Centrioles are discs built from microtubule material, with nine microtubules arranged in a cylinder. Each of the nine microtubules has two connected short microtubules. When the cell is not dividing, two centrioles are paired with each other at 90 degree angles. Along with some other proteins, the two centrioles combine to make the centrosome.
In addition to serving as a hub for microtubules during normal cell processes, the centrosome is critical for cell division. Because each daughter cell needs a centrosome, the centrosome must duplicate before cell division takes place. The two centrioles separate and produce copies of themselves. The result is two pairs where there used to be one, which combine to make two centrosomes.
