A threadlike structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes. Two pieces of DNA within a diploid organism which carry the same types genes, one from each parental source. The longest stage in the eukaryotic cell cycle during which the cell acquires nutrients, creates and uses proteins and other molecules, and starts the process of cell division by replicating the DNA.
The exchange of genetic material between two homologous chromosomes non-sister chromatids that results in recombinant chromosomes during sexual reproduction. The way in which different genes independently separate from one another when reproductive cells develop. During meiosis, the pairs of homologous chromosome are divided in half to form haploid cells, and this separation, or assortment, of homologous chromosomes is random.
A mature female reproductive cell, especially of a human or other animal, which can divide to give rise to an embryo usually only after fertilization by a male cell. The process whereby a haploid cell n is formed from a diploid cell 2n through meiosis and cell differentiation. Gametogenesis in the male is known as spermatogenesis and produces spermatozoa. Gametogenesis in the female is known as oogenesis and result in the formation of ova. All in the Family This family photo Figure 5.
Sexual Reproduction. Figure 5. Sexual Reproduction and Genetic Variation. In sexual reproduction , two parents produce gametes that unite in the process of fertilization to form a single-celled zygote. Gametes are haploid cells with one copy of each of the 23 chromosomes, and the zygote is a diploid cell with two copies of each of the 23 chromosomes.
Meiosis is the type of cell division that produces four haploid daughter cells that may become gametes. Meiosis occurs in two stages, called meiosis I and meiosis II, each of which occurs in four phases prophase, metaphase, anaphase, and telophase. Meiosis is followed by gametogenesis , the process during which the haploid daughter cells change into mature gametes. Males produce gametes called sperm in a process known as spermatogenesis , and females produce gametes called eggs in the process known as oogenesis.
Sexual reproduction produces genetically unique offspring. Crossing-over , independent alignment , and the random union of gametes work together to result in an amazing range of potential genetic variation.
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. A special type of cell division known as meiosis is responsible for your uniqueness.
Learn more here:. Sexual Reproduction Why do you look similar to your parents, but not identical? 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.
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. Meiosis I Prophase I : The nuclear envelope begins to break down, and the chromosomes condense. Centrioles start moving to opposite poles of the cell, and a spindle begins to form. Importantly, homologous chromosomes pair up, which is unique to prophase I.
In prophase of mitosis and meiosis II, homologous chromosomes do not form pairs in this way. During prophase I, crossing-over occurs. The significance of crossing-over is discussed in the next section called variations. Metaphase I : Spindle fibers attach to the paired homologous chromosomes. The paired chromosomes line up along the equator of the cell. This occurs only in metaphase I. In metaphase of mitosis and meiosis II, it is sister chromatids that line up along the equator of the cell.
Anaphase I : Spindle fibers shorten, and the chromosomes of each homologous pair start to separate from each other. One chromosome of each pair moves toward one pole of the cell, and the other chromosome moves toward the opposite pole. Telophase I and Cytokinesis: The spindle breaks down, and new nuclear membranes form.
The cytoplasm of the cell divides, and two haploid daughter cells result. The daughter cells each have a random assortment of chromosomes, with one from each homologous pair. Both daughter cells go on to meiosis II. The centrioles also start to separate. Sperm must be agile and highly motile in order to have the opportunity to fertilize the egg—and this is their sole purpose. For this reason, they hardly carry any cellular organelles excluding packs of mitochondria which fuel their rapid motion , mostly just DNA.
For this reason, only a single, well-fortified egg is produced by each round of meiosis. Meiosis is a process that is conserved, in one form or another, across all sexually-reproducing organisms. This means that the process appears to drive reproductive abilities in a variety of organisms and points to the common evolutionary pathway for those organisms that reproduce sexually.
It is vitally important for the maintenance of genetic integrity and enhancement of diversity. Since humans are diploid 2N organisms, failure to halve the ploidy before fertilization can have disastrous effects. For this reason, only very select types of abnormal ploidy survive and do so with noticeable defects ; most combinations containing abnormal ploidy never make it into the world. The correct reduction of the number of chromosomes insures that once fertilization takes place, the correct amount of genetic material is established in the fertilized egg and, eventually, in the person resulting from it.
Meiosis in Humans By: Inbar Maayan. Keywords: Human development , Meiosis. Meiosis in Humans Meiosis, the process by which sexually reproducing organisms generate gametes sex cells , is an essential precondition for the normal formation of the embryo. Gilbert, Scott F. Sunderland, MA: Sinauer, Hochwagen, Andreas. Klug, William S. Cummings, Charlotte Spencer, and Michael A.
San Francisco: Pearson, Tobin, Allan J. Asking About Life, Third Edition. Printer-friendly version PDF version. Meiosis starts 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 cell is haploid and must go through another meiotic cell division. The cell produced at the end of meiosis is called a spermatid and when it reaches the lumen of the tubule and grows a flagellum, 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. Figure 2.
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