Biochemistry: The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

The Setting
Evolution of Complex Organisms
Sex, Chromosomes, and Cell Division

Bacteria reproduce conventionally through binary fission. During this process, the single DNA molecule of the bacterial chromosome is duplicated, resulting in daughter Cells that each receive an identical chromosome. However, rudimentary forms of sexual reproduction are found in certain bacteria. The process responsible for "shuffling" bacterial genes in this case is genetic recombination (Ch. 15, Sec. G).

Sexual reproduction reaches its ultimate development in eukaryotes, where the growth of a multicellular Organism begins with the fusion of two haploid Gametes—an egg Cell and a sperm cell. Each gamete carries a complete set of genetic instructions; the fertilized egg (zygote) formed after nuclear fusion is diploid. A diploid cell contains two full sets of genetic templates inherited from two completely different parents. This provides tremendous advantages to the developing organism. Indeed, if a Gene inherited from one parent turns out to be defective, it is highly unlikely that the corresponding gene from the other parent will be defective as well. Furthermore, sexual reproduction serves as a mechanism for gene mixing, meaning that each of us receives not simply half of our genes from our mother and half from our father, but also certain genes from our grandparents, great-grandparents, and so on.

Before Cell Division begins, the DNA molecules in The Nucleus, which were previously dispersed throughout its volume, start to condense, forming dense bodies—Chromosomes—along with Proteins and Other Compounds. In some organisms, such as Ascaris (a roundworm), there are only 2 chromosomes. One chromosome contains the Genetic information that the "baby" Ascaris receives from the father, and the other from the mother. Thus, in this case, there is only a single homologous pair of chromosomes, each carrying the entire required set of genetic instructions. Both chromosomes of this pair are distributed during every cell division so that all Cells of the organism possess this homologous pair. Human body cells, like those of Ascaris, are diploid, but rather than just one, they contain 23 homologous pairs of chromosomes1. Human chromosomes vary in size, but their average length is 4–6 µm and their diameter is ~ 1 µm.



Last update: 06/08/2026

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