Sexual Reproduction

In order to maintain a species, its members have to produce offspring. It is often thought that sexual reproduction is the only method by which this can be achieved, but asexual reproduction is also a possibility. In many unicellular organisms offspring are produced because the cell simply divides itself in two. The result is two cells with exactly the same genetic characteristics (chromosomes) as their (one) parent. In humans asexual reproduction plays a role in the development of monozygotic, or identical twins. A fertilized ovum develops, by an asexual separation of the cells of a zygote*, into two genetically identical individuals. Sexual reproduction, however, is the prime method for producing offspring in humans. The uniting (fertilization) of male gametes (sperm) and female gametes (ova), marks the beginning of the development of a new individual. In essence sexual reproduction means that two separate sets of genetic material – 23 chromosomes from each parent – are brought together. This means that the new individual is not genetically identical with either of its parents. It is this genetic variability that also makes us different from our brothers and sisters, although there are also many similarities.

Meiosis

Every cell in our body contains a set of 23 pairs of chromosomes. Meiosis occurs during the formation of gametes. It takes place in order to maintain a constant number of chromosomes in offspring, and prevents the chromosome number doubling with each succeeding generation.

Meiotic division is a complex sequence of events. At the beginning of the process, every member of a chromosome pair in the nucleus combines with the other member. After this each member of every pair moves to one half of the nucleus. As a sex cell may contain any combination of chromosomes, the possible number of variations is enormous and because members of a chromosome pair also interchange their parts with each other the total number of possible variations is even greater. In this way every gamete or sex cell is of unique genetic composition. During the first meiotic division the parent cell splits into two, the products then divide again giving a total of four daughter cells each containing 23 chromosomes.

Sperm and eggs

Male and female gametes are different in structure and appearance.

The male gamete is called a spermatozoon (sperm). Millions of sperm cells are constantly being produced in finely coiled tubes within the testes during adult life. Each sperm is minute and shaped like a tadpole with a head and very long tail. The head contains the genetic information and the lashing tail, which is very powerful, provides the swimming propulsion which makes the sperm extremely motile. A major influence on sperm production is the temperature of the testis, which is about 3-5°F below that of the rest of the body. If this temperature rises, for example because of tight underclothing or an enforced stay in bed through illness, fertility is lowered.

The female gamete is the ovum or egg cell and is many times larger than the sperm. The ova are formed in the embryonic ovary at a very early stage of development and their genetic constitution is determined at that time. They are produced in much smaller numbers than the sperm, each ovary contains just 200 thousand immature ova. From puberty onwards the ova mature within the ovary, usually one each month, under hormonal control. The mature ovum is then shed from the ovary (ovulation) and from this time for about two days, until the egg dies, fertilization is possible.

Fertilization is the culmination of sexual intercourse during which about 100 million sperm are ejaculated into the female genital tract when the male reaches orgasm. The sperm are in a liquid called seminal fluid which provides a medium in which they can swim. Of the millions of sperm released only a few reach an ovum and only one actually succeeds in penetrating its outer membrane. The genetic material then combines to form the normal 23 pairs of chromosomes. The fertilized cell (zygote) begins to divide to form a ball of cells which eventually embeds itself in the endometrium which lines the wall of the uterus, going on to form the new individual.