Sex is not absolutely necessary. Single-celled organisms can reproduce by simple mitotic division, and many plants propagate vegetatively, by forming multicellular offshoots that later detach from the parent. Likewise, in the animal kingdom, a solitary multicellular Hydra can produce offspring by budding (Figure 20-1). Sea anemones and marine worms can split into two half-organisms, each of which then regenerates its missing half. There are even species of lizards that consist only of females and reproduce without mating. Although such asexual reproduction is simple and direct, it gives rise to offspring that are genetically identical to the parent organism. In sexual reproduction, on the other hand, the genomes from two individuals are mixed to produce offspring that differ genetically from one another and from both their parents. This mode of reproduction apparently has great advantages, as the vast majority of plants and animals have adopted it. Even many procaryotes and other organisms that normally reproduce asexually engage in occasional bouts of sexual reproduction, thereby creating offspring with new combinations of genes. This chapter describes the cellular machinery of sexual reproduction. Before discussing in detail how the machinery works, however, we shall pause briefly to consider why it exists and what benefits it brings.

Figure

Four sequential stages of fertilization are shown, beginning with a mature, but unfertilized human egg (top left). Next comes the post-fertilization incorporation of a sperm pronucleus, which in this example is moving inward from the left (top right). (more...)

Figure 20-1

Photograph of a Hydra from which two new organisms are budding (arrows). The offspring, which are genetically identical to their parent, will eventually detach and live independently. (Courtesy of Amata Hornbruch.)