During prophase, which is the first stage of mitosis, several key events occur as the cell prepares for division:
-
Chromatin Condensation: The chromatin, which is the relaxed form of DNA and proteins in the nucleus, begins to condense into discrete, visible structures known as chromosomes. Each chromosome is made up of two sister chromatids that are joined at a region called the centromere.
-
Nuclear Envelope Breakdown: The nuclear envelope, which encloses the nucleus, begins to disintegrate. This breakdown allows the spindle apparatus to interact with the chromosomes.
-
Spindle Formation: Structures called spindle fibers, which are composed of microtubules, begin to form from the centrosomes (organelles that organize microtubules). The spindle fibers extend towards the chromosomes, preparing to separate them in the next phases of mitosis.
-
Centrosome Movement: The centrosomes, which are located at opposite poles of the cell, move apart, and the spindle apparatus grows between them.
-
Attachment of Spindle Fibers: The spindle fibers extend towards the chromosomes, and eventually connect with the kinetochores, which are protein structures located at the centromeres of the chromosomes. This connection is crucial for the later separation of the sister chromatids.
Overall, prophase is a critical step in mitosis, setting the stage for the proper segregation of chromosomes into daughter cells.