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A diagram of the process of cell division by mitosis.
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Courtesy of National Center for Biotechnology Information, NIH

How New Cells Are Made: Mitosis

An animal, plant, or other multicellular organism needs to make more cells in order to grow larger. One cell divides into two during growth. Unicellular organisms, like protists, which have only one cell, often reproduce by splitting their one cell into two or more.

When two cells are made out of one during cell division, the genetic material must be duplicated so that there is the same number of chromosomes in the two cells spilt from one. After chromosomes are duplicated, dividing them up is a four-step process called mitosis.

Step 1:
During this step the cell gets ready for mitosis. Duplicated chromosomes are held together. By the end of this step they have changed from thin threads into thick rods. Fibers made of protein begin to form that will eventually help pull the pairs of chromosomes apart.

Step 2:
The membrane that surrounds the cellís nucleus brakes apart and the chromosome duplicates line up at the middle of the cell. The fibers that started to develop during the first phase have become stronger and attach at both ends of the cell as well as to each chromosome.

Step 3:
During this step, the action really happens! Those thick fibers attached to opposite ends of the cell pull the duplicated chromosomes apart into two groups.

Step 4:
A nucleus membrane forms around both groups of chromosomes and rest of the cell begins to divide. With the same genetic material, these two cells are just like the one they were made from!

Last modified April 13, 2004 by Lisa Gardiner.

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