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When two sex cells join during sexual reproduction, the two haploid sets form a full diploid set in the fertilized egg.
First Meiotic Division
In human beings and many other animals, meiosis takes place in specialized stem cells inside the sex glands—the female’s ovaries and the male’s testes. During the cell’s “resting” period, or interphase, DNA replicates itself to form two complete sets. As meiosis begins, the DNA becomes more coiled and twisted, and appears under the light microscope as thread-like chromosomes. Each chromosome is actually a double-strand of two identical chromatids, joined near the middle. The chromosomes come together in like pairs, one originally from the mother and one from the father. These pairings are bivalents. A chromatid of one chromosome in the bivalent may join together with a chromatid from the other chromosome, at points along their length called chiasmata. They may also exchange segments of DNA, known as crossing-over, at these points. This is one source of mixing or recombination of the genes into new and unique selections.

As in mitosis, the nuclear membrane around the chromosomes begins to break down. The pairs of chromosomes align along the middle of the cell, which are linked by rod-like fibres known as spindle fibres to pairs of centrioles at opposing ends, or poles, of the cell. The chromosomes separate and members of each pair move along the spindle fibres towards opposing poles. Whether a chromosome originally from the father or mother goes to a particular pole is entirely chance.

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