Chapter Summary

1. Reproduction may take place through natural vegetative propagation or by spores (asexual reproduction) or by sexual processes (sexual reproduction). In sexual reproduction, two gametes unite, forming a zygote, which is the first cell of a new individual.

2. The process of meiosis ensures that gametes will have half the chromosome number of the zygotes and also usually ensures that offspring will not be identical with the parents in every respect.

3. Meiosis takes place by means of two successive divisions, each of which, like mitosis, is divided into arbitrary phases even though the process is continuous. In prophase I, the chromosomes become paired, often exchange parts, and then separate. The similar chromosomes of each pair are referred to as being homologous. Exchange of parts of chromatids may be affected by the parts initially crossing over one another and forming chiasmata and then tearing apart.

4. In metaphase I, the chromosomes become aligned at the equator in pairs, and in anaphase I, whole chromosomes from each pair migrate to opposite poles. In telophase I, the chromosomes either partially revert to their interphase state or initiate the second division, which is essentially similar to mitosis.

5. In prophase II, the chromosomes of each of the two groups become shorter and thicker again, with both groups becoming aligned at their respective equators in metaphase II. In anaphase II, the chromatids of each chromosome separate and migrate to opposite poles, and in telophase II, the chromatids (now called chromosomes again) lengthen, and new nuclear envelopes and nucleoli appear for all four groups. New cell walls are produced between each of the four groups.

6. If the chromatids have exchanged parts earlier, none of the four groups may have identical combinations of DNA, and each group has half the original number of chromosomes. Each of the four cells constitutes a spore (sexual spore or meiospore), which may develop into a body or structure within which gametes may be produced by mitosis.

7. Any cell having one set of chromosomes is said to be haploid, or to have n chromosomes; any cell having two sets of chromosomes is said to be diploid, or to have 2n chromosomes.

8. In the life cycle of an organism that undergoes sexual reproduction, there is an alternation between a haploid phase and a diploid phase. The haploid body is called a gametophyte, and the diploid body is called a sporophyte.

9. The change from the haploid phase to the diploid phase occurs when two gametes (each n) unite, forming a zygote (2n) in the process of fertilization (syngamy). The change from the diploid to the haploid phase occurs as a result of a sporocyte (meiocyte) undergoing meiosis, when a 2n cell becomes four n cells. This switching of phases is referred to as Alternation of Generations.