In What Phases Does Crossing Over Occur Genetic Makeup

How is the same process responsible for genetic recombination and variety as well the cause of aneuploidy? Understanding the steps of meiosis is essential to learning how errors occur.

Organisms that reproduce sexually are thought to have an advantage over organisms that reproduce asexually, considering novel combinations of genes are possible in each generation. Furthermore, with few exceptions, each individual in a population of sexually reproducing organisms has a distinct genetic limerick. We have meiosis to thank for this multifariousness.

Meiosis, from the Greek word meioun, pregnant "to make small," refers to the specialized procedure by which germ cells divide to produce gametes. Because the chromosome number of a species remains the aforementioned from one generation to the next, the chromosome number of germ cells must exist reduced by half during meiosis. To accomplish this feat, meiosis, unlike mitosis, involves a single round of DNA replication followed past two rounds of jail cell division (Figure i). Meiosis as well differs from mitosis in that it involves a process known every bit recombination, during which chromosomes commutation segments with one some other. Equally a issue, the gametes produced during meiosis are genetically unique.

Researchers' initial understanding of meiosis was based upon careful observations of chromosome behavior using calorie-free microscopes. Then, in the 1950s, electron microscopy provided scientists with a glimpse of the intricate structures formed when chromosomes recombine. More recently, researchers have been able to identify some of the molecular players in meiosis from biochemical analyses of meiotic chromosomes and from genetic studies of meiosis-specific mutants.

Meiosis Is a Highly Regulated Process

Meiosis represents a survival mechanism for some uncomplicated eukaryotes such as yeast. When conditions are favorable, yeast reproduce asexually past mitosis. When nutrients become limited, however, yeast enter meiosis. The commitment to meiosis enhances the probability that the next generation will survive, because genetic recombination during meiosis generates four reproductive spores per cell, each of which has a novel genotype. The entry of yeast into meiosis is a highly regulated procedure that involves significant changes in cistron transcription (Lopez-Maury et al., 2008). By analyzing yeast mutants that are unable to consummate the various events of meiosis, investigators accept been able to identify some of the molecules involved in this complex process. These controls accept been strongly conserved during evolution, so such yeast experiments take provided valuable insight into meiosis in multicellular organisms likewise.

In most multicellular organisms, meiosis is restricted to germ cells that are set aside in early development. The germ cells reside in specialized environments provided by the gonads, or sexual activity organs. Within the gonads, the germ cells proliferate by mitosis until they receive the right signals to enter meiosis.

In mammals, the timing of meiosis differs greatly between males and females (Figure two). Male person germ cells, or spermatogonia, do not enter meiosis until after puberty. Fifty-fifty then, merely limited numbers of spermatogonia enter meiosis at any 1 time, such that adult males retain a puddle of actively dividing spermatogonia that acts equally a stem cell population. On the other mitt, meiosis occurs with quite different kinetics in mammalian females. Female germ cells, or oogonia, cease dividing and enter meiosis within the fetal ovary. Those germ cells that enter meiosis become oocytes, the source of future eggs. Consequently, females are born with a finite number of oocytes arrested in the first meiotic prophase. Within the ovary, these oocytes grow inside follicle structures containing large numbers of support cells. The oocytes volition reenter meiosis merely when they are ovulated in response to hormones. Human females, for example, are born with hundreds of thousands of oocytes that remain arrested in the kickoff meiotic prophase for decades. Over fourth dimension, the quality of the oocytes may deteriorate; indeed, researchers know that many oocytes die and disappear from ovaries in a process known equally atresia.

Meiosis Consists of a Reduction Sectionalisation and an Equational Division

Two divisions, meiosis I and meiosis II, are required to produce gametes (Figure iii). Meiosis I is a unique prison cell division that occurs only in germ cells; meiosis II is similar to a mitotic division. Before germ cells enter meiosis, they are generally diploid, meaning that they have two homologous copies of each chromosome. Then, merely before a germ cell enters meiosis, it duplicates its DNA and then that the cell contains iv DNA copies distributed between ii pairs of homologous chromosomes.

Meiosis I

Compared to mitosis, which tin take identify in a matter of minutes, meiosis is a tiresome process, largely because of the fourth dimension that the cell spends in prophase I. During prophase I, the pairs of homologous chromosomes come together to course a tetrad or bivalent, which contains iv chromatids. Recombination tin occur between whatsoever ii chromatids within this tetrad structure. (The recombination procedure is discussed in greater detail after in this article.) Crossovers betwixt homologous chromatids tin can be visualized in structures known as chiasmata, which appear late in prophase I (Figure 4). Chiasmata are essential for accurate meioses. In fact, cells that fail to class chiasmata may non be able to segregate their chromosomes properly during anaphase, thereby producing aneuploid gametes with abnormal numbers of chromosomes (Hassold & Hunt, 2001).

At the end of prometaphase I, meiotic cells enter metaphase I. Here, in sharp contrast to mitosis, pairs of homologous chromosomes line up opposite each other on the metaphase plate, with the kinetochores on sis chromatids facing the same pole. Pairs of sex chromosomes also align on the metaphase plate. In homo males, the Y chromosome pairs and crosses over with the 10 chromosome. These crossovers are possible considering the X and Y chromosomes have small regions of similarity near their tips. Crossover between these homologous regions ensures that the sexual practice chromosomes will segregate properly when the prison cell divides.

Side by side, during anaphase I, the pairs of homologous chromosomes carve up to different daughter cells. Earlier the pairs tin separate, still, the crossovers between chromosomes must be resolved and meiosis-specific cohesins must be released from the arms of the sister chromatids. Failure to separate the pairs of chromosomes to different daughter cells is referred to as nondisjunction, and it is a major source of aneuploidy. Overall, aneuploidy appears to exist a relatively frequent event in humans. In fact, the frequency of aneuploidy in humans has been estimated to be as high as 10% to 30%, and this frequency increases sharply with maternal age (Hassold & Chase, 2001).

Meiosis II

Post-obit meiosis I, the daughter cells enter meiosis Ii without passing through interphase or replicating their Dna. Meiosis II resembles a mitotic division, except that the chromosome number has been reduced past one-half. Thus, the products of meiosis II are four haploid cells that comprise a single copy of each chromosome.

In mammals, the number of viable gametes obtained from meiosis differs betwixt males and females. In males, four haploid spermatids of similar size are produced from each spermatogonium. In females, however, the cytoplasmic divisions that occur during meiosis are very asymmetric. Fully grown oocytes within the ovary are already much larger than sperm, and the future egg retains about of this volume every bit it passes through meiosis. As a issue, only i functional oocyte is obtained from each female meiosis (Figure 2). The other three haploid cells are pinched off from the oocyte as polar bodies that contain very little cytoplasm.

Recombination Occurs During the Prolonged Prophase of Meiosis I

Prophase I is the longest and arguably well-nigh important segment of meiosis, considering recombination occurs during this interval. For many years, cytologists have divided prophase I into multiple segments, based upon the appearance of the meiotic chromosomes. Thus, these scientists have described a leptotene (from the Greek for "thin threads") phase, which is followed sequentially by the zygotene (from the Greek for "paired threads"), pachytene (from the Greek for "thick threads"), and diplotene (from the Greek for "ii threads") phases. In recent years, cytology and genetics accept come together then that researchers now empathise some of the molecular events responsible for the stunning rearrangements of chromatin observed during these phases.

Recall that prophase I begins with the alignment of homologous chromosome pairs. Historically, alignment has been a hard trouble to arroyo experimentally, simply new techniques for visualizing individual chromosomes with fluorescent probes are providing insights into the procedure. Contempo experiments suggest that chromosomes from some species have specific sequences that human action equally pairing centers for alignment. In some cases, alignment appears to begin as early as interphase, when homologous chromosomes occupy the same territory within the interphase nucleus (Effigy 5). All the same, in other species, including yeast and humans, chromosomes exercise not pair with each other until double-stranded breaks (DSBs) appear in the Deoxyribonucleic acid (Gerton & Hawley, 2005). The formation of DSBs is catalyzed by highly conserved proteins with topoisomerase activity that resemble the Spo11 poly peptide from yeast. Genetic studies accept shown that Spo11 activity is essential for meiosis in yeast, because spo11 mutants fail to sporulate.

Following the DSBs, 1 Dna strand is trimmed back, leaving a iii′-overhang that "invades" a homologous sequence on another chromatid. As the invading strand is extended, a remarkable structure called synaptonemal complex (SC) develops around the paired homologues and holds them in close register, or synapsis. The stability of the SC increases as the invading strand offset extends into the homologue and then is recaptured by the cleaved chromatid, forming double Holliday junctions. Investigators have been able to discover the process of SC formation with electron microscopy in meiocytes from the Allium institute (Figure 6). Bridges approximately 400 nanometers long begin to form between the paired homologues following the DSB. Only a fraction of these bridges volition mature into SC; moreover, non all Holliday junctions will mature into crossover sites. Recombination will thus occur at only a few sites along each chromosome, and the products of the crossover will become visible as chiasmata in diplotene after the SC has disappeared (Zickler & Kleckner, 1999).

Figure 6: Visualization of chromosomal bridges in Allium fistulosum and Allium cepa (plant) meiocytes.

The sites of double-stranded pause (DSB) dependent homologue interaction tin can be seen every bit approximately 400 nm bridges betwixt chromosome axes. These bridges, which probably contain a DSB that is already engaged in a nascent interaction with its partner Dna, occur in large numbers. Their formation depends on the RecA (recombination poly peptide) homologues that are expressed in this species. In the side by side phase of homologue interaction, these nascent interactions are converted to stable strand-invasion events. This nucleates the formation of the synaptonemal complex (SC).

© 2005 Nature Publishing Group Gerton, J. L. & Hawley, R. S. Homologous chromosome interactions in meiosis: multifariousness amongst conservation. Nature Reviews Genetics six, 481 (2005). All rights reserved.

References and Recommended Reading

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Gerton, J. L., & Hawley, R. S. Homologous chromosome interactions in meiosis: Diversity amidst conservation. Nature Reviews Genetics half-dozen, 477–487 (2005) doi:10.1038/nrg1614 (link to article)

Hassold, T., & Hunt, P. To err (meiotically) is human: The genesis of human aneuploidy. Nature Reviews Genetics 2, 280–291 (2001) doi:10.1038/35066065 (link to article)

Lopez-Maury, L., Marguerat, S., & Bahler, J. Tuning gene expression to irresolute environments: From rapid responses to evolutionary accommodation. Nature Reviews Genetics 9, 583–593 (2008) doi:ten.1038/nrg2398 (link to commodity)

Marston, A. L., & Amon, A. Meiosis: Prison cell-bicycle controls shuffle and deal. Nature Reviews Molecular Prison cell Biological science v, 993–1008 (2004) doi:10.1038/nrm1526 (link to article)

Page, S. L., & Hawley, R. South. Chromosome choreography: The meiotic ballet. Scientific discipline 301, 785–789 (2003)

Petes, T. D. Meiotic recombination hot spots and cold spots. Nature Reviews Genetics 2, 360–369 (2001) doi:ten.1038/35072078 (link to article)

Zickler, D., & Kleckner, N. Meiotic chromosomes: Integrating structure and role. Annual Review of Genetics 33, 603–754 (1999)

Source: http://www.nature.com/scitable/topicpage/meiosis-genetic-recombination-and-sexual-reproduction-210

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