All cells come from pre-existing cells, so all cells must divide to make new cells. There are three basic types of cell division: Fission, Mitosis and Meiosis. Fission is limited to prokaryotic cells and mitosis and meiosis are types of cell division utilized by eukaryotic cells after DNA has replicated to form duplicated chromosomes (two copies joined to form an X).
Prokaryotic cells are the simpler unicellular bacteria cells. They are characterized by a lack of nucleus and a single circular DNA molecule. Fission is the form asexual reproduction in which a single parent bacterium replicates its DNA molecule and the cell splits to form two identical daughter cells. Fission results in two cells that are smaller copies of the original cell.
Eukaryotic cells may be unicellular or multi-cellular, but they too must make identical copies for asexual reproduction plus for growth or for replacement cells after injury. Eukaryotes are more complex because they package their double-stranded DNA into several chromosomes contained within a nucleus. Mitosis occurs in several stages to release the duplicated chromosomes from the nucleus and effectively sort the copied arms of the chromosomes to the opposite poles of the cell, ensuring that both new cells get one complete set of DNA instructions. Like fission, mitosis results in two daughter cells that are smaller copies of the original parent cell.
Sexual reproduction combines the genetic information of two parents to create genetically unique offspring. Meiosis is cell division to produce cells with half the normal number of chromosomes called gametes. Gametes are sex cells like eggs or sperm or pollen (male gametes in plants). Normal multi-cellular individuals have two versions of every chromosome because they inherited one from each parent. Because sexual reproduction involves combining chromosomes from two individuals, each parent can only contribute one of each type of chromosome to a new offspring. Meiosis produces half-set gametes which is necessary to avoid doubling the number of chromosomes in each generation of offspring. Meiosis is accomplished through two division stages called Meiosis I and Meiosis II. Meiosis I separates the two inherited versions of each chromosome to produce two daughter cells with a half set of chromosomes in each. However, each chromosome still is in duplicate form so both daughter cells divide again in Meiosis II to separate the DNA copies. Because two daughter cells from Meiosis I undergo Meiosis II, four total cells are produced. Meiosis results in four genetically unique daughter gametes that each contains only 1 copy of every chromosome. These gametes mature into one egg (and three discarded cells) or four sperm, depending on the gender of the individual.