Final answer:
To initiate shrinking at the growing end of a microtubule, GTP bound to β-tubulin subunits must be hydrolyzed to GDP, leading to a decreased stability and rapid depolymerization of the microtubule.
Step-by-step explanation:
For a microtubule to start shrinking at its growing end, a process known as dynamic instability must occur. This involves the transition from growth to shrinkage, and the key is the hydrolysis of GTP (guanosine triphosphate), which is bound to the β-tubulin subunit in the microtubule. When GTP is hydrolyzed to GDP (guanosine diphosphate), the stability of the β-tubulin bond in the microtubule diminishes, promoting disassembly and initiating the rapid depolymerization or shrinking of the microtubule.
Microtubules are dynamic structures that alter rapidly between phases of growth and shrinkage, which is crucial for their cellular functions such as providing structural support, and facilitating chromosome separation during cell division. The transition from a growth phase to a shrinking one is a fundamental aspect of how microtubules function within the cell, enabling them to perform tasks dynamically by adapting their length according to cellular needs.