Gametes are reproductive cells or sex cells that are produced by an organism for the purpose of sexual reproduction. In animals, female gametes are called ova or egg cells, and male gametes are called sperm. Gametes are haploid cells, meaning that each cell carries only one copy of each chromosome. During fertilization, a sperm and ovum unite to form a new diploid organism. Gametes are produced through a type of cell division called meiosis, in which a diploid parent cell undergoes one round of DNA replication followed by two separate cycles of nuclear division to produce four haploid cells. In species that produce two morphologically distinct types of gametes, and in which each individual produces only one type, a female is any individual that produces the larger type of gamete—called an ovum—and a male produces the smaller type, called a sperm.