Generation Upon Generation

The title of this chapter is the same title as Chap. 12 of Jacob Bronowski’s book The Ascent of Man, which accompanied the 1973 television documentary series. He points out that “sex produces diversity, and diversity is the propeller of evolution.” While there is DNA exchange in bacteria in what is called parasexuality, sexual reproduction evolved in the eukaryotes. The meiosis process allows for the production of the haploid gametes, sperm, and egg. Specific transcription factors signal entry into meiosis rather than mitosis. Crossing over, recombination and reduction division occurs in meiosis I. At the end of meiosis I, each cell contains a single set of chromosomes consisting of two chromatids. There is an independent assortment of the maternal and paternal chromosomes. At meiosis II, the chromatids separate so that each new cell becomes a haploid gamete. The male and female gamete cells come together in the process of fertilization to form the zygote and restore the diploid chromosome number and produce the next generation. The essential immortality of gamete DNA as opposed to somatic DNA is discussed, as well as trends in human fertility.