With the exception of a few odd species, all animals utilize basically the same life cycle, demonstrated here for us by Georgette Gene and her husband George.
Like all animals, George and Georgette are diploid. This means that all of the nuclei in their somatic cells have two sets of chromosomes. But like all animals, they produce a special set of haploid reproductive cells called gametes. Georgette's gametes are eggs (ova) and George's are sperm (spermatozoa). Like all cells, gametes are produced from other cells by a cell division event. When one of George's gametes meets up with one of Georgette's, the two fuse together (fertilization) to produce a zygote. This zygote is the first cell of a new individual. Since it combines a set of Georgette's chromosomes with a set of George's, the zygote has a unique combination of genetic information different from his or her mother's or father's.
This zygote then undergoes many cell divisions to produce an embryo, then a fetus, and finally baby Georgina or baby George, Jr. However, cell division isn't enough by itself. As all of these new cells are produced, many of them go through the process of differentiation. Differentiation is the process by which generalized cells become specialized for particular functions. It involves changes in the structure and chemistry of the cell, controlled by switching specific genes on and off.
As the cells are undergoing differentiation, the whole embryo goes through the process of development. This is the modification of masses of cells to form the different tissues and organs of the body. This again involves changes controlled by the switching on and off of various genes, influenced by signals sent from one group of cells to others.
After birth, baby Georgina continues to undergo cell division, growth and development. By the time she is an adult, she will have about 60 trillion cells, all with identical genetic information. The adult Georgina will take over her mother's role in the next generation. Her brother George Jr. will take over their father's position.
This life cycle involves a great deal of cell division. However, not all of the cell division steps are the same. Most of the cell division occurs in the growth and development of the individual, and the task of this kind of cell division is to produce many, many genetically identical cells.
The production of gametes is different (at least in the animal life cycle; plants do things a little bit differently). Gametes are produced from cells which are diploid, but it is necessary for the gametes themselves to be haploid, so this cell division has to involve reducing the chromosome number from diploid to haploid. Also, sexual reproduction is all about diversity; so this cell division, instead of producing identical cells, produces cells which are all different. Statistically speaking, although Georgette will produce millions of eggs in her lifetime, no two of them will be genetically identical.
The differences between these two types of cell division are all in the way the nucleus of the cell divides. To get identical nuclei, you divide your nucleus by mitosis. To reduce chromosome number and get genetically different daughters, you divide your nucleus by meiosis.
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