Lewis/Life - Review of Key Concepts

Review of Key Concepts - Chapter 10

Reproduction is essential to species' survival. Asexual reproduction, such as binary fission, can be successful in an unchanging environment. 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.
Some species reproduce both asexually and sexually, including yeast and plants. Plant sexual reproduction involves an alternation of generations and both haploid and diploid phases.
Conjugation, a form of gene transfer in some microorganisms, is sexual because one individual transfers genetic material to another; however, it is not reproduction because no additional individual forms. Chlamydomonas undergoes cell fusion, which may have been a forerunner of sexual reproduction.
In humans, gametes are produced in the male and female reproductive systems. These systems include paired gonads, where sperm and oocytes are manufactured, networks of tubes, and glands. Gametogenesis includes meiosis and maturation and produces sperm and oocytes.
Developing sperm originate in seminiferous tubules within the paired testes, passing through different stages of maturation as they approach the lumen of the tubule. Sperm mature in the epididymis and vasa deferentia, and they exit the body through the urethra during sexual intercourse. The prostate gland, seminal vesicles, and bulbourethral glands add secretions to sperm.
Oocytes originate in the ovaries. Each month after puberty, one ovary releases an oocyte into a fallopian tube, which leads to the uterus.
Meiosis halves the number of chromosomes in somatic cells to produce haploid gametes. A species' chromosome number stays constant because a gamete's DNA replicates once, but the cells divide twice.
Meiosis provides genetic variability by partitioning different combinations of genes into gametes through independent assortment. Crossing-over increases the variability.
Spermatogenesis begins with spermatogonia, which accumulate cytoplasm and replicate their DNA to become primary spermatocytes. After reduction division (meiosis I), the cells are haploid secondary spermatocytes. In an equational division (meiosis II) the secondary spermatocytes divide to each yield two spermatids, which then differentiate along the male reproductive tract.
In oogenesis, oogonia replicate their DNA to become 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 to yield the large ovum and another small polar body. The development of a sperm cell takes 74 days. Meiosis in the female completes at fertilization.