Gametes

Gametes are the specialized reproductive cells, such as sperm and eggs, that fuse during fertilization to form a zygote. They are haploid, carrying a single set of chromosomes, so that fusion restores the diploid number.

In animals gametes are made by meiosis, a division that halves the chromosome number. Crossing over and the random assortment of chromosomes mean that each gamete carries a unique mix of alleles, which is a main source of variation within a species. In humans each gamete has 23 chromosomes.

Most animals show anisogamy, with two kinds of gametes. The egg is large and holds nutrients and organelles for the early embryo, while the sperm is small, numerous and motile, and mainly delivers DNA. Some algae and fungi have isogamy, in which the gametes look alike.

In plants, gametes form by mitosis in a haploid stage called the gametophyte, and the male gametophyte is the pollen grain. Meiosis in such organisms makes spores rather than gametes. In all eukaryotes with sexual reproduction, the cycle of gamete formation and fusion alternates a haploid with a diploid phase.

• Meiosis — the division that makes gametes
• Fertilization — the fusion of gametes
• Pollen — the male gametophyte of seed plants