Process by Which Nonsex Cells Divide in Half Over and Over Again Is

Walker, Michelle Y. Department of Genetics, Academy of California, Davis, California.

Hawley, R. Scott Stowers Institute for Medical Research, Kansas Metropolis, Missouri.

The set of two successive cell divisions that serve to split up homologous chromosome pairs and thus reduce the total number of chromosomes by half. The process of meiosis (Fig. 1) includes 2 sequential nuclear divisions (meiosis I and Ii) that must occur prior to the formation of gametes [sperm and eggs (ova)]. The major purpose of meiosis is the precise reduction in the number of chromosomes by one-one-half, and then a diploid cell (diploid = having two consummate chromosome pairs in a nucleus) tin create haploid gametes (haploid = having one-half of the diploid or full complement of chromosomes). To accomplish this reduction, a unmarried parent cell undergoes two meiotic divisions to produce four daughter cells, with each having half of the original chromosome complement. The nonmeiotic (or somatic) cells of humans, for case, have 46 individual chromosomes, or 23 pairs of homologous chromosomes. However, following meiosis, human eggs or sperm have only 23 chromosomes (one fellow member of each pair). Reducing the number of chromosomes in the gametes to 23 allows the fusion of an egg with a sperm (in a process chosen fertilization) to result in an embryo with the requisite 46 chromosomes. Therefore, meiosis is a disquisitional component of sexual reproduction. See also: Creature reproduction; Cell (biology); Cell biology; Cell partition; Chromosome; Fertilization (animate being); Fertilization (institute); Gametogenesis; Genetics; Ovum; Plant reproduction; Sperm jail cell

Fig. ane Overview of meiosis. A single parent cell undergoes 2 meiotic divisions (meiosis I and 2) to produce four daughter cells (each with half of the original chromosome complement). The letter "n" stands for the number of chromosomes in a haploid cell. (Copyright © McGraw-Hill Education)

Chromosome behavior

For example, consider an organism that contains only two pairs of chromosomes. The chromosomes in each of these pairs are referred to individually as homologs; one is derived from the father of the organism and the other is derived from the mother. Both homologs bear the same array of genes. As the jail cell begins meiosis, each chromosome has already duplicated its deoxyribonucleic acid (DNA) and carries two identical copies of the Dna molecule. These are visible equally two lateral parts, called sister chromatids, which are connected by a centromere. See also: Deoxyribonucleic acrid (Deoxyribonucleic acid); Office of telomeres and centromeres in meiotic chromosome pairing; Sister chromatid cohesion

The bones events of meiosis are actually quite simple. Homologous pairs of chromosomes are offset identified and matched. This process, which occurs only in the first of the two meiotic divisions, is called pairing. The matched pairs are and so physically interlocked past recombination, which is too known as exchange or crossing-over. Later on recombination, the homologous chromosomes split from each other; then, at the get-go meiotic division, they are partitioned into different nuclei. The second meiotic partitioning begins with half of the original number of chromosomes. During this second meiotic division, the sister chromatids of each chromosome divide and migrate to dissimilar daughter cells.

The patterns past which genes are inherited are determined by the move of the chromosomes during the two meiotic divisions. Information technology is a key tenet of Mendelian inheritance that each individual carries ii copies of each gene—i derived from its father and one from its female parent. Moreover, each of that individual's gametes will carry only one copy of that cistron, which is chosen at random. The process by which the ii copies of a given gene are distributed into dissever gametes is referred to as segregation. Thus, if an individual is heterozygous at the A cistron for two different alleles (alternative forms of a gene), A and a, his or her gametes volition 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. Run across also: Allele

Mendel's constabulary of independent assortment states that the segregation of two different gene pairs occurs at random with respect to each other. Thus, for an private of the genotype AaBb, the gametes AB, Ab, aB, and ab will be formed with equal frequency. This result can be easily understood if the A and B genes lie on different chromosomes (Fig. ii). Because the chromosome pair begetting the A cistron orients independently of the homolog pair bearing the B factor, AB ↔ ab segregations are equally probable equally Ab ↔ aB segregations. Cases where independent assortment does not occur (an effect chosen linkage) tin can be understood as resulting from situations where the two gene pairs lie at unlike positions on the same pair of homologous chromosomes. See as well: Gene; Linkage (genetics); Mendelism

Fig. 2 Behavior of chromosomes at the first meiotic sectionalization explains independent assortment.

Meiotic divisions

The two meiotic divisions (meiosis I and II) may be divided into a number of distinct stages. Meiotic prophase refers to the menses later on the final bike of DNA replication; during prophase, homologous chromosomes pair and recombine. The end of prophase is signaled by the breakup of the nuclear envelope and the association of the paired chromosomes with the meiotic spindle. The spindle is made upwardly of microtubules that, with associated motor proteins, mediate chromosome movement. In some cases (for example, human sperm germination), the spindle is already formed at the point of nuclear envelope breakdown, and the chromosomes then attach to information technology. In other systems (for instance, human being female meiosis), the chromosomes themselves organize the spindle.

Metaphase I is the period before the commencement division when pairs of interlocked homologous chromosomes, called bivalents, line upwardly on the middle of the meiotic spindle. The chromosomes are primarily (only not exclusively) fastened to the spindle past their centromeres; the centromere of ane homolog is attached to spindle fibers emanating from i pole, and the centromere of its partner is attached to spindle fibers from the other pole (Fig. 3). The bivalents are physically held together by structures referred to as chiasmata, which are the event 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. It is important to call back that 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 betoken toward i pole of the spindle, whereas the maternally derived homolog in the next bivalent is oriented toward the aforementioned pole.

Fig. 3 Some of the stages of meiosis: (a) premeiotic interphase; (b) leptotene (note the advent of individualized chromosomes); (c) zygotene (signified past the alignment of homologs); (d) pachytene (the chromosomes making upwards each bivalent are intimately aligned); (e) diplotene/diakinesis; (f) metaphase I; (m) anaphase I; (h) metaphase 2; (i) anaphase II; (j) telophase 2.

Anaphase I refers to the indicate at which homologous chromosome pairs separate and motility to opposite poles. This is accomplished by the release of chiasmata. Although the sister chromatids remain attached around their centromeres, they release each other along the arms of the chromosome, allowing chiasmata to be resolved. Depending on the organism, there may or may non be a true telophase, or a time in which nuclei reform. In nigh organisms, the starting time cell sectionalisation occurs after the completion of anaphase I.

Post-obit the completion of the first meiotic division, the chromosomes align themselves on a new pair of spindles, with their sister chromatids oriented toward contrary poles. The stage at which each chromosome is then aligned is referred to as metaphase Two. In some (but not all) organisms, metaphase II is preceded past a brief prophase Ii. Dna replication does not occur during prophase 2; each chromosome yet consists of the two sis chromatids. In addition, there are no 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 motility of the ii sister chromatids to contrary poles. At telophase Two, the sisters have reached opposite poles and the nuclei begin to reform. The second jail cell division unremarkably occurs at this time. Thus, at the end of the second meiotic division, there will be iv daughter cells, with each having a single copy of each chromosome.

Details of meiotic prophase

Pairing and recombination occur during the beginning meiotic prophase, which is divided into a number of stages: leptotene, zygotene, pachytene, diplotene, and diakinesis (Fig. iii). Homolog recognition, alignment, and synapsis (pairing) occur during leptotene and zygotene. In the leptotene phase (an initial stage of chromosome individualization), initial homolog alignments are made. Past zygotene, homologous chromosomes have become associated at various points along their length. These associations facilitate a more than intimate pairing that results in the homologous chromosomes lying abreast of a tracklike structure called the synaptonemal complex. The beginning of pachytene is signaled past the completion of a continuous synaptonemal circuitous running the total length of each bivalent. During diplotene, the bonny forces that mediated homologous pairing disappear, and the homologs brainstorm to repel each other. However, homologs near always recombine, and those recombination events can be seen as chiasmata that tether the homologs together. In some organisms, those rare chromosome pairs that have failed to undergo recombination volition wing autonomously prematurely from each other at this phase. The terminal stage in meiotic prophase is diakinesis, during which the homologs shorten and condense in training for nuclear division.

Recombination

Meiotic recombination (Fig. iv) 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. Mechanistically, recombination occurs between homologous chromosomes and involves the precise breakage and rejoining of 2 nonsister chromatids. The result is the formation of 2 recombinant chromatids, with each conveying information from both of the original homologs. Come across also: Recombination (genetics)

Fig. 4 Recombination (crossing-over) involves the physical interchange of genetic material between 2 nonsister chromatids. This process creates new combinations and increases the genetic variability of the gametes. (Copyright © McGraw-Hill Teaching)

The actual mechanism of recombination involves a complex series of cutting and rejoining of homologous Dna. The number and position of recombination events are very precisely controlled. Exchange occurs simply in the gene-rich euchromatin that makes up most of the chromosome arms; it never occurs in the heterochromatin that surrounds the centromeres. Moreover, as a upshot of a process known as interference, the occurrence of one exchange in a given chromosomal region greatly decreases the probability of a second substitution in that region.

Sex differences in meiosis

In human male person meiosis, all four daughter cells of meiosis will go through a complicated cellular differentiation process called spermatogenesis to get mature functional sperm. In contrast, oogenesis in the human female results in only one of the four products of meiosis becoming an egg. The other 3 products donate their cytoplasm to the called oocyte, and so die. The oocyte and so completes the cellular differentiation process to go a mature egg. Run into as well: Oogenesis; Spermatogenesis

In man females, meiosis begins during fetal development. All of the oocytes (eggs) that a human being female will possess in her lifetime are produced during fetal development, but these oocytes are arrested at the end of pachytene. Thus, all of the meiotic recombination that a human female volition always do is completed before she is born. These arrested oocytes remain quiescent until the female enters puberty. At that point, a few oocytes are allowed to brainstorm the maturation process during each menstrual cycle; commonly simply a single oocyte is ovulated per cycle. The ovulated egg completes anaphase I and then gain through meiosis until it arrests again at metaphase II. The completion of the second meiotic partition is triggered just by fertilization.

Male meiosis begins at puberty and continues uninterrrupted throughout the life of the male person. The stem cells that will produce male meiotic cells continue to dissever throughout the male's life, constantly producing new populations of spermatocytes. Moreover, in one case meiosis is initiated in human spermatocytes, information technology unremarkably proceeds without intermission to produce four daughter cells; then, all of these differentiate to become mature spermatids.

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 jail cell to properly movement the segregating chromosomes on the meiotic spindle. The result of nondisjunction is the production of gametes that are aneuploid, that is, carrying the wrong number of chromosomes. When such a gamete is involved in a fertilization consequence, the resulting zygote is also aneuploid. Those cases where the embryo carries an extra re-create of a given chromosome are said to be trisomic, whereas those that carry but ane re-create are said to exist monosomic for that chromosome. Most aneuploid zygotes are not feasible and result in early on spontaneous abortion. At that place are no viable monosomies for the man autosomes (non-sex chromosomes). Nonetheless, a few types of trisomic zygotes are capable of survival; these are trisomies for the sex chromosomes (XXX, XXY, and XYY), trisomy 21 (Down syndrome), trisomy 18, and trisomy 13. Come across also: Chromosome aberration; Crossing-over (genetics); Down syndrome

Meiosis versus mitosis

The fundamental departure between meiosis and mitosis is that sister chromatids do not separate at the beginning meiotic division; rather, homologous chromosomes separate from each other with their sister chromatids still attached to each other. Although some chromosome pairing does occur in mitotic cells in some organisms, intimate synapsis along the entire length of chromosomes is commonly restricted to meiotic cells. Recombination is frequent in well-nigh meiotic cells; all the same, it occurs but rarely in mitotic cells, ordinarily as part of DNA repair events. Almost critically, Dna synthesis occurs only once inside the 2 meiotic divisions. In contrast, there is a complete replication before every mitotic division. This allows mitosis to produce ii genetically identical girl cells, whereas meiosis produces four girl cells, with each having just one-one-half of the number of chromosomes nowadays prior to meiosis. See likewise: DNA repair; Mitosis

R. Scott Hawley

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