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Prophase is the first phase of the four phases of mitosis. It is also the longest. Prophase can take up to sixty percent of the total time it takes the cell to divide.

The appearance of chromosomes signals the start of prophase. Chromosomes become visible when the chromatin condenses and coils. Under a light microscope, chromosomes are seen as two identical chromatids lying side by side.

The other important structures in prophase are the centrioles. Centrioles are tiny structures found in the cytoplasm near the nuclear envelope. In prophase, they seperate from each other and move to opposite ends of the nucleus. The spindle, a meshlike structure that will move the chromosomes apart later in mitosis, develops from the centrioles. The condensed chromosomes become attached to the spindle at or near the centromeres.

Near the end of prophase, the coiling of the chromosomes becomes tighter. The nucleolus disappears and the nuclear envelope breaks down.

Images of cells in prophase:

Image of a whitefish blastula cell in prophase
In this image, chromosomes are stained black.
Stained image of a cell in prophase
In this image, the spindle fibers are stained green, chromosomes are stained blue, and actin is stained red.

Once prophase is completed, the cell moves on to the next phase of mitosis, metaphase.

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