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Describe the process of meiosis in cells.

Meiosis is a specialized type of cell division that occurs in sexually reproducing organisms, resulting in the formation of gametes (sperm and egg cells). The process of meiosis involves two rounds of division, known as meiosis I and meiosis II, and ultimately produces four cells with only one copy of each chromosome (haploid cells). Here's a brief overview of the process:

  1. Duplication Chromosomes: Before meiosis begins, the chromosomes within the cell duplicate during the interphase, similar to the process in mitosis.

  2. Meiosis I: -phase I: During this phase, homologous chromosomes pair up and exchange genetic information through a process called chromosomal. This genetic recombination results in new combinations of genetic material on the chromosomes.

    • Metaphase I: The of homologous chromosomes line up along the cell's equ.
    • Anaphase I: The homologous chromosomes separate and move towards opposite poles of the cell.
    • Telophase I: The cell undergoes cytokinesis, forming daughter cells, each with half the number of chromosomes as the original cell. Importantly, each chromosome still of two sister chromatids at this stage.

  3. Meiosis II:

    • Prophase II: The chromosomes within the two daughter cells condense again.
    • Metaphase II: The chromosomes line up along the equator of the cell - Anaphase II: The sister chromatids of each chromosome are pulled apart and move towards poles of the cell.
    • Telophase II: The cell undergoes cytokinesis again, resulting in the formation of a total of four haploid daughter cells, each with one copy of each chromosomeThe four daughter cells produced by meiosis are gametes, such as sperm or egg cells, with only one set of chromosomes. During fertilization, two haploid gametes fuse to form a diploid zygote, restoring the full complement of paired chromosomes. This process of meiosis is crucial for genetic diversity and is a key aspect of sexual reproduction.

Additionally, errors in meiosis can result in aneuploidy, which can lead to miscarriages and developmental disabilities. Therefore, meiosis plays a critical role in ensuring the proper distribution of genetic material during the formation gametes.

For the figure of speech, see Meiosis (figure of speech). For the process whereby cell nuclei divide to produce two copies of themselves, see Mitosis. For excessive constriction of the pupils, see Miosis. For the parasitic infestation, see Myiasis. For muscle inflammation, see Myositis. In meiosis, the chromosomes duplicate (during interphase) and homologous chromosomes exchange genetic information (chromosomal crossover) during the first division, called meiosis I. The daughter cells divide again in meiosis II, splitting up sister chromatids to form haploid gametes. Two gametes fuse during fertilization, forming a diploid cell (zygote) with a complete set of paired chromosomes. A video of meiosis I in a crane fly spermatocyte, played back at 120 the recorded speed Meiosis (; from Ancient Greek (mesis)'lessening', since it is a reductional division)[1][2] is a special type of cell division of germ cells and apicomplexans in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately result in four cells with only one copy of each chromosome (haploid). Additionally, prior to the division, genetic material from the paternal and maternal copies of each chromosome is crossed over, creating new combinations of code on each chromosome.[3] Later on, during fertilisation, the haploid cells produced by meiosis from a male and a female will fuse to create a cell with two copies of each chromosome again, the zygote. Errors in meiosis resulting in aneuploidy (an abnormal number of chromosomes) are the leading known cause of miscarriage and the most frequent genetic cause of developmental disabilities.[4] In meiosis, DNA replication is followed by two rounds of cell division to produce four daughter cells, each with half the number of chromosomes as the original parent cell.[3] The two meiotic divisions are known as meiosis I and meiosis II. Before meiosis begins, during S phase of the cell cycle, the DNA of each chromosome is replicated so that it consists of two identical sister chromatids, which remain held together through sister chromatid cohesion. This S-phase can be referred to as "premeiotic S-phase" or "meiotic S-phase". Immediately following DNA replication, meiotic cells enter a prolonged G2-like stage known as meiotic prophase. During this time, homologous chromosomes pair with each other and undergo genetic recombination, a programmed process in which DNA may be cut and then repaired, which allows them to exchange some of their genetic information. A subset of recombination events results in crossovers, which create physical links known as chiasmata (singular: chiasma, for the Greek letter Chi ()) between the homologous chromosomes. In most organisms, these links can help direct each pair of homologous chromosomes to segregate away from each other during meiosis I, resulting in two haploid cells that have half the number of chromosomes as the parent cell. During meiosis II, the cohesion between sister chromatids is released and they segregate from one another, as during mitosis. In some cases, all four of the meiotic products form gametes such as sperm, spores or pollen. In female animals, three of the four meiotic products are typically eliminated by extrusion into polar bodies, and only one cell develops to produce an ovum. Because the number of chromosomes is halved during meiosis, gametes can fuse (i.e. fertilization) to form a diploid zygote that contains two copies of each chromosome, one from each parent. Thus, alternating cycles of meiosis and fertilization enable sexual reproduction, with successive generations maintaining the same number of chromosomes. For example, diploid human cells contain 23 pairs of chromosomes including 1 pair of sex chromosomes (46 total), half of maternal origin and half of paternal origin. Meiosis produces haploid gametes (ova or sperm) that contain one set of 23 chromosomes. When two gametes (an egg and a sperm)

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Learning Objectives By the end of this section, you will be able to: Describe the behavior of chromosomes during meiosis Describe cellular events during meiosis Explain the differences between meiosis and mitosis Explain the mechanisms within meiosis that generate genetic variation among the products of meiosis Sexual reproduction requires fertilization, the union of two cells from two individual organisms. If those two cells each contain one set of chromosomes, then the resulting cell contains two sets of chromosomes. Haploid cells contain one set of chromosomes. Cells containing two sets of chromosomes are called diploid. The number of sets of chromosomes in a cell is called its ploidy level. If the reproductive cycle is to continue, then the diploid cell must somehow reduce its number of chromosome sets before fertilization can occur again, or there will be a continual doubling in the number of chromosome sets in every generation. So, in addition to fertilization, sexual reproduction includes a nuclear division that reduces the number of chromosome sets. Most animals and plants are diploid, containing two sets of chromosomes. In each somatic cell of the organism (all cells of a multicellular organism except the gametes or reproductive cells), the nucleus contains two copies of each chromosome, called homologous chromosomes. Somatic cells are sometimes referred to as body cells. Homologous chromosomes are matched pairs containing the same genes in identical locations along their length. Diploid organisms inherit one copy of each homologous chromosome from each parent; all together, they are considered a full set of chromosomes. Haploid cells, containing a single copy of each homologous chromosome, are found only within structures that give rise to either gametes or spores. Spores are haploid cells that can produce a haploid organism or can fuse with another spore to form a diploid cell. All animals and most plants produce eggs and sperm, or gametes. Some plants and all fungi produce spores. The nuclear division that forms haploid cells, which is called meiosis, is related to mitosis. As you have learned, mitosis is the part of a cell reproduction cycle that results in identical daughter nuclei that are also genetically identical to the original parent nucleus. In mitosis, both the parent and the daughter nuclei are at the same ploidy leveldiploid for most plants and animals. Meiosis employs many of the same mechanisms as mitosis. However, the starting nucleus is always diploid and the nuclei that result at the end of a meiotic cell division are haploid. To achieve this reduction in chromosome number, meiosis consists of one round of chromosome duplication and two rounds of nuclear division. Because the events that occur during each of the division stages are analogous to the events of mitosis, the same stage names are assigned. However, because there are two rounds of division, the major process and the stages are designated with a I or a II. Thus, meiosis I is the first round of meiotic division and consists of prophase I, prometaphase I, and so on. Meiosis II, in which the second round of meiotic division takes place, includes prophase II, prometaphase II, and so on. Meiosis I Meiosis is preceded by an interphase consisting of the G1, S, and G2 phases, which are nearly identical to the phases preceding mitosis. The G1 phase, which is also called the first gap phase, is the first phase of the interphase and is focused on cell growth. The S phase is the second phase of interphase, during which the DNA of the chromosomes is replicated. Finally, the G2 phase, also called the second gap phase, is the third and final phase of interphase; in this phase, the cell undergoes the final preparations for meiosis. During DNA duplication in the S phase, each chromosome is replicated to produce two identical copies, called sister chromatids, that are held together at the centromere by cohesin proteins. Cohesin holds the chromatids togeth

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Home Health & Medicine Anatomy & Physiology Also known as: reduction division Written and fact-checked by Also called: reduction division meiosis, division of a germ cell involving two fissions of the nucleus and giving rise to four gametes, or sex cells, each possessing half the number of chromosomes of the original cell.A brief treatment of meiosis follows. For further discussion, see cell: Cell division and growth. More From Britannica cell: Meiosis Observe the meiotic reproduction of a diploid cell into four haploid gametesGametes are formed through meiosis (reduction division).See all videos for this articleThe process of meiosis is characteristic of organisms that reproduce sexually. Such species have in the nucleus of each cell a diploid (double) set of chromosomes, consisting of two haploid sets (one inherited from each parent). These haploid sets are homologousi.e., they contain the same kinds of genes, but not necessarily in the same form. In humans, for example, each set of homologous chromosomes contains a gene for blood type, but one set may have the gene for blood type A and the other set the gene for blood type B.Prior to meiosis, each of the chromosomes in the diploid germ cell has replicated and thus consists of a joined pair of duplicate chromatids. Meiosis begins with prophase I and the contraction of the chromosomes in the nucleus of the diploid cell. Homologous paternal and maternal chromosomes pair up along the midline of the cell. Each pair of chromosomescalled a tetrad, or a bivalentconsists of four chromatids. At this point, the homologous chromosomes exchange genetic material by the process of crossing over (see linkage group). The homologous pairs line up along the midline of the cell in metaphase I and then separate in anaphase I, with each pair being pulled to opposite ends of the cell. In telophase I the elongated cell then pinches in half to form two daughter cells. Each daughter cell of this first meiotic division contains a haploid set of chromosomes. The chromosomes at this point still consist of duplicate chromatids.In the second meiotic division, each haploid daughter cell divides. There is no further reduction in chromosome number during this division, as it involves the separation of each chromatid pair into two chromosomes, which are pulled to the opposite ends of the daughter cells. Each daughter cell then divides in half, thereby producing a total of four different haploid gametes. When two gametes unite during fertilization, each contributes its haploid set of chromosomes to the new individual, restoring the diploid number. See also mitosis. The Editors of Encyclopaedia BritannicaThis article was most recently revised and updated by Melissa Petruzzello.

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