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14

The Nature of Living Things

The union of a maternal and a paternal gene is typical of eukaryotes, a corollary

of which is that siblings share half their genes with each other. The social insects

are an important (recall that ants may comprise about a quarter of the animal mass

on earth) exception. The queen is only fertilized once in her lifetime, storing the

sperm in her body. She lays two kinds of eggs: fertilized with the stored sperm just

before laying, and which become females; and unfertilized, which become males.

The males therefore have only one set of chromosomes (i.e., they are haploid); in a

certain sense, the males have no father. Hence, they transmit all their genes to their

progeny, which are invariably female. In consequence, sisters share three-quarters of

their genes with each other, but they only have a quarter of their genes in common

with their brothers. ¹⁷

Mitosis

The simple process of gene replication is called mitosis. ¹⁸ This is the type of cell

division that produces two genetically identical (in theory) cells from a single parent

cell. It applies to bacteria and to the somatic (body) cells of eukaryotes.

Prior to division, homologous pairs (of the maternal and corresponding pater-

nal genes for each chromosome) form. They are attached at one zone, near the

centre of the chromosome, by a large multiprotein complex called the centromere.

The attached chromosomes then compactify, forming the characteristic “X”-shaped

structures easily seen in the optical microscope after staining. The remainder of the

process is described in Table 14.1.

Meiosis

Meiosis is a more complex process than mitosis. It starts with an ordinary diploid

cell and leads to the formation of gametes (germline cells).

First, the two chromosomes (paternal and maternal) are duplicated (as in mito-

sis) to produce four double helices. Then the four double helices come into close

proximity and recombination (see below) is possible. Thereupon, the cell divides

without further DNA replication. The chromosomes are segregated; hence, each cell

contains two double helices (diploid). A given double helix may have sections from

the father and from the mother. Finally, there is a further division without further

DNA replication. Each cell contains one double helix (haploid). They are the gametes

(germ cells).

17 This fact is used to “explain” social insect behaviour.

18 McIntosh et al. (2012).