Cytokinesis is the division of one cell into two and is the final step following the four-stage mitotic cell cycle. During cytokinesis the nuclear envelope, or nuclear membrane, that encloses the nucleus’s genetic material remains unchanged, as it was dissolved and reformed into two separate membranes in an earlier mitosis phase.
Importance of Mitosis
The ability of cells to divide and replicate through the process of mitosis allows for an organism’s growth and repair. Humans can grow, for example, only because their cells are able to replicate. Mitosis also allows multicellular organisms to have cells with specialized functions, such as muscle cells. Furthermore, mitosis makes possible the repair or replacement of damaged or dead cells. Skin tissue, for example, is constantly regenerating through mitosis, which can repair damage from cuts or abrasions. In simpler creatures, the regenerative benefits of mitosis can result in the regrowth of lost appendages.
Role of the Nuclear Envelope
The nuclear envelope is essential to healthy cell function. A membrane of two layers fused together with nuclear pores, the envelope serves as an essential architectural framework to enclose DNA from the exterior cytoplasm. At the same time, the envelope serves as gatekeeper for molecules, from proteins to water, which might pass between the nucleus and cytoplasm. The envelope also contributes to important genetic functions, such as DNA replication.
Prophase and Nuclear Envelope Dissolution
The first stage of mitosis, known as prophase, begins as paired copies of DNA, known as sister chromatids, condense within the dividing, or parent, cell to become visible by microscope. As this condensation begins, the nuclear envelope begins to disintegrate. Since this dissolution ends prophase, some models consider it the beginning of an intermediate prometaphase. This breakdown of the envelope allows the DNA pairs to align with the central axis, or equatorial plate, of the cell, the key step of the subsequent metaphase. Next, in anaphase, the sister chromatids separate and migrate to opposite ends of the cell, identified by the centrioles.
Telophase, Nuclear Envelope Reformation and Cytokinesis
The result of this separation is two equal sets of DNA grouped at either pole of the cell, making it ready for the reappearance of the nuclear envelope and coinciding with the final stage of mitosis, called telophase. The nuclear envelope reforms around each new bundle of DNA, creating two independent nuclei and triggering the cytokinetic division of the parent cell into two new daughter cells.