Metaphase

Metaphase is a stage of cell division in which duplicated chromosomes align along the metaphase plate at the cell’s equator. It follows prometaphase, during which spindle microtubules attach to kinetochores, and precedes anaphase, when sister chromatids or homologous chromosomes separate. The mitotic or meiotic spindle, composed of microtubules and motor proteins, organizes the chromosomes and positions them for separation. Kinetochores, protein structures at centromeres, attach microtubules from opposite spindle poles, generating tension that promotes proper alignment. Nonkinetochore microtubules and motor forces help maintain the arrangement.

In mitosis, metaphase features the alignment of all replicated chromosomes at the metaphase plate at the cell center. Each chromosome is held in place by opposing kinetochore microtubules, creating tension that bi-orients sister chromatids toward opposite poles. The metaphase is monitored by the spindle assembly checkpoint to ensure correct attachments before proceeding to anaphase.

In meiosis, metaphase occurs twice. Metaphase I arranges homologous chromosomes as tetrads at the metaphase plate, with kinetochores from homologous chromosomes attached to opposite poles, enabling their separation in anaphase I. Metaphase II resembles mitotic metaphase, with sister chromatids aligned at the plate and kinetochores attached to opposite poles, preparing for separation in anaphase II.

The accurate alignment and tension across kinetochores are critical for error-free chromosome segregation; defects can lead to aneuploidy. The process is regulated by spindle assembly checkpoints and various motor proteins that coordinate microtubule dynamics.