A zygote is formed when the sperm cell from a human male and the ovum from a human female meet in the fallopian tubes inside the female body. The sperm cells and ova are different to all the other cells in the human body; they contain only 23 chromosomes (half the number of the full complement of chromosomes). Why is it that the sex cells only contain 23 chromosomes? This is because a normal cell contains 46 chromosomes. If the sex cells sheltered this many chromosomes, when they came to fuse in the fallopian tubes, the resulting zygote would contain 92 chromosomes! This is far too many, and the zygote would be invalid and never ‘come to term’. In order to overcome this problem, the sex cells contain 23 chromosomes, so that when they meet to form a zygote, the zygote contains 46 chromosomes; 23 from each parent. But how do the sex cells come to have only 23 chromosomes? The answer can be found in the process called meiosis.
Meiosis occurs only in the sex cells. It is similar to mitosis, although, meiosis produces four daughter cells and each daughter cell contains half the number of chromosomes found in a normal, somatic cell. The same stages that are found in mitosis, i.e. prophase, metaphase, anaphase, telophase and interphase, are also found in meiosis but unlike mitosis these stages occur twice in meiosis. It is the fact that these steps occur twice that allow for four daughter cells to be produced. The reduction of the number of chromosomes in the daughter cells occurs during the second interphase. Unlike the first sequence, chromosomes are not replicated during the second interphase. This means that when the chromosomes are pulled apart during the anaphase II, each pole receives only half the usual number of chromosomes. Hence, each daughter cell consists of 23 chromosomes.
However, it is what happens during prophase that is significant in explaining the biological role of sex. The fertilised zygote, containing 23 unique chromosomes from its mother and 23 unique chromosomes from its father, replicates these during interphase. In prophase, the chromosomes condense and ‘pair up’. In doing so, they form adhesions along their lengths, where parts of the homologous pairs ‘cross over’. When, during anaphase, these ‘crossings’ are pulled apart, the genetic information on one chromosome is swapped with that of its homologous partner. It is this crossing over that allows for genetic variation.
Genetic variation is important. Charles Darwin, now the father of evolutionary biology, published his book ‘The Origin of Species’ in 1859, explaining his theory of evolution and natural selection. Darwin believed that we often produce many more offspring than is necessary to ensure that our genes are carried on after we are gone. He also believed that not all the offspring we produce survive; they may die from disease or predation, and so producing lots of progeny allows our genes a greater chance of surviving. Finally, he believed that those young with the better genetic make up, and who are best adapted to their environment, are more likely to survive than any others. Thus, during meiosis, thousands upon thousands of different combinations of genes can be made by crossing over, independent assortment and random fertilisation. This makes each child born unique genetically. Some children have a better chance of surviving than others, depending upon their genetic make up.
In 1871, Darwin published ‘The Descent of Man’, in which he said he believed all humans evolved from apes. This has taken millions of years, and we are still evolving even now. In order to evolve, and to evolve towards a better, stronger, more intelligent species, different genetic sequences are produced during meiosis to give exclusive individuals, some more advantageous than others on the evolutionary scale.
The biological role of sex is to procreate. It is to drive already existing organisms to mate and produce new organisms of the same species so that the species is able to continue living. It is to allow for genetic variation to be generated during meiosis. It is to allow for this genetic variation to encourage evolution amongst species.