9.1 Why Reproduce?
1. Reproduction is essential to species' survival.
2. Asexual reproduction, such as binary fission, can be successful in an unchanging environment.
3. Sexual reproduction mixes traits and therefore provides species' protection in a changing environment. Sexual reproduction also increases the number of organisms. It occurs when haploid gametes fuse, restoring the diploid state.
9.2 Variations on the Sexual Reproduction Theme
4. Meiosis halves the genetic material, and fertilization is the joining of haploid nuclei. Meiosis and fertilization are two key events in sexual life cycles.
5. Somatic growth occurs between meiosis and fertilization, or between fertilization and meiosis.
6. Sexual reproduction in plants involves an alternation of generations and generations with multicellular haploid and diploid phases.
7. Conjugation, a form of gene transfer in some microorganisms, is sexual because one individual transfers genetic material to another, but it is not reproduction because no additional individual forms. Chlamydomonas undergoes cell fusion, which may have been a forerunner of sexual reproduction.
8. In animals and some other organisms, cells undergoing meiosis are germ cells. After meiosis, haploid gametes specialize in a process called maturation.
9.3 How Does Meiosis Halve the Chromosome Number?
9. Meiosis halves the number of chromosomes in somatic cells, producing haploid gametes. A species' chromosome number stays constant because in gamete-producing cells, the DNA replicates once, but the cells divide twice.
10. In meiosis I, reduction division, the chromosome number is halved. In meiosis II, equational division, the two products of meiosis I undergo essentially a mitotic division. Each division proceeds through stages of prophase, metaphase, anaphase, and telophase.
11. Meiosis provides genetic variability by partitioning different combinations of genes into gametes through independent assortment. Crossing over, which occurs in prophase I as homologous pairs synapse, increases the variability.
9.4 The Sculpting of Gametes
12. Spermatogenesis begins with spermatogonia, which accumulate cytoplasm and replicate their DNA to become primary spermatocytes. After meiosis I, the cells are haploid secondary spermatocytes. In meiosis II the secondary spermatocytes divide to each yield two spermatids, which differentiate along the male reproductive tract to yield sperm.
13. Sperm develop within the testes in the walls of the seminiferous tubules and mature in the epididymis. Sperm join secretions and leave the body through the vasa deferentia.
14. In oogenesis, oogonia replicate their DNA, becoming primary oocytes. In meiosis I, the primary oocyte divides, apportioning cytoplasm to one large secondary oocyte and a much smaller polar body. In meiosis II, the secondary oocyte divides, yielding the large ovum and another small polar body. Oogenesis occurs in the ovaries.
15. The development of a sperm cell takes 74 days. Meiosis in the female begins before birth and completes at fertilization.