A diagram of the process of cell division by mitosis. Click on image for full size (24K GIF) Courtesy of National Center for Biotechnology Information, NIH

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. The process by which the genetic material within the cell nucleus (chromosomes) divides is called mitosis.

Human cells each have 46 chromosomes and when they divide each of the two new cells has 46 chromosomes. Where do the additional chromosomes come from? The genetic material is duplicated so that the same number of chromosomes is in the next generation of cells. After chromosomes are duplicated, dividing them up is a four-step process.

Step 1: Prophase
During this step the cell gets ready for mitosis. Duplicated chromosomes are held together. By the end of prophase 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: Metaphase
The membrane around the nucleus brakes apart and the chromosomes line up at the middle of the cell. The fibers that started to develop during prophase have become stronger and attach at both ends of the cell as well as to each chromosome. If the cell were a globe, the fibers would be attached at the north and south poles while the chromosomes would be lined up along the equator.

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

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

A diagram of the process of cell division by mitosis. Click on image for full size (24K GIF) Courtesy of National Center for Biotechnology Information, NIH

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 cell’s 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!

A diagram of the process of cell division by mitosis. Click on image for full size (24K GIF) Courtesy of National Center for Biotechnology Information, NIH

Some organisms, like animals and plants need to make more cells to grow larger. To grow, one cell divides into two identical cells. Other organisms, like protists, have only one cell. They can reproduce by splitting their one cell into two or more.

Before dividing, the cell’s genetic material, called chromosomes, needs to duplicate forming two identical sets. The two sets of chromosomes are divided during a process called mitosis, which is a four-step process.

Step 1:
During this step the cell gets ready. Duplicated chromosomes are held together and change from thin threads into thick rods. Fibers form to help pull the pairs of chromosomes apart.

Step 2:
The thin wall called a membrane that surrounds the nucleus brakes apart. The chromosomes are no longer trapped within it. Instead, they line up at the middle of the cell. The fibers 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 two ends of the cell pull the chromosomes into two groups.

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