Unit Three. The Continuity of Life
Humans, like most animals and plants, reproduce sexually. That is how you came into being: Your father contributed a sperm cell that united with an egg cell from your mother to form a cell called a zygote, containing both sets of chromosomes. Dividing repeatedly by mitosis, this zygote cell eventually gave rise to your adult body, made up of an astonishing number of cells—some 10 to 100 trillion. The sperm and egg that joined to form your initial cell were the products of a special form of cell division called meiosis, the subject of this chapter. Far more intricate than mitosis, the details of meiosis are not as well understood. The basic process, however, is clear. A cell dividing by meiosis goes through two nuclear divisions, replicating the DNA before the first division but not between the two divisions. In the photo above, chromosomes are lining up and getting ready to be pulled to opposite ends of the cell by microtubules too tiny to be visible to our eyes. When the two meiotic divisions are all over, there will be four cells, each with only half as much DNA as the initial cell. Confused? So were biologists when they first discovered meiosis. Hopefully, this chapter will make things clearer. It is important that you understand meiosis clearly, because meiosis and sexual reproduction play key roles in generating the tremendous genetic diversity that is the raw material of evolution.
9.1. Discovery of Meiosis
Only a few years after Walther Fleming’s discovery of chromosomes in 1879, Belgian cytologist Pierre-Joseph van Beneden was surprised to find different numbers of chromosomes in different types of cells in the roundworm Ascaris. Specifically, he observed that the gametes (eggs and sperm) each contained two chromosomes, whereas the somatic (nonreproductive) cells of embryos and mature individuals each contained four.
From his observations, van Beneden proposed in 1887 that an egg and a sperm, each containing half the complement of chromosomes found in other cells, fuse to produce a single cell called a zygote. The zygote, like all of the somatic cells ultimately derived from it, contains two copies of each chromosome. The fusion of gametes to form a new cell is called fertilization, or syngamy.
It was clear even to early investigators that gamete formation must involve some mechanism that reduces the number of chromosomes to half the number found in other cells. If it did not, the chromosome number would double with each fertilization, and after only a few generations, the number of chromosomes in each cell would become impossibly large. For example, in just 10 generations, the 46 chromosomes present in human cells would increase to over 47,000 (46 x 210) chromosomes.
The number of chromosomes does not explode in this way because of a special reduction division that occurs during gamete formation, producing cells with half the normal number of chromosomes. The subsequent fusion of two of these cells ensures a consistent chromosome number from one generation to the next. This reduction division process, known as meiosis, is the subject of this chapter.
The Sexual Life Cycle
Meiosis and fertilization together constitute a cycle of reproduction. Two sets of chromosomes are present in the somatic cells of adult individuals, making them diploid cells (Greek, di, two), but only one set is present in the gametes, which are thus haploid (Greek, haploos, one). Figure 9.1 shows how two haploid cells, a sperm cell containing three chromosomes contributed by the father and an egg cell containing three chromosomes contributed by the mother, fuse to form a diploid zygote with six chromosomes. Reproduction that involves this alternation of meiosis and fertilization is called sexual reproduction. Some organisms however, reproduce by mitotic division and don’t involve the fusion of gametes. Reproduction in these organisms is referred to as asexual reproduction. Binary fission of prokaryotes shown in chapter 8 is an example of asexual reproduction. Some organisms are able to reproduce both asexually and sexually. The strawberry plant pictured in figure 9.2 reproduces sexually by fertilization that occurs in its flowers. Strawberries also reproduce asexually by sending out runners, stems that grow along the ground and produce new roots and shoots that give rise to genetically identical plants.
Figure 9.1. Diploid cells carry chromosomes from two parents.
A diploid cell contains two versions of each chromosome, a maternal homologue contributed by the mother's haploid egg, and a paternal homologue contributed by the father's haploid sperm.
Figure 9.2. Sexual and asexual reproduction.
Reproduction in an organism is not always either sexual or asexual. The strawberry plant reproduces both asexually (runners) and sexually (flowers).
Key Learning Outcome 9.1. Meiosis is a process of cell division in which the number of chromosomes in certain cells is halved during gamete formation.