Sexual Reproduction of Organisms
General Process of Procreation
Reproduction (procreation) is the production of new individuals of the same species by asexual or sexual means. The term "reproduction" usually refers to sexual reproduction which involves the joining together (fusion) of special sex cells, one of which is female (known as the ovum or egg) and the other is male (sperm in animals and pollen in flowering plants for example). The sex cells of any organism are known as gametes.
Many organisms produce gametes within special reproductive organs. In the flower of a seed plant, the male sex organs are the stamens which produce pollen. The carpels are female and produce ovules which later develop into seeds after fertilization. Gametes are special cells which contain half the number of chromosomes of the parent, and this cell division is known as haploid (more deeper information about the definition of cell division is provided below). When a male and female gamete join together, called fusion, fertilization takes place and a single cell, called zygote, is produced which contains the full number of chromosomes, and this part is known as diploid. This goes on to produce a new individual, usually by a process of many cell divisions.
Cell Division Explained
Cell division is the formation of two daughter cells from one parent cell, with the nucleus dividing first by a process known as mitosis. The end result is that the nucleus of each daughter cell contains the same number of chromosomes, in other words, carrying the genes, as the original parent cell. The chromosomes are in the form of strands of material which come together in pairs. These eventually arrange themselves across the center of the nucleus and the nuclear membrane disappears. Each member of a pair is now composed of two strands, known as chromatids, which are joined at a single point called a centromere. As mitosis progresses, the centromeres split and one chromatid from each of the pairs moves to opposite ends.
A nuclear membrane forms around each group of chromatids, now the new chromosomes and then the cytoplasm itself divides to form two daughter cells each with its own nucleus. Another type of cell division occurs in the formation of sex cells which are sperm and eggs, and this is known as meiosis. In meiosis, the stages of mitosis are repeated again after a resting phase. The end result is that four daughter cells are produced, each with half the number of chromosomes of the parent cell, a condition known as haploid as already mentioned above.
At fertilization, two haploid cells, a sperm and an egg, fuse together so that the new individual has the full number of chromosomes, half from each parent, and this condition is known as diploid. Most cells are diploid with only the sex cells which are gametes, being haploid.
In mammals, as well as many other animals, the male sex organs are the testes and female ones are the ovaries. These reproductive organs are called gonads, and are specialized structures, the cells of which produce the sperm and ova under the influence of hormones. Most adult female mammals have a reproductive cycle, which is called an estrous cycle (or oestrous cycle), during which time the eggs develop and the animal becomes ready to mate. This mating period is known as heat or estrous.
Mating occurs at the time when the animal is most likely to become pregnant. Some mammals have a definite breeding season and only one estrous cycle in a year, this is known as monoestrous. Other animals have several cycles and this is known as polyoestrous. In female humans, the menstrual cycle replaces estrous cycle. Mating normally results in the fertilization of one or more egg. The cells rapidly divide and become embryos which grow and develop inside a muscular organ called the womb or uterus. The animal is now said to be pregnant.
A special organ of pregnancy, called the placenta, develops and this attaches the embryos to the uterus by the umbilical cord. This provides for the passage of food, oxygen and some other substances like vitamins and antibodies to the embryos while waste products, such as carbon dioxide and urea, pass in the opposite direction and are removed by the blood circulation of the mother.
When the young are fully developed they are born, in other words, it means that they are pushed to the outside through a passage leading from the womb called the vagina. This is brought about by contractions of the muscles in the wall of the uterus under the influence of hormones. After giving birth, the placenta is also pushed out immediately.