In prophase, the chromatin condenses into discrete chromosomes. 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. Nematode Ascaris megalocephala in Prophase.
Nuclear membrane is barely visible.
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 chromosomes begin to migrate toward the cell center. Nematode Ascaris megalocephala in late Prophase. Nuclear membrane is no longer visible.