Meiosis I Meiosis II
Interphase I: Identical to Interphase in mitosis. Prophase I: Identical to Prophase in mitosis. Metaphase I: Instead of all chromosomes pairing up along the midline of the cell as in mitosis, homologous chromosome pairs line up next to each other. This is called synapsis. Homologous chromosomes contain the matching alleles donated from mother and father. This is also when meiotic recombination, also know as "crossing over" (see below) occurs. This process allows for a genetic shuffling of the characteristics of the two parents, creating an almost infinite variety of possible combinations. See the close-up diagram below. Anaphase I: Instead of chromatids splitting at the centromere, homologous chromosome pairs (now shuffled by crossing over) move along the spindle fibers to opposite poles. Telophase I: The cell pinches and divides.
Prophase II: It is visibly obvious that replication has not occurred. Metaphase II: The paired chromosomes line up. Anaphase II: The chromatids split at the centromere and migrate along the spindle fibers to opposite poles. Telophase II: The cells pinch in the center and divide again. The final outcome is four cells, each with half of the genetic material found in the original. In the case of males, each cell becomes a sperm. In the case of females, one cell becomes an egg and the other three become polar bodies which are not used.
Your parents each have at least one pair of alleles (versions of a gene) for every trait (and many pairs of alleles for each polygenic trait). You ended up with half of mom's paired genes and half of dad's paired genes. But each non-identical-twin child of these parents ends up with a different combination. Imagine, for example, that eye color was controlled by a single gene, and that mom could have B, the allele for brown eyes or b, the allele for blue eyes, and dad could also have B or b. This leads to four possibilities: You could get B from mom and B from dad, or B from mom and b from dad, or b from mom and B from dad, or b from mom and b from dad. Each sperm and egg will end up with either B or b from mom and either B or b from dad. It's a flip of the coin. But this happens independently for each trait, so just because you got your dad's brown eyes doesn't mean you'll get his blond hair too. Each sibling is 50% mom and 50% dad, but which 50% of each can vary in the siblings. This shuffling process is known as recombination or "crossing over" and occurs while the chromome pairs are lined up in Metaphase I. In Metaphase I, homologous chromosome pairs line up. Homologous chromosomes can exchange parts in a process called "crossing over."
Meiosis is a process where a single cell divides twice to produce four cells containing half the original amount of genetic information. These cells are our sex cells – sperm in males, eggs in females. Meiosis can be divided into nine stages. These are divided between the first time the cell divides (meiosis I) and the second time it divides (meiosis II): 1. Interphase:
2. Prophase I:
3. Metaphase I:
4. Anaphase I:
5. Telophase I and cytokinesis:
Meiosis II6. Prophase II:
7. Metaphase II:
8. Anaphase II:
9. Telophase II and cytokinesis:
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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. Figure \(\PageIndex{1}\): Family photo
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. Figure \(\PageIndex{2}\): Crossing over occurs during meiosis I, and is the process where homologous chromosomes pair up with each other and exchange different segments of their genetic material to form recombinant chromosomes. In some species crossing over is essential for the normal segregation of chromosomes during meiosis. Crossing over also increases genetic variation, because due to the swapping of genetic material during crossing over, the chromatids held together by the centromere are no longer identical. So, when the chromosomes go on to meiosis II and separate, some of the daughter cells receive daughter chromosomes with recombined alleles. Due to this genetic recombination, the offspring have a different set of alleles and genes than their parents do. In the diagram, genes B and b are crossed over with each other, making the resulting recombinants after meiosis Ab, AB, ab, and aB.l.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. Figure \(\PageIndex{3}\): Overview of Meiosis. During meiosis, homologous chromosomes separate and go to different daughter cells. This diagram shows just the nuclei of the cells. Notice the exchange of genetic material that occurs prior to the first 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.
Figure \(\PageIndex{5}\): A human sperm is a tiny cell with a tail. A human egg is much larger. Both cells are mature haploid gametes that are capable of fertilization. What process is shown in this photograph? 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. Figure \(\PageIndex{7}\): Oogenesis The process of oogenesis occurs in the ovary's outermost layer. A primary oocyte begins the first meiotic division but then arrests until later in life when it will finish this division in a developing follicle. This results in a secondary oocyte, which will complete meiosis if it is fertilized.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: |