Although both ends of a microtubule can gain or lose...

Although both ends of a microtubule can gain or lose subunits, one end (called the plus end) polymerizes and depolymerizes at a higher rate than the other end (the minus end). For spindle microtubules, the plus ends are in the center of the spindle, and the minus ends are at the poles. Motor proteins that move along microtubules specialize in walking either toward the plus end or toward the minus end; the two types are called plus end-directed and minus end-directed motor proteins, respectively. Given what you know about chromosome movement and spindle changes during anaphase, predict which type of motor proteins would be present on (a) kinetochore microtubules and (b) nonkinetochore microtubules.

Key Concepts

Chromosome Movement during Anaphase

During anaphase, chromosomes are segregated into daughter cells by two major mechanisms: the shortening of kinetochore microtubules to pull chromosomes toward the poles (anaphase A) and the elongation of the spindle, often driven by sliding of overlapping nonkinetochore microtubules (anaphase B). Both processes rely on the coordinated action of motor proteins that generate directional forces along microtubules.

Kinetochore versus Nonkinetochore Microtubules

Kinetochore microtubules attach directly to chromosomes via kinetochores and are involved in the movement of chromosomes during anaphase, typically relying on minus end-directed motor proteins to pull chromosomes poleward. In contrast, nonkinetochore microtubules, such as interpolar microtubules, interact in the spindle midzone where plus end-directed motors slide them apart to facilitate spindle elongation, contributing to the separation of the poles.

Microtubule Polarity

Microtubules have an inherent structural polarity with a plus end and a minus end, where the plus end typically undergoes faster rates of polymerization and depolymerization. This polarity is fundamental for directional processes in the cell, including the assembly and disassembly of the spindle apparatus during cell division.

Motor Protein Directionality

Motor proteins move along microtubules with specificity for either the plus end or the minus end. Plus end-directed motors generally transport cargos towards the cell periphery or spindle center, whereas minus end-directed motors move cargos in the opposite direction, a property crucial for the segregation of chromosomes during mitosis.

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