User:Rosieredfield/Sandbox

Mitosis sandbox:

(Hatnote to main Wikipedia entry)

Introduction:

Mitosis is the Cell division mechanism used by all organisms except Bacteria and the bacteria-like Archaea. Its main function is to make sure that the two new cells each have the same genetic information as the original 'parent' cell. It does this by first duplicating the DNA and proteins of the parent cell's chromosomes, and then pulling one copy of each chromosome into each new cell.

All cells in the body arise by mitosis from earlier cells, all the way back to the original fertilized egg. The only exceptions are the cells that produce the sperm and egg (the gametes), which are formed by a special division called meiosis

Why cells need mitosis:

The non-chromosome components of cells do not need a special sorting mechanism to get them into daughter cells. Each cell contains many of each component, and simply pinching the cell into two equal-sized halves is good enough to make sure all the components are present. But cells of animals, plants and other Eukaryotes typically have many kinds of chromosomes, each present in only one copy. Mitosis makes sure each daughter cell gets one copy of each chromosome. Bacteria and Archaea do not need mitosis because they have only a single chromosome and can easily make sure each daughter cell gets a copy.

Events of mitosis

The events of mitosis are usually taught as four or five 'phases'. By tradition mitosis is considered to only divide the nucleus and its chromosomes. The final step, division of the cell into two identical daughter cells, is treated as a separate process called cytokinesis.

DNA replication: Before mitosis can take place, the chromosomes in the nucleus must be duplicated so that each daughter cell can get a copy of each chromosome. The DNA of each chromosome is replicated, and new proteins are attached to each DNA molecule. The duplicate copies of each chromosome are tied together along their length by loops of a string-like protein.

Prophase

During prophase, the proteins and DNA of each chromosome become much more tightly packed together. The chromosomes become much shorter and wider.

Prometaphase

The nuclear envelope around the chromosomes breaks down. The chromosomes are now very short, and fat enough be seen with a microscope. Each pair of chromosome copies appears as a single rod-shaped structure. Fibers grow from opposite ends of the cell and attach to the two parts of the paired chromosomes.

Metaphase

Tugging in both directions by the fibers pulls each pair of chromosome copies towards the middle of the cell. Once this has happened to all of the pairs of chromosome copies, a cutting protein is activated.

Anaphase

The cutting protein cuts all the string-like proteins that held the chromosome copies together. The fibers continue pulling, so the separated chromosome copies now move apart and towards the ends of the cell. The cell begins to stretch out as other fibers push the ends of the cell apart.

Telophase

One complete set of chromosomes is now at each end of the cell. Each set is identical. The fibers begin to disappear, and a nuclear membrane forms around each set of chromosomes. The compact packing of the DNA and proteins relaxes and the chromosomes become long and thin again, and invisible even under a microscope.

Cytokinesis: Finally, the cell itself splits into two smaller cells, each with a nucleus with a complete set of chromosomes like those in the original cell. The details of this splitting are different between plant and animal cells, because of the stiff plant cell wall. Main article: Cytokinesis

Interphase: Between one division and the next, cells usually grow back to their mature size. See cell cycle.

Resources for learning more about mitosis:

The Facts of Life: Mitosis: jargon-free candymation video for students. (Put it on Wikimedia Commons)

References

OpenStax textbook