Question

If the concentration of tubulin monomers is above the critical concentration of the plus end but below the critical concentration of the minus end, what will happen to in vitro microtubule assembly?

1) The microtubules will get longer.
2) The microtubules will get shorter.
3) The microtubules will lose subunits off both the plus end and the minus end.
4) Microtubule length will remain generally constant, but cellular position will shift toward the plus end.

Answer 1

In vitro, microtubule assembly with tubulin monomer concentration above the critical level for the plus end and below for the minus end will result in microtubules getting longer (option 2) at the plus end and shorter at the minus end.

Step-by-step explanation:

If the concentration of tubulin monomers is above the critical concentration of the plus end but below the critical concentration of the minus end, the microtubules will undergo a process of differential assembly and disassembly known as 'dynamic instability'. Specifically, at the plus end where the concentration of tubulin monomers is above the critical level, microtubules will gain subunits and therefore get longer. In contrast, at the minus end where the monomer concentration is below the critical level, microtubules will undergo net disassembly or 'shrink'. Usually, the plus end of microtubules assembles faster than the minus end, and the minus end is anchored in microtubule organizing centers (MTOCs) like centrioles.

A key application of this principle is seen during cell division, where microtubules pull genetically material to ensure proper cell division, organizing into spindle fibers that attach to chromatids via kinetochores. This balance of assembly and disassembly is essential for many cellular processes including mitotic spindle formation and the motion of chromosomes during mitosis. In this context, microtubule dynamic instability is a critical aspect of cellular function and division.