Wednesday, January 23, 2013

Centrosome and Centriole

Centrosome is another organelle in all eukaryotic cells. It is present in the cytoplasm of the cell usually close to the nucleus.  It has a key role in organization of microtubule in all animal cells and hence also referred to as the Microtubule Organizing Center (MTOC).
Simple Structure of a Centriole
The centrosome is generally composed of two centrioles (centriole figure on the right) which are at right angles or perpendicular to one another as can be seen in the figure below. They are surrounded by amorphous mass of protein called the  Pericentriolar Material (abbreviated as PCM).
Diagram showing Two Centrioles
Perpendicular to One Another
 This PCM contain proteins which are responsible for microtubule nucleation and anchoring. So, the two centrioles (perpendicular to one another) along with PCM constitutes the centrosome.
You might be thinking, what are centrioles? The centriole is cylindrical shaped structure composed of nine triplets of microtubule. This can be clearly seen in the cross section of centriole (diagram below).
Cross Section of Centriole
showing Nine Triplets of Microtubule

Centrosomes play a key role in the process of mitosis. As we all know, during the interphase stage of mitosis, the nuclear membrane breaks down, the microtubules interacts with the chromosomes to build the mitotic spindle. Each daughter cell receives only one centrosome containing two centrioles. This is because of the simple reason that centrosome is copied only once per cell cycle. How? The reason is simple just as how some other organelles divide in the cell during mitosis. The centrosome replicates in the S phase. Now, there are two centrosomes. Next, during prophase, each of the centrosome migrate to the opposite pole of the cell. The mitotic spindle forms between both of them. Upon dividing, each daughter cell inherits one centrosome (containing two centrioles).

Centrioles are a very important part of centrosomes, which are involved in organizing microtubules in the cytoplasm.The position of the centriole determines the position of the nucleus and plays a crucial role in the spatial arrangement of the cell. Experiments have shown that centrioles are not required for the progression of mitosis. If the centrioles are not present, the microtubules of the spindle are focused by motors which allows them to form the bipolar spindle. So, it has been shown that many cells can completely undergo interphase without centrioles.