Promfázis

Promfázis, or prometaphase, is a stage of mitosis in eukaryotic cells that follows prophase and precedes metaphase. In many cells, the nuclear envelope breaks down during promfázis, allowing spindle fibers to interact with chromosomes. Chromosomes condense further and become more distinct under light microscopy as their kinetochores become the attachment points for spindle microtubules.

The defining events of promfázis are the attachment and movement of chromosomes. Kinetochores, protein structures at centromeres, capture microtubules and establish connections to opposite spindle poles. Chromosomes begin to move toward the center of the cell, guided by forces generated by microtubule dynamics and motor proteins such as dynein and kinesin. Non-kinetochore microtubules lengthen and help push spindle poles farther apart, while the nuclear envelope remains largely broken down.

Regulation of promfázis involves cell cycle control mechanisms that ensure proper progression. Cyclin B–dependent kinases trigger entry into mitosis, and the spindle assembly checkpoint monitors correct kinetochore-microtubule attachments and tension across sister chromatids. If attachments are incorrect or incomplete, the checkpoint delays advancement to metaphase to prevent mis-segregation.

Variations exist among organisms. In many plants and some fungi, spindle organization occurs without classic centrosomes, and microtubule organizing centers differ, but the fundamental sequence—nuclear envelope breakdown, kinetochore capture, and chromosome congression—remains conserved.

Significance lies in preparing chromosomes for accurate segregation. By the end of promfázis, most chromosomes are attached and aligned for proper bi-orientation, ensuring each daughter cell receives an exact set of chromosomes during the subsequent anaphase.