meiosis(redirected from Anaphase II)
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meiosis (mīŏˈsĭs), process of nuclear division in a living cell by which the number of chromosomes is reduced to half the original number. Meiosis occurs only in the process of gametogenesis, i.e., when the gametes, or sex cells (ovum and sperm), are being formed. Because fertilization 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 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.
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
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.
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.
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
maturation division, a method of cell division by which the number of chromosomes is reduced by half, each diploid cell (containing two sets of chromosomes) giving rise, after two rapidly successive divisions, to four haploid cells (containing one chromosomal set each). The diploid number of chromosomes is restored by fertilization.
Meiosis is an essential element in the sexual process and requisite for the formation of sex cells (gametes). The biological function of meiosis is to maintain the constancy of the karyotype throughout the generations of a given species and to provide for the possibility of chromosomal and genie recombination in the sexual process. It is one of the key mechanisms of heredity and hereditary variability. The behavior of the chromosomes during meiosis is such that the main laws of inheritance can be carried out.
Three types of meiosis are distinguished, according to the place of meiosis in the life cycle of the organism. Gamete, or terminal, meiosis, which occurs in all multicellular animals and a number of lower plants, takes place in the gonads and results in the formation of gametes. Zygote, or initial, meiosis, which occurs in many fungi and algae, takes place in the zygote immediately after fertilization and results in the formation of a haploid mycelium, or thallome, and then of spores and gametes. Spore, or intermediate, meiosis, which occurs in higher plants, takes place just before flowering and results in the formation of the haploid gametophytes in which the gametes are subsequently formed. All three types of meiosis are found in protozoans.
The amount of DNA in a cell doubles before meiosis and divides evenly between the four daughter cells in the course of the two meiotic divisions. The pairs of homologous chromosomes separate as a result of the first, or reduction, division, the members of each pair going to one of two daughter cells (reduction in the number of chromosomes). Each chromosome retains its two longitudinal halves, or chromatids. As a result of the second, or equational, division, the chromatids separate into different cells; each of the four sister cells receives one chromatid. Thus, the first meiotic division is fundamentally different from mitosis, whereas the second is, in effect, mitosis in cells containing the haploid number of chromosomes. During meiosis but before reduction, an exchange of sections of homologous chromosomes, or crossing over, results in the redistribution of allelic genes.
Meiosis takes much longer than mitosis—for example, it lasts 24 hours in wheat, nine to 12 days in lilies, 11 to 14 days in mice, and 24 days in man. In a number of animals, including man, the meiositic process ceases temporarily—which may mean several years—during the formation of the female gametes and remains uncompleted until fertilization.
The first phase of meiosis, prophase I, which is the most complex and the longest of the phases (22.5 days in man, eight to ten in lilies), is subdivided into five stages. The leptotene is the stage at which the chromosomes appear as slender threads minimally spiralized and at their longest. Thickenings of the chromosomes, called chromomeres, are also observed. The zygotene is the stage of the initial side-by-side pairing of homologous chromosomes (synapsis, or conjugation). Homologous chromomeres draw together and arrange themselves exactly opposite each other. The pachytene is the stage at which the chromosomal threads appear thickest. Homologous chromosomes are joined firmly in pairs, or bivalents, the number of which equals the haploid number of chromosomes. Electron microscopy reveals a complex ultrastructure at the point of contact in the bivalent of the two homologous chromosomes, called the synaptonemal complex, which begins to form as early as the zygotene. Each chromosome of the bivalent contains two chromatids. Thus, a bivalent (or tetrad, in earlier terminology) consists of four homologous chromatids. Crossing over, which occurs during this stage, takes place at the molecular level; the cytological results are manifested in the following stage.
The diplotene is the stage at which the chromosomal threads separate from one another. Homologous chromosomes begin to repel each other but remain connected, usually at two or three points of the bivalent, where chiasmata, the cytological manifestation of crossing over, can now be seen. Diakinesis is the stage at which the homologous chromosomes, previously joined as bivalents by chiasmata (which move to the ends of the chromosomes—terminalization), move away from one another. The chromosomes are maximally shortened and thickened (from spiralization) and form characteristic figures, such as crosses and rings.
The next phase of meiosis is metaphase I, throughout which the chiasmata are retained; the bivalents align themselves at the middle of the spindle of the dividing cell and orient themselves toward opposite poles by the centromeres of the homologous chromosomes.
In anaphase I, the homologous chromosomes move to opposite poles with the help of the threads of the spindle. Each chromosome of the pair may proceed to either of the two poles, regardless of the behavior of the chromosomes of other pairs. The number of possible combinations during separation is therefore 2n;, where n is the number of chromosomal pairs. Unlike mitotic anaphase, the centromeres do not split, continuing to hold both of the chromatids of the chromosome moving toward the pole.
In telophase I, the chromosomes at each pole begin to despiralize and the daughter nuclei and cells begin to form. A short interphase (interkinesis) follows, without reduplication of DNA, and the second meiotic division begins.
Prophase II, metaphase II, anaphase II, and telophase II take place quickly. The centromeres split at the end of metaphase II; the chromatids of each chromosome move apart to the poles in anaphase II.
The classical scheme of meiosis described has exceptions. For example, in plants of the genus Luzula and insects of the family Coccidae, the chromatids move apart during the first meiotic division and the homologous chromosomes during the second; however, even in these cases, the result is a reduction in the number of chromosomes. The differences between spermatogenesis and oogenesis in animals and between the formation of microspores and megaspores in plants are not reflected in chromosomal behavior during meiosis, even though the size and fate of the sister cells are different.
Certain anomalies are possible in meiosis. All chromosomes in interspecific hybrids and unpaired chromosomes in aneuploids are unable to conjugate and remain univalent. Combinations of more than two chromosomes, or multivalents, are formed in autopolyploids. The number of chromosomes in anaphase I cannot undergo reduction properly in any of these cases. The gametes formed, possessing unbalanced chromosome sets, are either themselves not viable or productive of nonviable or deformed progeny. The absence of chiasmata usually has the same effects. However, in the males of some species of flies, including drosophila, chiasmata are always absent, although the gametes are formed normally.
The reasons for the change by cells from mitotic division to meiotic in the course of the life cycle of every organism are being investigated, as are the molecular mechanisms of conjugation of homologous chromosomes and the phenomenon of crossing over
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