1. Know the phases of meiosis and briefly describe what occurs in each of them.
2. Understand clearly what features meiosis and mitosis have in common and how they differ.
3. Explain the significance of crossing-over to offspring.
5. Relate meiosis and Alternation of Generations to the process of DNA replication.
s a young child, I used to augment my allowance
A by growing a few vegetables and other plants from seed in my back yard; the produce was then sold—mostly to my mother. Occasionally, an unharvested plant would go to seed, and I would save the seeds for the next growing season. I noticed that in at least some instances, the second generation of plants did not always quite resemble the parents, either in appearance or taste. Although I was mildly curious, I did not understand until many years later that a primary basis for the variation was a process known as meiosis.
In Chapter 3, we noted that plants grow through an increase in the number of cells brought about by mitosis and cell division. Mitosis ensures, in very precise fashion, that the number of chromosomes and the nature of the DNA in the daughter cells will be identical with those of the parent cell. Living organisms, however, do not grow indefinitely, although a few may remain alive for a long time. In order to perpetuate the species, they must reproduce, or they will become extinct.
This reproduction may take place through natural vegetative propagation, which is discussed in Chapter 14, or by means of special cells called vegetative spores, which are produced by mitosis. Processes involving cells that are identical in their chromosomes with the cells from which they arise are forms of asexual reproduction (the prefix a of asexual means "without"; asexual reproduction, therefore, simply means reproduction without sex).
Nearly all plants, however, also undergo sexual reproduction, which in flowering and cone-bearing plants ultimately results in the formation of seeds. In sexual reproduction, sex cells called gametes are produced. Two gametes, called egg and sperm in higher plants and animals, form a single cell called a zygote when they fuse together. The zygote is the first cell of a new individual (Fig. 12.1).
If gametes had the same number of chromosomes as all the other cells of the organism, then the zygote, which results from the union of two gametes, would have double the number of its parents' chromosomes. If such zygotes were then to develop into mature organisms, each of the new organism's cells would have double the original number of chromosomes, and when they produced gametes, the zygotes resulting from the union of this next generation of gametes would have four times the number of chromosomes of the grandparents. If that sort of thing were to continue for 20 generations, a species having 16 chromosomes in each cell to begin with would end up with no fewer than 8,388,608 chromosomes per nucleus! The problem does not actually arise, however, because at a certain point in the life cycle of all sexually reproducing organisms, the unique process of meiosis occurs.
Meiosis and Alternation of Generations 223
The process of meiosis brings about the development of gametes that have only half the number of chromosomes of any body developing from the zygote. When the gametes form zygotes as they unite in pairs, the original chromosome number is restored.
Since some aspects of meiosis are similar to those of mitosis (the two processes are compared in Fig.12.2), it is strongly recommended that you review mitosis in Chapter 3 before going through the phases of meiosis outlined in the following section.
Keep in mind that all living cells, before undergoing meiosis, have two sets of chromosomes—one from the male parent, the other from the female parent. Generally, the members of each pair of chromosomes are identical with each other in length, in the amount of DNA present, and in having a centromere at the same precise location. However, as discussed in Chapter 13, their genes may control contrasting characteristics. Such chromosomes are called homologues, or homologous chromosomes.
When a cell undergoes meiosis, four cells are produced from two successive divisions, which generally take place without pausing. Because of the remarkable events that occur during the process, the four cells, depending on the organism involved, are rarely, if ever, identical with the original cell or with each other.
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