Back To CourseCollege Biology: Help and Review
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Katy teaches biology at the college level and did her Ph.D. work on infectious diseases and immunology.
Just as our skeletons give our bodies' structure and shape, the cytoskeleton gives cells structure and shape. The cytoskeleton is responsible for lots of important cellular functions:
Clearly, things just wouldn't be the same without the cytoskeleton.
In eukaryotic cells, the cytoskeleton is made up of three major kinds of filaments: actin filaments, intermediate filaments, and microtubules. Each of these filaments is a polymer, meaning that it is made up of many single subunits, like a child's building blocks snapped together to form a long chain. The subunits are called monomers, and each type of cytoskeletal filament is built out of a different kind of monomer.
The polymeric structure of cytoskeletal filaments means that they can be disassembled and rearranged at any time. This means that the cell can respond to signals in its environment and rapidly change its shape, motion or attachment accordingly. You can imagine it like this: if the buildings in a city were made out of easily rearranged monomers, it would be easy to take them down and make new buildings in different places. We usually don't need to do this, but our cells do!
In this lesson, we'll focus on one type of cytoskeletal filament, microtubules, and learn about their structure and functions within the cell.
Microtubules are the largest cytoskeletal filaments in cells, with a diameter of 25 nanometers. They are made out of subunits called tubulin. Each tubulin subunit is made up of one alpha and one beta tubulin that are attached to each other, so technically tubulin is a heterodimer, not a monomer. As you can see, it really does look like a tube, hence the name micro'tubule.'
In a microtubule structure, tubulin monomers are linked both at their ends and along their sides (laterally). This means that microtubules are quite stable along their lengths. Imagine that you have some plastic building blocks that are all identical and can attach to each other both at their ends and laterally. If you arranged them into a microtubule structure, and then wanted to take the structure apart, you can imagine that it would be really hard to take it apart somewhere in the middle, because how would you get the first block out? If you wanted to take it apart, you'd have to start at the ends. And indeed, this is how microtubules are assembled and disassembled, only from their ends.
Since the tubulin subunits are always linked in the same direction, microtubules have two distinct ends, called the plus (+) and minus (-) ends. On the minus end, alpha tubulin is exposed, and on the plus end, beta tubulin is exposed.
Microtubules preferentially assemble and disassemble at their plus ends. An important consequence of this fact is that microtubule minus ends can be clustered together in a so-called microtubule-organizing center, or centrosome. The centrosome stays stable as the plus ends of the microtubules grow and shrink.
Microtubules are used in many important cellular functions.
One of those function is helping to separate sister chromatids during cell division. In this process, each daughter cell needs to get one complete set of chromosomes. The replication and separation of chromosomes is called mitosis.
How does it work? During mitosis, there are two centrosomes, one at each end of the cell, and each with an array of microtubules stretching out from it. The replicated chromosomes, called chromatids, are lined up in pairs in the center of the cell. Microtubules bind to the centers of each one and then, all at the same time, the microtubules begin to disassemble and shrink.
The plus ends are where the dis-assembly occurs and the minus ends stay stable at the centrosomes. Thus, as the microtubules shrink, the chromatids are pulled apart in the directions of the two centrosomes. This neatly divides the replicated chromosomes into one set per daughter cell. Perfect!
Microtubules are important in intracellular transport, too. In cells that are not dividing, microtubules reach out in a star-like shape from the cell's single centrosome and form intracellular highways to transport vesicles and organelles around. Cells take advantage of microtubules' two different ends to allow 'directional' transport, just like northbound and southbound lanes of a highway.
This directional transport is accomplished by two motor proteins that attach to cargo and then 'walk' along microtubules. Kinesin walks toward the plus end of a microtubule, and dynein walks toward the minus end.
Directional vesicular transport is important in endocytosis and exocytosis, which is when cells take in or push out molecules. So, dynein is important in endocytosis, or movement of vesicles towards the center of the cell (the minus ends), and kinesin is important in exocytosis, or movement of vesicles towards the outside of the cell (the plus ends).
This is particularly impressive in neurons, or nerve cells. These cells are situated in the spinal cord and brain but need to extend out to the peripheral parts of the body to sense stimuli and move our muscles. Thus, they have long, thin projections called 'axons' that can be up to a meter in length in humans. Vesicles and organelles must be transported from the cell body all the way to the end of the axon, and this transport occurs along microtubules.
Microtubules also form important movable appendages on cells, like flagella and cilia. Flagella are whip-like structures that allow single cells, like sperm and bacteria, to swim around. Cilia are like tiny moving hairs on stationary cells. They beat constantly to move liquids across cell surfaces. This is important in our respiratory tract, where cilia move mucus up to where we can cough it out.
In summary, microtubules are tube-like filaments made up of tubulin heterodimer subunits. They have two distinct ends, the plus and minus ends. The minus ends are anchored at the cell's centrosome. The plus end is where growth and shrinkage preferentially occurs as tubulin subunits attach and detach. Cells make use of this property in cell division, when microtubules shrink, separating the replicated chromosomes that will go into the two daughter cells.
Microtubules also form intracellular highways, along which vesicles can be directionally transported by microtubule motor proteins. Finally, microtubules form moveable appendages on cells, such as flagella and cilia, which help cells swim and move liquids across their surfaces.
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Back To CourseCollege Biology: Help and Review
24 chapters | 433 lessons