The simplest type of reproduction entails the division of a “parent” cell into two “daughter” cells. This occurs as part of the cell cycle, a series of events that prepares a cell to divide followed by the actual division process, called mitosis. The eukaryotic cell cycle commonly is represented as four stages. The chromosomes and the DNA they carry are copied during the S (synthesis) phase. The replicated chromosomes separate during the M (mitotic) phase, with each daughter cell getting a copy of each chromosome during cell division. The M and S phases are separated by two gap stages, the G1 phase and G2 phase, during which mRNAs and proteins are made. In single-celled organisms, both daughter cells often (though not always) resemble the parent cell. In multicellular organisms, stem cells can give rise to two different cells, one that resembles the parent cell and one that does not. Such asymmetric cell division is critical to the generation of different cell types in the body.
During growth the cell cycle operates continuously, with newly formed daughter cells immediately embarking on their own path to mitosis. Under optimal conditions bacteria can divide to form two daughter cells once every 30 minutes. At this rate, in an hour one cell becomes four; in a day one cell becomes more than 1014, which if dried would weigh about 25 grams. Under normal circumstances, however, growth cannot continue at this rate because the food supply becomes limiting.
Most eukaryotic cells take considerably longer than bacterial cells to grow and divide. Moreover, the cell cycle in adult plants and animals normally is highly regulated. This tight control prevents imbalanced, excessive growth of tissues while assuring that worn-out or damaged cells are replaced and that additional cells are formed in response to new circumstances or developmental needs. For instance, the proliferation of red blood cells increases substantially when a person ascends to a higher altitude and needs more capacity to capture oxygen. Some highly specialized cells in adult animals, such as nerve cells and striated muscle cells, rarely divide, if at all. The fundamental defect in cancer is loss of the ability to control the growth and division of cells.
Mitosis is an asexual process since the daughter cells carry the exact same genetic information as the parental cell. In sexual reproduction, fusion of two cells produces a third cell that contains genetic information from each parental cell. Since such fusions would cause an ever-increasing number of chromosomes, sexual reproductive cycles employ a special type of cell division, called meiosis, that reduces the number of chromosomes in preparation for fusion. Cells with a full set of chromosomes are called diploid cells. During meiosis, a diploid cell replicates its chromosomes as usual for mitosis but then divides twice without copying the chromosomes in-between. Each of the resulting four daughter cells, which has only half the full number of chromosomes, is said to be haploid.
Sexual reproduction occurs in animals and plants, and even in unicellular organisms such as yeasts. Animals spend considerable time and energy generating eggs and sperm, the haploid cells, called gametes, that are used for sexual reproduction. A human female will produce about half a million eggs in a lifetime, all these cells form before she is born; a young human male, about 100 million sperm each day. Gametes are formed from diploid precursor germ-line cells, which in humans contain 46 chromosomes. In humans the X and Y chromosomes are called sex chromosomes because they determine whether an individual is male or female. In human diploid cells, the 44 remaining chromosomes, called autosomes, occur as pairs of 22 different kinds. Through meiosis, a man produces sperm that have 22 chromosomes plus either an X or a Y, and a woman produces ova (unfertilized eggs) with 22 chromosomes plus an X. Fusion of an egg and sperm (fertilization) yields a fertilized egg, the zygote, with 46 chromosomes, one pair of each of the 22 kinds and a pair of X’s in females or an X and a Y in males. Errors during meiosis can lead to disorders resulting from an abnormal number of chromosomes. These include Down’s syndrome, caused by an extra chromosome 21, and Klinefelter’s syndrome,caused by an extra X chromosome.