ProphaseIn prophase, the chromatin condenses into discrete chromosomes. The nuclear envelope breaks down and spindles form at opposite "poles" of the cell. Many consider prophase (versus interphase) to be the first true step of the mitotic process.
View image of a cell in prophase.
Changes that occur in a cell during prophase:
- Chromatin fibers become coiled into chromosomes with each chromosome having two chromatids joined at a centromere.
- The mitotic spindle, composed of microtubules and proteins, forms in the cytoplasm.
- In animal cells, the mitotic spindle initially appears as structures called asters which surround each centriole pair.
- The two pair of centrioles (formed from the replication of one pair in Interphase) move away from one another toward opposite ends of the cell due to the lengthening of the microtubules that form between them.
In late prophase:
- The nuclear envelope breaks up.
- Polar fibers, which are microtubules that make up the spindle fibers, reach from each cell pole to the cell's equator.
- Kinetochores, which are specialized regions in the centromeres of chromosomes, attach to a type of microtubule called kinetochore fibers.
- The kinetochore fibers "interact" with the spindle polar fibers connecting the kinetochores to the polar fibers.
- The chromosomes begin to migrate toward the cell center.