Developmental Biology Online



Displayed below are a series of photomicrographs of whitefish blastula cells at various stages of mitosis.



This is how all cells look before mitosis. Note: Interphase is a phase of the cell cycle, but NOT a stage of mitosis.


Early Prophase

To begin mitosis, the nuclear membrane breaks down, while the chromosomes shorten and thicken (here, a chromosome is two chromatids, bound at a point called the centromere, making an "X" shape). The other structures important for mitosis are also forming (i.e. the centrioles).



The mitotic spindle apparatus has now formed and lies on the poles of the nucleus (but remember, the nuclear membrane has broken down, so there is no distinctly delineated nucleus). The chromosomes are lined up along the cell's equator, also known as the equatorial plate, and are attached to the mitotic spindle apparatus via microtubules (to try and visualize the microtubules extending from the poles to the chromosomes on the equator, think of the Earth - it's as if rope was extending from the chilly north and south poles to the chromosomes basking in the sun at the equator). Here's the confusing part - When the individual chromatids ( of the "X") are separated from the chromosome (the "X"), they are now each referred to as a chromosome (i.e. In metaphase, the chromosome, composed of two chromatids, separates into the individual chromatids, which are then renamed chromosomes, even though they were only one half of a chromosome only moments before!) - Whew!



The newly formed chromosomes (which were recently chromatids while they were still of the "X") are pulled along the microtubules toward opposite poles of the cell (like Monarch butterflies migrating back to Canada and the southern tip of South America (toward the poles) from Mexico (near the equator).



The chromosome have finished their migration to the poles and the mitotic structures breakdown. The plasma membrane of the cell pinches down along the equator creating two separate cells. At this time, the chromosomes become indistinct (as they are during Interphase), the nuclear membrane forms again and the nucleolus reappears.

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maintained by Colin DeMill