SCIENTIFIC INQUIRY Although both ends of a microtubule can...

SCIENTIFIC INQUIRY 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 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.

SCIENTIFIC INQUIRY 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
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

Mechanisms of Chromosome Segregation

Chromosome segregation during anaphase is driven by coordinated actions of forces generated through microtubule dynamics and motor protein activity. Forces produced by plus end-directed motors can facilitate sliding and elongation of overlapping microtubules, while minus end-directed motors often help pull chromosomes toward the spindle poles. The interplay between these mechanisms ensures that chromosomes are accurately separated into daughter cells.

Spindle Apparatus Dynamics in Mitosis

The spindle apparatus is a dynamic structure composed largely of microtubules and associated proteins, including motor proteins. Its organization, with microtubules extending from spindle poles and interacting with chromosomes, underlies the precise movements during mitosis. Different populations of microtubules, such as kinetochore microtubules (which attach directly to chromosomes) and nonkinetochore microtubules (which often interdigitate and help form the spindle structure), are subject to distinct regulatory mechanisms during anaphase to ensure faithful chromosome segregation.

Microtubule Polarity and Dynamic Instability

This concept involves the structural polarity of microtubules, which have distinct plus and minus ends. The plus end is typically more dynamic, undergoing rapid phases of polymerization and depolymerization, whereas the minus end is often more stable or anchored. This polarity plays a crucial role in determining the direction and rate at which cellular components are transported and how the microtubule network remodels during processes like cell division.

Motor Protein Directionality

Motor proteins are specialized molecules that move along microtubules, and their directionality—whether they move toward the plus end or the minus end—is fundamental to their function. Plus end-directed motor proteins typically transport cargo or exert forces toward the cell periphery or spindle midzone, while minus end-directed proteins move toward the microtubule-organizing centers or spindle poles. This directional movement is critical in orchestrating the mechanical forces required for chromosome alignment and segregation during mitosis.

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