Centrioles during cell division

[Rissa]

Hi, I'm new here and this is my first post. I hope I'm posting in the right thread!

I just read in my biology textbook that spindle "microtubules grow out of the centrosomes" in the G2 phase, but I was wondering, what is the role of the centrioles in the centrosomes here? When they say that the tubules grow out of the "centrosomes", do they mean that the centrioles elongate to become the spindle microtubules? If not, then where do the microtubules come from?

Thanks in advance.

 

[rodrigoniskier]

Hi. The centrioles don’t elongate to become the spindle microtubules.

The formation of a microtubule requires the interaction of several constitutively produced tubulin heterodimers in the cytoplasm.

The concentration of tubulin subunits required for spontaneous microtubule nucleation is very high. Therefore, nucleation of the microtubules requires the help of other factors. While alpha and beta tubulins are fundamental units of the microtubules, another type of tubulin, called gamma-tubulin, is present in much smaller amounts than alpha and beta-tubulin and is involved in nucleating the growth of microtubules in different organisms.

Microtubules are generally nucleated from a specific intracellular location known as a microtubule organizing center (MTOC) where gamma-tubulin is found in higher concentration.

The organization of microtubules varies greatly between different species and cell types. Together with a large number of accessory proteins, the centrioles organize the pericentriolar material, where microtubules nucleate.

However, in budding yeasts, the microtubules are nucleated in an MTOC that is inserted into the nuclear envelope, in the form of a small multilayer structure called the polar body of the spindle, which is also found in other fungi and diatoms.

The cells of higher plants appear to perform microtubule nucleation at sites distributed throughout the nuclear envelope and the cellular cortex.

Neither fungi nor most plant cells contain centrioles. Despite these differences, all these cells appear to use gamma-tubulin to nucleate their microtubules.