Prophase
Prophase is the initial stage of karyokinesis, which is the nuclear division during mitosis, occurring following DNA synthesis in the S phase of the cell cycle. In this phase, several critical events take place:
- Chromatin Condensation: The chromatin fiber condenses to form distinct mitotic chromosomes, each consisting of two sister chromatids connected at a region known as the centromere.
- Centrosome Movement: The centrosome, which has duplicated during the S phase, begins migrating towards opposite poles of the cell. Each centrosome radiates microtubules, creating a structure called the mitotic spindle.
- Nuclear Envelope Disintegration: As prophase progresses, the nuclear envelope, along with the nucleolus, begins to break down, leading to the dispersal of nuclear contents into the cytoplasm.
- Spindle Formation: The aster arrays of microtubules formed by the centrosomes and the spindle fibers indicate the setup for chromosome alignment in later phases of mitosis. At the end of prophase, the chromosomal material is compacted, making it easier to visualize under a microscope.
The significance of prophase cannot be understated as it sets the groundwork for successful chromosome segregation and ensures genetic fidelity in daughter cells.