Sexual Reproduction: Meiosis, Germ Cells, and Fertilization
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 21–1), andsea anemones and marine worms can split into two half-organisms, each of which then regenerates its missing half. There are even some lizard species that consist only of females that reproduce without mating. Although such asexual reproduction is simple and direct, it gives rise to offspring that are genetically identical to their parent. Sexual reproduction, by contrast, mixes the genomes fromtwo individuals to produce offspring that differ genetically from one another and from both parents. This mode of reproduction apparently has great advantages, as the vast majority of plants and animals have adopted it. Even many procaryotes and eucaryotes that normally reproduce asexually engage in occasional bouts of genetic exchange, thereby producing 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 will briefly consider what sexual reproduction involves and what its benefits might be.
In This Chapter
OVERVIEW OF SEXUAL REPRODUCTION MEIOSIS PRIMORDIAL GERM CELLS AND SEX DETERMINATION IN MAMMALS EGGS SPERM FERTILIZATION 1269 1272 1282 1287 1292 1297OVERVIEW OF SEXUAL REPRODUCTION
Sexual reproduction occurs in diploid organisms, in which each cell contains two sets of chromosomes, one inherited from each parent. The specialized cells that carry out sexual reproduction, however, are haploid; that is, they each contain only one set of chromosomes. In the final step of sexual reproduction, a haploid cell of one individual fuses with a haploid cellof another, mixing the two genomes and restoring the diploid state. Sexual reproduction, therefore, requires a specialized type of cell division called meiosis, in which a diploid precursor cell gives rise to haploid progeny cells, rather than to diploid cells as occurs in ordinary mitotic cell division. In sexually reproducing multicellular organisms, the haploid cells produced by meiosisdevelop into highly specialized gametes—eggs (or ova), sperm (or spermatozoa), pollen, or spores. In animals, females typically produce large and nonmotile eggs, whereas males typically produce small and motile sperm (Figure 21–2). At fertilization, a haploid sperm fuses with a haploid egg to form a diploid cell (a fertilized egg, or zygote), which contains a new combination of chromosomes. The zygotethen develops into a new multicellular organism through repeated rounds of ordinary mitosis, followed by cell specialization, which includes the production of gametes (Figure 21–3A).
The Haploid Phase in Higher Eucaryotes Is Brief
In most organisms that reproduce sexually, diploid cells proliferate by mitotic cell division, and the haploid cells that form by meiosis do notproliferate. Some simple organisms, such as fission yeasts, are exceptional in that haploid cells
Figure 21–1 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.)
Chapter 21: Sexual Reproduction: Meiosis, Germ Cells, andFertilization
Figure 21–2 Scanning electron micrograph of an egg with many human sperm bound to its surface. Whereas the egg is immotile, the sperm are highly motile. Although many sperm are bound to the egg, only one will fertilize it, as we discuss later. (Courtesy of D. Phillips/ Science Photo Library.)
proliferate by mitotic cell division, and the diploid cells formed by the fusion of...