Mitosis begins with prophase, a stage characterized by chromosome condensation and the initial steps of sister chromatid resolution.
Mitosis, the process of cell division that results in two identical daughter cells, doesn't have a single "start" point in the sense of a switch being flipped. Instead, it's a continuous process triggered by specific cellular signals and checkpoints. However, the observable initiation of mitosis is generally considered to be the stage called prophase.
Here's a breakdown of how prophase initiates mitosis:
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Chromosome Condensation: The process fundamentally starts with changes to the genome within the cell nucleus. During prophase, chromosomes, which were previously diffuse and extended during interphase, begin to condense. This condensation is largely facilitated by condensin proteins. Condensin works to coil and pack the DNA more tightly, making the chromosomes shorter and thicker, and therefore easier to segregate later in mitosis. This also prevents tangling.
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Cohesin Dynamics: Simultaneously, another protein complex called cohesin, which holds sister chromatids together, undergoes changes. While cohesin remains present at the centromere (the region where sister chromatids are most closely associated), much of the cohesin is removed from the chromosome arms. This removal allows the sister chromatids to begin to resolve, or become distinguishable from each other.
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Spindle Formation: Outside the nucleus, the microtubule organizing centers (MTOCs), also called centrosomes, begin to migrate to opposite poles of the cell. As they migrate, they organize microtubules to form the mitotic spindle, a structure essential for chromosome segregation.
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Nuclear Envelope Breakdown (in some organisms): In many but not all eukaryotic organisms, the nuclear envelope begins to break down at the end of prophase (sometimes considered part of prometaphase). This allows the microtubules of the mitotic spindle to access the chromosomes.
In summary, the start of mitosis, characterized by the onset of prophase, is marked by coordinated events including chromosome condensation (mediated by condensin), cohesin redistribution, the initiation of the mitotic spindle formation, and ultimately nuclear envelope breakdown, setting the stage for accurate chromosome segregation. The precise triggering mechanisms are complex and involve a variety of signaling pathways responding to cellular cues that indicate the cell is ready to divide.
