No matter which pathway is used, a number of enzymes are required to complete the steps of recombination. The genes that code for these enzymes were first identified in E. coli by the isolation of mutant cells that were deficient in recombination. This research revealed that the recA gene encodes a protein necessary for strand invasion. Meanwhile, the recB, recC, and recD genes code for three polypeptides that join together to form a protein complex known as RecBCD; this complex has the capacity to unwind double-stranded DNA and cleave strands. Two other genes, ruvA and ruvB, encode enzymes that catalyze branch migration, while Holliday structures are resolved by the protein resolvase, which is product of the ruvC gene. Several enzymes involved in DNA replication, such as ligase and DNA polymerase, also contribute to recombination (Clark, 1973). In eukaryotes, recombination has been perhaps most thoroughly studied in the budding yeast Saccharomyces cerevisiae. Many of the enzymes identified in this yeast have also been found in other organisms, including mammalian cells. Such studies reveal that the Rad genes (named for the fact that their activity was found to be sensitive to radiation) play a key role in eukaryotic recombination. In particular, the Rad51 gene, which is homologous to recA, encodes a protein (called Rad51) that has recombinase activity. This gene is highly conserved, but the accessory proteins that assist Rad51 appear to vary among organisms. For example, the Rad52 protein is found in both yeast and humans, but it is missing in Drosophila melanogaster and C. elegans. In eukaryotic cells, single-stranded DNA (ssDNA) becomes rapidly coated with the protein RPA (replication protein A). RPA has a higher affinity for ssDNA than Rad51, and it therefore can inhibit recombination by blocking Rad51's access to the single strand needed for invasion. In yeast, however, binding of Rad51 to ssDNA is enhanced by the proteins Rad52 and the complex Rad55-Rad57. Once access has been gained, Rad51 polymerizes on the DNA strand to form what is called a presynaptic filament, which is a right-handed helical filament containing six Rad51 molecules and 18 nucleotides per helical repeat. The search for DNA homology and formation of the junction between homologous regions is then carried out within the catalytic center of the filament. In addition to proteins that assist Rad51 activity, there are also some proteins that inhibit it. In yeast, for instance, the helicase Srs2 dismantles the Rad51-ssDNA complex, while the proteins Sgs1 and BLM inhibit the complex. It is thought that these proteins play a role in preventing recombination during DNA replication when it is not needed. In humans, the tumor suppressor genes BRCA1 and BRCA2 also play a role in regulating recombination. Individuals who are heterozygous for BRCA2 are subject to increased risk for breast and ovarian cancer; loss of both alleles causes Fanconi's anemia, a genetic disease characterized by predisposition to cancer, among other defects. BRCA2 appears to promote Rad51 binding to ssDNA (Li & Heyer, 2008; Modesti & Kanaar, 2001). Meiosis is defined as the cellular and nuclear processes that reduce the chromosomal content per nucleus from two sets to one set. From: Encyclopedia of Genetics, 2001
The family photo in Figure \(\PageIndex{1}\) illustrates an important point. Children in a family resemble their parents and each other, but the children are never exactly the same unless they are identical twins. Each of the children in the photo inherited a unique combination of traits from the parents. In this concept, you will learn how this happens. It all begins with sex — sexual reproduction, that is.
Why do you look similar to your parents, but not identical? First, it is because you have two parents. Second, it is because of sexual reproduction. Whereas asexual reproduction produces genetically identical clones, sexual reproduction produces genetically diverse individuals. Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. As both parents contribute half of the new organism’s genetic material, the offspring will have traits of both parents, but will not be exactly like either parent. Organisms that reproduce sexually by joining gametes, a process known as fertilization, must have a mechanism to produce haploid gametes. This mechanism is meiosis, a type of cell division that halves the number of chromosomes. During meiosis, the pairs of chromosomes separate and segregate randomly to produce gametes with one chromosome from each pair. Meiosis involves two nuclear and cell divisions without interphase in between, starting with one diploid cell and generating four haploid cells. Each division, named meiosis I and meiosis II, has four stages: prophase, metaphase, anaphase, and telophase. These stages are similar to those of mitosis, but there are distinct and important differences. Prior to meiosis, the cell’s DNA is replicated, generating chromosomes with two sister chromatids. A human cell prior to meiosis will have 46 chromosomes, 22 pairs of homologous autosomes, and 1 pair of sex chromosomes. Homologous chromosomes (Figure \(\PageIndex{2}\)), or homologs, are similar in size, shape, and genetic content; they contain the same genes, though they may have different alleles of those genes. The genes/alleles are at the same loci on homologous chromosomes. You inherit one chromosome of each pair of homologs from your mother and the other one from your father. Sexual reproduction is the primary method of reproduction for the vast majority of multicellular organisms, including almost all animals and plants. Fertilization joins two haploid gametes into a diploid zygote, the first cell of a new organism. The zygote enters G1 of the first cell cycle, and the organism begins to grow and develop through mitosis and cell division.
The process that produces haploid gametes is called meiosis. Meiosis is a type of cell division in which the number of chromosomes is reduced by half. It occurs only in certain special cells of an organism. In mammals, Meiosis occurs only in gamete producing cells within the gonads. During meiosis, homologous (paired) chromosomes separate, and haploid cells form that have only one chromosome from each pair. Figure \(\PageIndex{3}\) gives an overview of meiosis. As you can see from the meiosis diagram, two cell divisions occur during the overall process, so a total of four haploid cells are produced. The two cell divisions are called meiosis I and meiosis II. Meiosis I begins after DNA replicates during interphase. Meiosis II follows meiosis I without DNA replicating again. Both meiosis I and meiosis II occur in four phases, called prophase, metaphase, anaphase, and telophase. You may recognize these four phases from mitosis, the division of the nucleus that takes place during routine cell division of eukaryotic cells.
At the end of meiosis, four haploid cells have been produced, but the cells are not yet gametes. The cells need to develop before they become mature gametes capable of fertilization. The development of diploid cells into gametes is called gametogenesis. It differs between males and females.
Spermatogenesis occurs in the wall of the seminiferous tubules, with stem cells at the periphery of the tube and the spermatozoa at the lumen of the tube. Immediately under the capsule of the tubule are diploid, undifferentiated cells. These stem cells, called spermatogonia (singular: spermatagonium), go through mitosis with one offspring going on to differentiate into a sperm cell, while the other gives rise to the next generation of sperm. Figure \(\PageIndex{6}\): Spermatogenesis During spermatogenesis, four sperm result from each primary spermatocyte, which divides into two haploid secondary spermatocytes; these cells will go through a second meiotic division to produce four spermatids. Meiosis begins with a cell called a primary spermatocyte. At the end of the first meiotic division, a haploid cell is produced called a secondary spermatocyte. This haploid cell must go through another meiotic cell division. The cell produced at the end of meiosis is called a spermatid. When it reaches the lumen of the tubule and grows a flagellum (or "tail"), it is called a sperm cell. Four sperm result from each primary spermatocyte that goes through meiosis. Stem cells are deposited during gestation and are present at birth through the beginning of adolescence but in an inactive state. During adolescence, gonadotropic hormones from the anterior pituitary cause the activation of these cells and the production of viable sperm. This continues into old age.
Oogenesis occurs in the outermost layers of the ovaries. As with sperm production, oogenesis starts with a germ cell, called an oogonium (plural: oogonia), but this cell undergoes mitosis to increase in number, eventually resulting in up to one to two million cells in the embryo. The cell starting meiosis is called a primary oocyte. This cell will begin the first meiotic division, but be arrested in its progress in the first prophase stage. At the time of birth, all future eggs are in the prophase stage. At adolescence, anterior pituitary hormones cause the development of a number of follicles in an ovary. This results in the primary oocyte finishing the first meiotic division. The cell divides unequally, with most of the cellular material and organelles going to one cell, called a secondary oocyte, and only one set of chromosomes and a small amount of cytoplasm going to the other cell. This second cell is called a polar body and usually dies. A secondary meiotic arrest occurs, this time at the metaphase II stage. At ovulation, this secondary oocyte will be released and travel toward the uterus through the oviduct. If the secondary oocyte is fertilized, the cell continues through the meiosis II, completing meiosis, producing a second polar body and a fertilized egg containing all 46 chromosomes of a human being, half of them coming from the sperm.
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A special type of cell division known as meiosis is responsible for your uniqueness. Learn more here: Ever wonder why some babies have Down Syndrome? Check out this video: |