. But they also have the ability to have structure and change their shape, which permits movement and many other functions.
When animals cells want to change their shapes, complexes of proteins move the cell’s membranes along a network of microtubules – stiff “skeletal” fibers which can bend and change shape in response to intra- and extra-cellular signals.
The largest changes to a cell’s membrane shape occurs during mitosis, when the entire cell splits in two to form daughter cells.
Mitosis is also when centrosomes play a starring role as the organizers of the microtubules that pull sister chromatids apart, ensuring that each daughter cell gets a full compliment of the parent cells’ DNA.
Centrosomes can also orchestrate large changes to cell membrane shape under other circumstances, such as phagocytosis.
This process, which comes from the Greek words for “cell eating,” occurs when the cell changes shape to completely wrap itself around and “swallow” another cell or item in its environment.
Controversy Over Necessity
For many years, it was believed that animal cells could not divide successfully without centrosomes coordinating the separation of sister chromatids, the changes to the cytoskeleton, etc.. Upholding this theory, some cells in the lab were observed to stop dividing altogether, or to divide incorrectly, when their centrosomes were destroyed.
But in recent years, it has been discovered that some species of animals can develop normally, even if they are genetic mutants who have no centrosomes at all. Fruit flies and flatworms are among those that accomplish successful cell division without centrosomes.
This has raised questions about the real utility of centrosomes, and whether the cell can “make up for” their absence through other mechanisms. Some scientists propose that centrosomes might assist the processes described in this article, but not be vital to them.
More data is needed before scientists can say for sure whether centrosomes are essential to cell division, and what they can do that cells don’t have other ways to accomplish. But in the meantime, it’s better to assume that they are important than that they aren’t!