Mitosis is a fundamental biological process ensuring the accurate distribution of genetic material during cell division. This mechanism facilitates growth, repair, and the reproduction of single-celled organisms, yielding two genetically identical daughter cells from a single parent cell. The precision of mitosis ensures genetic stability and proper organismal function.
Interphase: Preparing for Mitosis
Before a cell embarks on mitosis, it undergoes a preparatory stage known as interphase. Although not technically part of mitosis itself, interphase is a period of significant growth and DNA replication, laying the groundwork for successful division. Interphase consists of three distinct phases: G1, S, and G2.
During the G1 phase, the cell experiences substantial growth, synthesizing proteins and organelles. Following G1, the cell enters the S phase, where DNA replication occurs. Here, each chromosome is duplicated, resulting in two sister chromatids that remain joined. The final stage of interphase, G2, involves further cell growth and the synthesis of additional proteins and organelles. The cell also checks its DNA for errors and makes any necessary repairs, ensuring it is fully prepared to enter mitosis.
Prophase: Chromosome Condensation
Prophase marks the beginning of mitosis, initiating visible changes within the cell’s nucleus. During this phase, the loosely organized genetic material, known as chromatin, undergoes a transformation, coiling and condensing into compact, visible chromosomes. Each chromosome at this stage consists of two sister chromatids, tightly linked.
As chromosomes become more defined, the nucleolus, a dense structure, begins to disappear. Concurrently, the nuclear envelope starts to break down. Meanwhile, in animal cells, centrosomes, duplicated during interphase, move to opposite ends of the cell, initiating the formation of the mitotic spindle. This spindle, composed of microtubules, guides chromosome movement.
Metaphase: Chromosome Alignment
Following prophase, the cell enters metaphase, a highly organized stage characterized by the precise alignment of all chromosomes. The condensed chromosomes migrate to the cell’s center, forming a plane known as the metaphase plate or equatorial plate. This alignment ensures that each new daughter cell receives an accurate set of chromosomes.
Spindle microtubules, kinetochore microtubules, extend from the centrosomes at the cell poles and attach to specialized protein structures called kinetochores, located at the centromere of each sister chromatid. These attachments create tension, pulling the chromosomes into alignment along the metaphase plate. The proper attachment and alignment are monitored by cellular checkpoints to prevent errors in chromosome segregation.
Anaphase: Sister Chromatid Separation
Anaphase is a dynamic stage where the sister chromatids finally separate. This separation is triggered by the cleavage of cohesin proteins, which hold the sister chromatids together. Once cleaved, each chromatid is now considered an individual chromosome.
The spindle fibers, attached to the kinetochores, shorten and pull these newly separated chromosomes towards opposite poles of the cell. This coordinated movement ensures that an identical set of chromosomes moves to each pole, preparing the cell for division into two daughter cells. The precise and simultaneous separation ensures the correct chromosome number.
Telophase and Cytokinesis: Completing the Division
The final stages of cell division involve telophase and cytokinesis, which complete the formation of two daughter cells. During telophase, the separated chromosomes arrive at the opposite poles of the cell. The chromosomes begin to decondense, returning to their chromatin state.
Concurrently, a new nuclear envelope forms around each set of chromosomes at the poles, creating two nuclei. The mitotic spindle disassembles as its role concludes. Following nuclear division, cytokinesis, the division of the cytoplasm, begins, pinching the parent cell into two daughter cells. In animal cells, cytokinesis involves the formation of a cleavage furrow, an indentation that deepens as a contractile ring of actin and myosin filaments tightens, pinching the cell in two. Plant cells, with their cell walls, form a cell plate in the middle of the cell, which grows outward to become a new cell wall, dividing the two daughter cells.