This image illustrates the human life cycle. Adults produce gametes (reproductive cells termed sperm and eggs) by meiosis , and each gamete contains only one-half the number of chromosomes found in the nonreproductive cells of the parent. The nonreproductive cells (containing 46 chromosomes) are called diploid cells , and are abbreviated as "2n ." The gametes (containing 23 chromosomes) are called haploid cells , and are abbreviated as "n ." When a haploid ovum and a haploid sperm unite through fertilization , a diploid zygote is produced. This life cycle describes how diploid (2n ) adults produce haploid (n ) gametes, which then unite to form a new diploid (2n ) zygote.
This alternation of meiosis and fertilization is found in the sexual life cycles of all sexually reproducing organisms, but its timing can differ considerably depending on the organism. The human life cycle typifies the most common life cycle found in animals, where the gametes are the only haploid cells. They combine to form a zygote, which divides by mitosis and grows to form a new individual.