..... Click the link for more information. is reduced to half the original number. Meiosis occurs only in the process of gametogenesis, i.e., when the gametes, or sex cells (ovum ovum (ō`vəm), in biology, specialized plant or animal sex cell, also called the egg, or egg cell.
..... Click the link for more information. and sperm sperm or spermatozoon (spûr'mətəzō`ən, –zō`ŏn)
..... Click the link for more information. ), are being formed. Because fertilization Cross-fertilization indicates fusion of a sperm of one hermaphroditic plant or animal with an ovum of another, as distinguished from self-fertilization, in which ovum and sperm of the same individual are fused.
..... Click the link for more information. consists of the fusion of two separate nuclei, one from each of the sex cells, meiosis is necessary to prevent the doubling of the chromosome number in each successive generation. An ordinary body cell is diploid; i.e., it contains two of each type of chromosome. The members of each pair are known as homologous chromosomes. An ovum or sperm is haploid; i.e., it contains only a single chromosome of each type and, therefore, half the number of chromosomes of the diploid cell. When the two haploid cells fuse, the diploid number is restored, and the plant or animal growing from the fertilized egg (zygote) has the usual diploid number of chromosomes in its cells. Just before meiosis each chromosome replicates to form two identical copies in the form of strands called chromatids joined together at a point called the centromere. In the first stage of meiosis, called the reduction division, the members of each pair of homologous chromosomes lie side by side and crossing over crossing over, process in genetics by which the two chromosomes of a homologous pair exchange equal segments with each other. Crossing over occurs in the first division of meiosis .
..... Click the link for more information. occurs. Each member of the pair then moves away from the other toward opposite ends of the dividing cell, and two nuclei, each with the haploid number of double-stringed chromosomes, are formed. Thus at the beginning of the second meiotic sequence, called the equational division, each cell nucleus contains one chromosome from each homologous pair and each chromosome is of two strands that are identical (except where crossing over has occurred). Then the chromosomes separate into their single strands which move toward opposite ends of the dividing nucleus. The result of meiotic division is four cells, each haploid, with one chromosome of each pair.
meiosis
or reduction divisionDivision of a gamete-producing cell in which the nucleus splits twice, resulting in four sex cells (gametes, or eggs and sperm), each possessing half the number of chromosomes of the original cell. Meiosis is characteristic of organisms that reproduce sexually and have a diploid set of nuclear chromosomes (see ploidy). Before meiosis, chromosomes replicate and consist of joined sister strands (chromatids). Meiosis begins as homologous paternal and maternal chromosomes line up along the midline of the cell. The chromosomes exchange genetic material by the process of crossing-over (see linkage group), in which chromatid strands from homologous pairs entangle and exchange segments to produce chromatids containing genetic material from both parents. The pairs then separate and are pulled to opposite ends of the cell, which then pinches in half to form two daughter cells, each containing a haploid set (half the usual number) of double-stranded chromosomes. In the second round of meiotic division, the double-stranded chromosomes of each daughter cell are pulled apart, resulting in four 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.
meiosis
meiosis [mī′ō·səs]
Meiosis
The set of two successive cell divisions that serve to separate homologous chromosome pairs prior to the formation of gametes (sperm and eggs). The major purpose of meiosis is the precise reduction in the number of chromosomes by one-half, so that a diploid cell can create haploid gametes. Meiosis is therefore a critical component of sexual reproduction. See Gametogenesis
The basic events of meiosis are actually quite simple. As the cell begins meiosis, each chromosome has already duplicated its deoxyribonucleic acid (DNA) and carries two identical copies of the DNA molecule. These are visible as two lateral parts, called sister chromatids, which are connected by a centromere. Homologous pairs of chromosomes are first identified and matched. This process, which occurs only in the first of the two meiotic divisions, is called pairing. The matched pairs are then physically interlocked by recombination, which is also known as exchange or crossing-over. After recombination, the homologous chromosomes separate from each other, and at the first meiotic division are partitioned into different nuclei. As a consequence, the second meiotic division begins with half of the original number of chromosomes. During this second meiotic division, the sister chromatids of each chromosome separate and migrate to different daughter cells. See Chromosome
The patterns by which genes are inherited are determined by the movement of the chromosomes during the two meiotic divisions. It is a fundamental tenet of mendelian inheritance that each individual carries two copies of each gene, one derived from its father and one derived from its mother. Moreover, each of that individual's gametes will carry only one copy of that gene, which is chosen at random. The process by which the two copies of a given gene are distributed into separate gametes is referred to as segregation. Thus, if an individual is heterozygous at the A gene for two different alleles, A and a, his or her gametes will be equally likely to carry the A allele or the a allele, but never both or neither. The fact that homologous chromosomes, and thus homologous genes, segregate to opposite poles at the first meiotic division explains this principle of inheritance. See Cell cycle
Meiotic divisions
The two meiotic divisions may be divided into a number of distinct stages. Meiotic prophase refers to the period after the last cycle of DNA replication, during which time homologous chromosomes pair and recombine. The end of prophase is signaled by the breakdown of the nuclear envelope, and the association of the paired chromosomes with the meiotic spindle. The spindle is made up of microtubules that, with associated motor proteins, mediate chromosome movement. In some cases (such as human sperm formation), the spindle is already formed at the point of nuclear envelope breakdown, and the chromosomes then attach to it. In other systems (such as human female meiosis), the chromosomes themselves organize the spindle.
Metaphase I is the period before the first division during which pairs of interlocked homologous chromosomes, called bivalents, line up on the middle of the meiotic spindle. The chromosomes are primarily (but not exclusively) attached to the spindle by their centromeres such that the centromere of one homolog is attached to spindle fibers emanating from one pole, and the centromere of its partner is attached to spindle fibers from the other pole (see illustration). The bivalents are physically held together by structures referred to as chiasmata that are the result of meiotic recombination events. In most meiotic systems, meiosis will not continue until all of the homolog pairs are properly oriented at the middle of the spindle, the metaphase plate. The orientation of each pair of homologs on the spindle occurs in a random fashion, such that the paternally derived homolog of one bivalent may point toward one pole of the spindle, while in the adjacent bivalent the maternally derived homolog is oriented toward the same pole.
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Anaphase I refers to the point at which homologous chromosome pairs separate and move to opposite poles. Depending on the organism, there may or may not be a true telophase, or a time in which nuclei reform. In most organisms, the first cell division occurs after the completion of anaphase I.
Following the completion of the first meiotic division, the chromosomes recondense and align themselves on a new pair of spindles, with their sister chromatids oriented toward opposite poles. The stage at which each chromosome is so aligned is referred to as metaphase II. In some, but not all, organisms, metaphase II is preceded by a brief prophase II. DNA replication does not occur during prophase II; each chromosome still consists of the two sister chromatids. Nor are there opportunities for pairing or recombination at this stage due to the prior separation of homologs at anaphase I.
The start of anaphase II is signaled by the separation of sister centromeres, and the movement of the two sister chromatids to opposite poles. At telophase II, the sisters have reached opposite poles and the nuclei begin to reform. The second cell division usually occurs at this time. Thus, at the end of the second meiotic division, there will be four daughter cells, each with a single copy of each chromosome.
Details of meiotic prophase
Because pairing and recombination occur during the first meiotic prophase, much attention has been focused on this stage of the process. The prophase of the first meiotic division is subdivided into five stages: leptotene, zygotene, pachytene, diplotene, and diakinesis (see illustration). Homolog recognition, alignment, and synapsis occur during leptotene and zygotene. In the leptotene, initial homolog alignments are made. By zygotene, homologous chromosomes have become associated at various points along their length. These associations facilitate a more intimate pairing that results in the homologous chromosomes lying abreast of a tracklike structure called the synaptonemal complex. The beginning of pachytene is signaled by the completion of a continuous synaptonemal complex running the full length of each bivalent. During diplotene, the attractive forces that mediated homologous pairing disappear, and the homologs begin to repel each other. However, homologs virtually always recombine, and those recombination events can be seen as chiasmata that tether the homologs together. The final stage in meiotic prophase is diakinesis, during which the homologs shorten and condense in preparation for nuclear division.
Recombination
Meiotic recombination involves the physical interchange of DNA molecules between the two homologous chromosomes, thus allowing the creation of new combinations of alleles for genes located on that pair of chromosomes. Recombination involves the precise breakage and rejoining of two nonsister chromatids. The result is the formation of two recombinant chromatids, each of which carries information from both of the original homologs. The number and position of recombination events is very precisely controlled. Exchange occurs only in the gene-rich euchromatin that makes up most of the chromosome arms, never in the heterochromatin that surrounds the centromeres. Moreover, as a result of a process known as interference, the occurrence of one exchange in a given chromosomal region greatly decreases the probability of a second exchange in that region. See Recombination (genetics)
Errors of meiosis
The failure of two chromosomes to segregate properly is called nondisjunction. Nondisjunction occurs either because two homologs failed to pair and/or recombine or because of a failure of the cell to properly move the segregating chromosomes on the meiotic spindle. The result of nondisjunction is the production of gametes that are aneuploid, carrying the wrong number of chromosomes. When such a gamete is involved in a fertilization event, the resulting zygote is also aneuploid. Those cases where the embryo carries an extra copy of a given chromosome are said to be trisomic, while those that carry but one copy are said to be monosomic for that chromosome. Most aneuploid zygotes are not viable and result in early spontaneous abortion. There are no viable monosomies for the human autosomes; however, a few types of trisomic zygotes are capable of survival. These are trisomies for the sex chromosomes (XXX, XXY, XYY), trisomy 21 (Down syndrome), trisomy 18, and trisomy 13. See Crossing-over (genetics)
Meiosis versus mitosis
The fundamental difference between meiosis and mitosis is that at the first meiotic division, sister chromatids do not separate; rather, homologous chromosomes separate from each other with their sister chromatids still attached to each other. Recombination is frequent in most meiotic cells; however, it occurs only rarely in mitotic cells, usually as part of DNA repair events. Most critically, DNA synthesis occurs only once within the two meiotic divisions, while there is a complete replication before every mitotic division. This allows mitosis to produce two genetically identical daughter cells, while meiosis produces four daughter cells, each of which have only one-half the number of chromosomes present prior to meiosis. See Cell division, Gene, Mitosis
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