Final answer:
Tubulin addition occurs at the plus end with higher affinity for heterodimers, regulated by GTP hydrolysis resulting in decreased affinity.
Step-by-step explanation:
Tubulin addition happens preferentially at the plus end because it has a higher affinity for tubulin heterodimers. GTP hydrolysis acts as a molecular switch to decrease microtubule affinity for the addition of heterodimers. The correct answer to fill in the blanks in the student's question is: a) plus; higher; GTP; decrease.
Microtubules are dynamic structures important for cell shape, cell motion, and the segregation of chromosomes during cell division. The stabilization and destabilization of microtubules are due to the intrinsic polarity of tubulin dimers and the GTP hydrolysis that regulates their assembly.