The growth of an organism occurs as a result of cell division in a process known as mitosis. Many cells are
SOURCE: "Centromere," in Talking Glossary of Genetic Terms, U.S. Department of Health and Human Services, National Institutes of Health, National Human Genome Research Institute, http://www.genome.gov/Pages/Hyperion//DIR/VIP/Glossary/Illustration/centromere.shtml (accessed February 3, 2005)
relatively short lived, and mitosis allows for regular renewal of these cells. It is also the process that generates the millions of cells needed to grow an organism from birth to adulthood.
SOURCE: "New Combinations of Alleles Result from the Crossing Over between Homologous Chromosomes," adapted from Robinson, 2001, in Genetics, vol. 3, K–P, Macmillan Reference USA, Gale Group, 2002
Mitosis is a continuous process that occurs in several stages. Between cell divisions, the cells are in interphase, during which there is cell growth, and the genetic material—DNA—contained in the chromosomes is duplicated so that when the cell divides, each new cell has a full-scale version of the same genetic material. The process of mitosis involves exact duplication—gene by gene—of the cell's chromosomal material and a systematic method for evenly distributing this material. It concludes with the physical division known as cytokinesis, when the identical chromosomes pull apart and each heads for the nucleus of one of the new daughter cells. Mitosis occurs in all eukaryotic cells, except the gametes (sperm and egg), and always produces genetically identical daughter cells with a complete set of chromosomes.
If, however, mitosis occurred in the gametes, then when fertilization—the joining of sperm and egg—took place, the offspring would receive a double dose of hereditary information. To prevent this from occurring, the gametes undergo a process of reduction division known as meiosis. Meiosis reduces the number of chromosomes in the gametes by half, so that when fertilization occurs the normal number of chromosomes is restored. For example, in humans the gametes produced by meiosis are haploid—they have just one copy of each of the twenty-three chromosomes. In addition to preventing the number of chromosomes from doubling with each successive generation, meiosis also provides genetic diversity in offspring.
During meiosis, the chromosomal material replicates and concentrates itself into homologous chromosomes (doubled chromosomes), each of which is joined at a central spot called the centromere. Figure 2.11 shows chromosomes joined at the centromere, and Figure 2.13
Meiosis in an organism with six chromosomes
SOURCE: "Meiosis in an Organism with Six Chromosomes. Replication Precedes Meiosis," adapted from Curtis, 1994, in Genetics, vol. 3, K–P, Macmillan Reference USA, Gale Group, 2002
shows the location of the centromere in the chromosome. Pairing up along their entire lengths, they are able to exchange genetic material in a process known as crossing over. Figure 2.14 shows homologous chromosomes crossing over during meiosis to create new gene combinations. Crossing over results in much of the genetic variation observed among parents and their offspring. The pairing of homologous chromosomes and crossing over occur only in meiosis.
The process of meiosis also creates another opportunity to generate genetic diversity. During one phase of meiosis, called metaphase, the arrangement of each pair of homologous chromosomes is random, and different combinations of maternal and paternal chromosomes line up with varying orientations to create new gene combinations on different chromosomes. This action is called "independent assortment." Figure 2.15 shows the process of meiosis in an organism with six chromosomes. Since recombination and independent assortment of parental chromosomes takes place during meiosis, the daughter cells are not genetically identical to one another.
In male animals gamete formation, known as spermatogenesis, begins at puberty and takes place in the testes. Spermatogenesis involves a sequence of events that begin with the mitosis of primary germ cells to produce primary spermatocytes. (See Figure 2.16.) Four primary spermatocytes undergo two meiotic divisions and as they undergo spermatogenesis they lose much of their cytoplasm and develop the spermatozoa's characteristic tail, the motor apparatus that provides the propulsion necessary to reach the egg cell. Like oocytes (egg cells) produced by the female, the mature sperm cells will be haploid, possessing just one copy of each chromosome.
In female animals gamete formation, known as oogenesis, takes place in the ovaries. Primary oogocytes are produced by mitosis in the fetus before birth. Unlike the male, which continues to produce sperm cells throughout life, in the female the total number of eggs ever to be produced is present at birth. At birth, or shortly before, meiosis begins and primary oocytes remain in the pro-phase of meiosis until puberty. At puberty, the first meiotic division is completed, a diploid cell becomes two haploid daughter cells; one large cell becomes the secondary oocyte and the other the first polar body. The secondary oocyte undergoes meiosis a second time but the meiosis does not continue to completion without fertilization. Figure 2.17 shows the sequence of events leading to the production of a mature ovum.
Like gamete formation, fertilization is a process, as opposed to a single event. It begins when the sperm and egg first come into contact and fuse together and culminates with the intermingling of two sets of haploid genes to reconstitute a diploid cell with the potential to become a new organism.
Of the millions of sperm released during an ejaculation, less than 1% survives to reach the egg. Of the few
Process of spermatogenesis
SOURCE: Danton H. O'Day, "Spermatogenesis: The Sequence of Events," University of Toronto at Mississauga, Canada
Process of oogenesis
SOURCE: Danton H. O'Day, "Oogenesis: The Sequence of Events," University of Toronto at Mississauga, Canada
hundred sperm that reach the egg, only one will successfully fertilize it. While the sperm are in the female reproductive tract, swimming toward the egg, they undergo a process known as capacitation, during which they acquire the capacity to fertilize the egg. As the sperm near the egg they become hyperactivated, and in a frenzy of mechanical energy the sperm attempt to burrow their way through the outer shell of the egg called the zona pellucida.
The cap of the sperm, known as the acrosome, contains enzymes that are crucial for fertilization. These acrosomal enzymes dissolve the zona pellucida by making a tiny hole in it, so that one sperm can swim through and reach the surface of the egg. At this time, the egg transforms the zona pellucida by creating an impenetrable barrier, so that no other sperm may enter.
Sperm penetration triggers the second meiotic division of the egg. With this division the chromosomes of the sperm and egg form a single nucleus. The resulting cell—the first cell of an entirely new organism—is called a zygote. The zygote then divides into two cells, which, in turn, continue to divide rapidly, producing a ball of cells now called the blastocyst. The blastocyst is an early stage of embryogenesis, the process that describes the development of the fertilized egg as it becomes an embryo. In humans the developing baby is considered an embryo until the end of the eighth week of pregnancy.
SOURCE: "Phenotype," in Talking Glossary of Genetic Terms, U.S. Department of Health and Human Services, National Institutes of Health, National Human Genome Research Institute, http://www.genome.gov/Pages/Hyperion//DIR/VIP/Glossary/Illustration/phenotype.shtml (accessed February 3, 2005)