Meiosis and Mitosis Explained
When body cells begin dying, which they do on a daily basis, they are replenished by the process of cell division. When cells divide they create new cells to take their place in a process called mitosis. The other process of cell division is called meiosis. Mitosis creates an exact copy of one cell. That is, one cell becomes two identical cells. Meiosis is the cellular process of sexual reproduction. The cells are not exact replicas and involve only female eggs and male sperm cells. The process of mitosis reproduces all other body, or somatic, cells, like blood, hair, and skin cells.
Each human being begins with one egg cell that combines with one sperm cell. By the time we are born, we have trillions of cells in our body. Each human cell is made up of certain parts, including a cell membrane, a nucleus and cytoplasm. The cell membrane is the cell's outer protective covering that holds the cell parts in place. The nucleus is located in the center of each cell and contains the genetic ladder, DNA. The jelly-like substance located between the nucleus and the cell membrane is the watery cytoplasm where all the other cell parts are located. Cells are the smallest unit of living matter. Each cell is a living entity.
Some cell terms to know to help you to understand mitosis and meiosis:
- Chromosomes: threadlike structures. Each contains DNA.
- Organelles: structures within a cell.
- Centrioles: specialized organelles.
- Chromatid: one of two identical strands of a split chromosome.
- Spindle fibers: pull the chromatids into alignment.
- Gametes: reproductive cells that form a zygote.
- Zygote: two joined reproductive cells, the egg and the sperm cell.
- Homologous: same, as in homologous chromosomes, chromosomes with the same characteristics.
- Haploid: Meiosis produces four of these cells containing one set of chromosomes each.
- Diploid: Mitosis produces cells with two sets of homologous chromosomes.
- Cytokinesis: The separation of cells by partitioning, without this process cell division would not happen.
The average person loses about three billion cells each minute. That is, those cells die. In that same minute, those three billion cells are replaced by identical new cells. This is the process of mitosis. The four phases of mitosis are:
- Prophase: one cell doubles its "insides" or its chromosomes and organelles. The centrioles become visible.
- Metaphase: chromosomes align in the center of the cell. The spindle fiber from the centrioles connect to each chromatid.
- Anaphase: chromatids separate by pulling apart, the parent cell membrane begins to pinch together in the middle creating a "twisted balloon" effect.
- Telophase: the cell further separates at that pinched center until the cell completely separates becoming two diploid "daughter" cells (cytokinesis).
One parent cell creates a new cell. This is called asexual reproduction. In other words, the parent cell does not need any other cell in order to replicate itself. The cells are genetically identical, meaning the amount of chromosomes in each new cell is identical to the amount of chromosomes in the parent cell. Mitotic cell division repairs the body and provides growth.
The process of meiosis is a sexual process, part of the reproductive process in which the division results in four new cells. There are seven stages in meiosis:
- Interphase I: doubling of genetic material.
- Prophase I: genetic material recombination, the reason for genetic diversity.
- Metaphase I: equatorial lining up of homologous chromosomes.
- Anaphase I: cells lengthen and separate to form two cells.
- Telophase I: two cells begin to split. The splitting happens in Cytokinesis I.
- Metaphase II, Anaphase II, and Telophase II repeats the process for both cells, with an end result of four haploid cells or 1n cells.
When meiosis occurs in men, four sperm cells are created. Meiosis in women creates one egg and three polar bodies (cells with discarded genetic material that eventually disappear). The genetic material in the resulting cells are not identical to the parent, but retain certain genetic traits while creating new genetic combinations unique to that cell and to that new person.
To learn more about mitosis and meiosis:
What is a Cell? A thinkquest rendition of what makes up a cell.
Asexual and Sexual Cell Production The process of mitosis and meiosis depicted by phase in both picture and text.
Cell Division and Phase Chart The basics of genetic cell division with slide photos.
Genetic Traits Dominant and recessive characteristics housed in cell DNA.
Mitosis Animated Watch an animated cell division. The phases are on the side with an arrow depicting the process as your watch. Click and freeze the frame at any point in the process.
Meiosis Animation Introduction, animation and an explanation of genetic diversity.
How Cells Make a Baby PBS's Nova explores the steps of meiosis and mitosis.
Stem Cells Cell division and stem cells.
Graphics Gallery Labeled diagrams and information summaries regarding cells and biotechnology.
Glossary Cell biology glossary with links to other biological glossaries.