As part of the cycle of sexual reproduction, haploid germ cells are produced from diploid cells. During this process, a diploid cell undergoes a round of DNA replication, followed by not one but two rounds of cell division, resulting in four haploid cells.
The mechanistic differences between mitosis, which produces somatic cells, and meiosis, is best understood by considering the case of mitosis first. During a mitotic cell cycle, chromosomes are duplicated but remain closely aligned, and these twin copies are called sister chromatids. Note that diploid cells have two sets of homologous chromsomes. DNA replication generates sister chromatids from each chromosome, and in the interval between DNA replication and cell division, the cell bears four copies of each chromosome. At metaphase, sister chromatids align on the mitotic spindle. At anaphase, these sister chromatids separate, each migrating toward an opposite pole of the spindle. The two new cells that result bear one copy of each homologous chromosome.
The initial stages of meiosis appear quite similar to the mitotic cell cycle: a diploid cell undergoes DNA replication, producing two sister chromatids for each of two homologous chromosomes. However, the mechanism of chromosome partitioning is drastically different in the first meiotic cell division. Each pair of homologous chromosomes, rather than one sister chromatid from each homologous chromosome, migrates toward a different pole. The resulting cells have two copies of each chromosome, but correspond to the sister chromatid pair. Prior to the second meiotic division, DNA replication does not occur. At cell division, the sister chromatids separate, and the resulting cells have one copy of each chromosome.