For the formation of ova, pollen, sperm, and other gametes, the division process is
slightly different (see Fig. 2.4b).
When the ovum nucleus that is the source of gametes transforms into a chromosome, homologous chromosomes first attach to form what appears outwardly to
be a single chromosome. The homologous chromosome then divides once again to
form two separate nuclei as the cell divides in two. If the parent cell’s number of
chromosomes is 2n, then the number of chromosomes for the two cells formed
through initial division in half that, or n. Another division then occurs, so that four
gametes with n chromosomes are formed from a single ovum. Because the number
of chromosomes is halved when gametes are formed in this way, the division
process is known as “meiosis.”
Fertilization in both plants and animals consists of the combination of male and
female gametes with this halved number of chromosomes n, so that the new number
of chromosomes is a multiple or sum 2n. The somatic cell of an organism that has
matured from a fertilized egg thus contains 2n chromosomes, half of them from the
Fig. 2.3 Mimetic diagrams for animal cells
2.2 Where Do Cells Exist in the Body?
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