Can You Identify the Sequence of Events in Meiosis I?

How is the same process responsible for genetic recombination and diversity besides 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, because 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 composition. We have meiosis to thank for this variety.

Meiosis, from the Greek word meioun, pregnant "to make small-scale," refers to the specialized process by which germ cells dissever to produce gametes. Considering the chromosome number of a species remains the same from i generation to the side by side, the chromosome number of germ cells must be reduced by half during meiosis. To accomplish this feat, meiosis, unlike mitosis, involves a unmarried round of Dna replication followed by two rounds of jail cell division (Figure one). Meiosis likewise differs from mitosis in that it involves a process known equally recombination, during which chromosomes exchange segments with one another. As a result, the gametes produced during meiosis are genetically unique.

Researchers' initial understanding of meiosis was based upon careful observations of chromosome behavior using light microscopes. Then, in the 1950s, electron microscopy provided scientists with a glimpse of the intricate structures formed when chromosomes recombine. More recently, researchers accept 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 simple eukaryotes such as yeast. When weather are favorable, yeast reproduce asexually past mitosis. When nutrients get limited, nevertheless, 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 process that involves meaning changes in gene transcription (Lopez-Maury et al., 2008). Past analyzing yeast mutants that are unable to complete the diverse events of meiosis, investigators have 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 have provided valuable insight into meiosis in multicellular organisms likewise.

In most multicellular organisms, meiosis is restricted to germ cells that are fix bated in early on development. The germ cells reside in specialized environments provided by the gonads, or sex activity organs. Within the gonads, the germ cells proliferate past mitosis until they receive the correct signals to enter meiosis.

In mammals, the timing of meiosis differs greatly betwixt males and females (Figure two). Male germ cells, or spermatogonia, do non enter meiosis until after puberty. Even and so, only express numbers of spermatogonia enter meiosis at any one fourth dimension, such that adult males retain a puddle of actively dividing spermatogonia that acts as a stem prison cell population. On the other mitt, meiosis occurs with quite different kinetics in mammalian females. Female germ cells, or oogonia, finish dividing and enter meiosis within the fetal ovary. Those germ cells that enter meiosis get oocytes, the source of time to come eggs. Consequently, females are built-in with a finite number of oocytes arrested in the commencement meiotic prophase. Within the ovary, these oocytes grow within follicle structures containing big numbers of support cells. The oocytes will reenter meiosis simply when they are ovulated in response to hormones. Human being females, for example, are born with hundreds of thousands of oocytes that remain arrested in the first meiotic prophase for decades. Over time, the quality of the oocytes may deteriorate; indeed, researchers know that many oocytes dice and disappear from ovaries in a process known as atresia.

Meiosis Consists of a Reduction Division and an Equational Sectionalization

Ii divisions, meiosis I and meiosis II, are required to produce gametes (Effigy 3). Meiosis I is a unique cell division that occurs only in germ cells; meiosis II is similar to a mitotic partition. Before germ cells enter meiosis, they are generally diploid, meaning that they have two homologous copies of each chromosome. So, just before a germ cell enters meiosis, it duplicates its Dna so that the cell contains four DNA copies distributed between ii pairs of homologous chromosomes.

Meiosis I

Compared to mitosis, which can take place in a matter of minutes, meiosis is a deadening process, largely because of the time that the cell spends in prophase I. During prophase I, the pairs of homologous chromosomes come together to grade a tetrad or bivalent, which contains 4 chromatids. Recombination can occur between whatever two chromatids inside this tetrad construction. (The recombination procedure is discussed in greater detail later in this commodity.) Crossovers between homologous chromatids tin 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 form 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 abrupt dissimilarity to mitosis, pairs of homologous chromosomes line up opposite each other on the metaphase plate, with the kinetochores on sister chromatids facing the same pole. Pairs of sexual practice chromosomes also align on the metaphase plate. In human being males, the Y chromosome pairs and crosses over with the X chromosome. These crossovers are possible because the Ten and Y chromosomes have small regions of similarity near their tips. Crossover between these homologous regions ensures that the sex chromosomes will segregate properly when the cell divides.

Next, during anaphase I, the pairs of homologous chromosomes separate to different daughter cells. Before the pairs can divide, however, the crossovers between chromosomes must be resolved and meiosis-specific cohesins must be released from the arms of the sister chromatids. Failure to dissever the pairs of chromosomes to different girl cells is referred to as nondisjunction, and information technology is a major source of aneuploidy. Overall, aneuploidy appears to be a relatively frequent effect 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 & Hunt, 2001).

Meiosis Ii

Following 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 by half. Thus, the products of meiosis 2 are four haploid cells that comprise a single copy of each chromosome.

In mammals, the number of viable gametes obtained from meiosis differs between males and females. In males, four haploid spermatids of like size are produced from each spermatogonium. In females, nevertheless, the cytoplasmic divisions that occur during meiosis are very asymmetric. Fully grown oocytes within the ovary are already much larger than sperm, and the futurity egg retains most of this volume as it passes through meiosis. Every bit a consequence, simply one functional oocyte is obtained from each female person meiosis (Figure ii). 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 most important segment of meiosis, considering recombination occurs during this interval. For many years, cytologists have divided prophase I into multiple segments, based upon the advent of the meiotic chromosomes. Thus, these scientists take 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 "two threads") phases. In contempo years, cytology and genetics have come together so that researchers now empathize 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 difficult trouble to approach experimentally, only new techniques for visualizing private chromosomes with fluorescent probes are providing insights into the procedure. Recent experiments suggest that chromosomes from some species take specific sequences that act every bit pairing centers for alignment. In some cases, alignment appears to begin as early on as interphase, when homologous chromosomes occupy the aforementioned territory within the interphase nucleus (Figure 5). However, in other species, including yeast and humans, chromosomes do not pair with each other until double-stranded breaks (DSBs) appear in the DNA (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 have shown that Spo11 action is essential for meiosis in yeast, because spo11 mutants fail to sporulate.

Post-obit the DSBs, one Deoxyribonucleic acid strand is trimmed back, leaving a 3′-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 annals, or synapsis. The stability of the SC increases every bit the invading strand first extends into the homologue and then is recaptured past the broken chromatid, forming double Holliday junctions. Investigators have been able to observe the process of SC formation with electron microscopy in meiocytes from the Allium plant (Effigy 6). Bridges approximately 400 nanometers long begin to form betwixt the paired homologues following the DSB. Only a fraction of these bridges will mature into SC; moreover, not all Holliday junctions will mature into crossover sites. Recombination will thus occur at only a few sites forth each chromosome, and the products of the crossover will become visible equally 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 break (DSB) dependent homologue interaction can exist seen equally approximately 400 nm bridges between chromosome axes. These bridges, which probably comprise a DSB that is already engaged in a nascent interaction with its partner DNA, occur in big numbers. Their formation depends on the RecA (recombination poly peptide) homologues that are expressed in this species. In the side by side stage of homologue interaction, these nascent interactions are converted to stable strand-invasion events. This nucleates the formation of the synaptonemal circuitous (SC).

© 2005 Nature Publishing Group Gerton, J. L. & Hawley, R. S. Homologous chromosome interactions in meiosis: multifariousness amidst conservation. Nature Reviews Genetics vi, 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 amid conservation. Nature Reviews Genetics 6, 477–487 (2005) doi:10.1038/nrg1614 (link to article)

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

Lopez-Maury, L., Marguerat, South., & Bahler, J. Tuning cistron expression to irresolute environments: From rapid responses to evolutionary accommodation. Nature Reviews Genetics 9, 583–593 (2008) doi:10.1038/nrg2398 (link to article)

Marston, A. L., & Amon, A. Meiosis: Cell-bike controls shuffle and bargain. Nature Reviews Molecular Cell Biological science 5, 993–1008 (2004) doi:10.1038/nrm1526 (link to article)

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

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

Zickler, D., & Kleckner, Due north. Meiotic chromosomes: Integrating construction and function. Annual Review of Genetics 33, 603–754 (1999)

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

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