Human Life Cycle Mitosis & Meiosis
Sexual Reproduction & Mitotic and Meiotic Cellular Division
It all begins with the fusion of a haploid sperm cell provided by the father and the haploid ovum of the mother. Gametes unite when the nuclei of sperm and egg merge.
Human Life Cycle
The fertilized egg that results from the fusion of sperm and egg DNA is called a zygote. This new cell is “diploid” because it contains the two haploid sets of chromosomes, one set supplied by the mother and one by the father. These chromosomes bear the ancestral genes that represent the maternal and paternal family lines.
After fertilization, the zygote begins a process of cellular division in which additional cells which are genetic copies of the original fertilized egg are generated. This process of making copies of somatic cells (the cells of the body) is called mitosis and it results in a zygote eventually developing into a sexually mature adult.
Mitotic Generation of Somatic Cells
Growth, Development & Repair
So each of us grew from a zygote, or fertilized into an organism with trillions of specialized cells. Mitosis is the process that enabled us to grow and develop to adulthood after that fateful meeting of egg and sperm. But throughout out life, cell division is also required for maintenance, cell turnover and replacement.
Rate of Mitosis
Some cell types of your body, once formed, do not undergo much division, like neurons (nerve cells), for example. Others, like the skin cells, undergo a high rate of mitosis. Most other cell types are in between, with a moderate rate of cell division.
Meiosis is Sex Cell (Gamete) Formation
The only cells in the human body that are not made through the process of mitosis are sex cells, or gametes. In sexually reproducing organisms, some cells are able to divide by another method called meiosis. This type of cell division results in the production of gametes (eggs or sperm).
Meiosis is much more complex than mitosis. Whereas mitosis involves the duplication and subsequent division of chromosomes, meiosis involves two divisions of genetic material. Gametes are haploid (1n) with half the number of chromosomes than the progenitor cell that they arose from. These haploid sex cells arise in specialized reproductive tissue called the gonads. Ovaries (female gonads) and testes (male gonads) are the sites of meiosis.
Ploidy of Mitosis & Meiosis
Most of the cells in our bodies are somatic, or non sex cells, and have a diploid (2n) chromosome number; meaning that chromosomes come in pairs called homologues. Every somatic cell in your body has 46 chromosomes. You received a set of 23 from your mother’s egg and a matching set of 23 via your father’s sperm, and now these chromosomes are the genetic material inside nearly every cell of your body.
More Information
See Cells Alive or the website Science Prof Online for more about cell biology, or look to additional Suite101 biology articles, including Mitosis & Meiosis Difference and Sexual & Asexual Reproduction.
Sources
Campbell, N. A. & Reece J. B. (2005) Biology, seventh edition. Pearson Education Inc.
Campbell, N. A., Reece J. B. & Simon, E. (2004) Essential Biology with Physiology. Pearson Education Inc.
