Definition
The spindle fiber is a microscopic protein structure that divides genetic material during cell division and organizes the components of a cell. A centrosome, also known as a microtubule-organizing center, or MTOC, forms spindle fibers.
Overview
Spindle fibers are formed from microtubules with many accessory proteins which help guide the process of genetic division. Each spindle fiber forms during cellular division near the poles of the dividing cell. As they extend across the cell, they search for the centromere of each chromosome.
Once attached, the spindle fiber is pulled back. With each fiber comes the chromosome it is attached to, which separates the chromosomes into each daughter cell. The process can be seen in the image above.
The spindle fibers can be seen extending in all directions from the centrosomes in step 6. Each spindle fiber is formed from several microtubules. The spindle fibers act like small machines during cell division. They carefully assemble and divide the chromosomes, and have been doing so for billions of years. But how does this complex process take place?
Structure of Spindle Fibers
The centrosome, or MTOC, always has some microtubules preassembled. On the surface of the MTOC are small proteins, responsible for lengthening or shortening the microtubules. These proteins respond to signals from the cell, and when it is time for cell division, they begin lengthening the spindle fibers.
To do this, they must add subunits of alpha-tubulin and beta-tubulin. Together, these two small proteins form the structure of a microtubule. Many individual microtubules together are called spindle fibers. A single microtubule can be seen in the graphic below.
Functions of Spindle Fibers
Shrinkage and Growth
The main feature of microtubules, and therefore of larger fibers, is that the proteins which control them can extend or contract the microtubule by adding or removing tubulin dimers. At first, the MTOCs must add many of these dimers to the microtubule, to extend it across the cell. As the microtubule travels, it eventually reaches a chromosome.
Special proteins within the centromere of the chromosome can attach to the microtubule. Here, there are also proteins which can shorten and extend the spindle fibers.
This is one of the main ways that the chromosomes get aligned on the metaphase plate, a hypothetical middle of the cell. It is also the main way they are separated during anaphase of mitosis or meiosis.
While the addition and subtraction of dimers is one of the main ways that spindle fibers help carry chromosomes about the cell, there are two other primary methods.
Spindle Fibers: Sliding
When two fibers from opposite poles of the cell meet, they are bound together by a special protein. Instead of grabbing onto a chromosome, they more or less attach to each other via the protein. This protein is a specialized motor protein, which reacts to signals from the cell. At the appropriate time during cell division, the motor protein will begin crawling along each microtubule it is attached to.
This “sliding action” causes pressure to be exerted against the poles and helps drive the poles apart. This action of the spindle fibers is what forces the cell apart and allows for it to be divided in half during telophase.
Spindle Fibers: Anchors
The final action carried out by some spindle fibers is that of anchoring to the cell surface. On the inside surface of the cell membrane, specialized proteins are placed to anchor the microtubules. While these anchors cannot assemble dimers into the microtubule, they can bind onto it.
Then, when the MTOC starts removing microtubule dimers, the whole spindle fiber shortens. In this way, it pulls the cell membrane toward the MTOC and starts to define the area of the newly forming cell.