Where is meiosis occur

These results suggest that AM1 is required for meiosis initiation and may also regulate meiotic progression. These differences among species may indicate that the AM1-related genes have undergone species-specific diversification. Using Agilent 44K microarrays, the authors compared transcriptomes in 1-mm and 1. In 1-mm anthers when meiosis is about to start in the wild-type, genes were missing and genes were ectopically expressed in am anthers.

These genes are considered to contribute to the initiation of meiosis or the suppression of mitosis. These results redefine the role of AM1 in the modulation of transcript accumulation for many meiotic genes rather than simply switching them on or off Nan et al. Recently, microarray analyses on laser-captured germinal and somatic initials from maize 0. Surprisingly, more than meiotic genes are expressed in the mitotic amplification period that is long before the onset of meiosis initiation.

This finding raises a possibility that precocious expression of meiotic genes permits gradual dilution of mitotic chromatin components, a hypothesis recently proposed for the mouse germ-line Hackett et al. Another possibility is that those PMC precursors are preparing for meiosis at the transcriptional level, and may store some meiotic transcripts for translation at later developmental stages Zhang et al.

Regardless, this finding suggests that the decision to start meiosis is a series of consecutive steps rather than a single switch.

Perhaps, the expression of meiotic genes may be one of the earliest actions, and the following regulatory cascade finally governs the initiation and progression of meiosis. Thus, which transcription factors are responsible for early meiotic gene expression and whether meiotic genes are under translational control are interesting questions for further study.

In addition, identification of components in the regulatory cascade will provide better understanding of this process. A small proportion of PMCs can escape from the defects and undergo meiosis with a significant delay or continued mitotic cycles. How an RRM protein affects the initiation of meiosis is unclear at the molecular level, but this result implied a possible link between mRNA processing, transport or stability, and entry into meiosis in plants.

Studies in yeast have shown that the final trigger to start meiosis is the activation of specific cyclin—CDK complexes to initiate the meiotic S phase. Arabidopsis has at least 50 cyclins and only a few of them are specifically expressed in the inflorescence Bulankova et al. Mutant analyses revealed that some of these cyclins contribute to distinct meiosis-related processes, but none of cyclin mutants showed meiosis initiation defects, which was attributed to gene redundancy.

Thus, it will be interesting to know which, if any, cyclin—CDK complex is responsible for the transition. Besides cyclin—CDK complexes, some meiosis-specific regulators, such as replication factor MUM2 and cohesion protein REC8, are involved at the meiotic S phase although much of the basic replication apparatus is employed Strich, Therefore, what is special about the pre-meiotic S phase and which are the specific genes that differ from the mitotic S phase in plants? Understanding of these meiosis-specific components at meiotic S phase will help us to illustrate the molecular mechanisms of meiosis initiation.

A proteomics study may offer valuable information on this aspect. To date, mutants directly affecting meiosis initiation showed similar phenotypes in that some of reproductive cells fail to enter meiosis in either female, male, or both sexes.

Although some of these mutants produce unreduced daughter cells by mitosis-like division, there is no evidence that these resulting diploid cells in ovules would undergo the apomictic pathway without fertilization. Most of the progeny were triploid, suggesting that unreduced female daughter cells after mitosis-like division are able to develop further and be fertilized by haploid male gametes Ravi et al.

Apomixis is a type of asexual reproduction through seeds that avoid both meiosis and fertilization. In the apomictic pathway, differentiated MMCs or other somatic cells in ovules that gain germinal cell fate are able to bypass meiosis or undergo an abnormal meiosis to produce unreduced spores that further divide mitotically to form an embryo sac Figure 1 ; Koltunow, ; Carman, Although apomixis is genetically regulated and occurs naturally in more than species of flowering plants, its implementation at the molecular level is still unclear.

Over the past few years, there has been increasing evidence to show that epigenetic control may regulate apomixis. In Arabidopsis, argonaute 9 ago9 mutants exhibit multiple MMCs compared to a single MMC in the wild-type ovule, and additional MMCs in the mutant are able to initiate gametogenesis without undergoing meiosis, resembling apospory Figure 1 ; Olmedo-Monfil et al. Similarly, maize AGO, the homolog of Arabidopsis AGO9, is found to regulate reproductive fate despite some differences between maize ago and Arabidopsis ago9 phenotypes Singh et al.

The maize ago mutant has a single MMC; however, defective female meiosis with aberrant condensation results in functional female gametes with an unreduced chromosome set, resembling diplospory Figure 1.

Thus, loss of RdDM seems to direct somatic cells to distinct reproductive cells with an apomictic fate seen in Arabidopsis ago9 mutant or lead to apomixis in correctly specified MMCs seen in maize ago mutant.

Interestingly, both AGO9 in Arabidopsis and AGO in maize are specifically expressed in surrounding somatic nucellar cells, and not in the reproductive cells, implying that both genes control the apomictic pathway in a non-cell-autonomous manner.

These results suggest a link between siRNA-dependent chromatin remodeling and the apomictic pathway Garcia-Aguilar et al. It encodes an AGO5 protein that is required for maintaining germ cell identity and normal meiosis progression.

Interestingly, the mel1 mutant also shows defective chromosome condensation with abnormal pericentromere histone modification Nonomura et al. Further investigation is needed to understand the epigenetic regulation of plant reproduction. Over the past few years, the identification of mutants has shed light on genetic control of epigenetic mechanisms involved in apomixis. However, it is still not clear how the RdDM-dependent process affects cell fate specification, meiosis, and gametophyte development?

Why is there a need for transposes-derived siRNA in the germ line? Perhaps, identifying the targets of the RdDM pathway at different stages will be essential for further definition of their roles. It is worth noting that alterations in histone modification were observed in the swi1 mutant Boateng et al. Many exciting questions are awaiting further investigation.

Understanding the initiation of meiosis and apomixis in plants will be enlightening, and may have many potential applications for plant breeding and in agriculture including developing a strategy for acquiring apomixis in crops, and allowing manipulation of the meiotic cell cycle.

It will be crucial to identify more participants in the mitosis—meiosis decision and the apomictic pathway and to explore their molecular functions. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. We would like to thank the members of the Wang lab for insightful discussions and critical comments. Anderson, E.

Stra8 and its inducer, retinoic acid, regulate meiotic initiation in both spermatogenesis and oogenesis in mice. Barcaccia, G. Apomixis in plant reproduction: a novel perspective on an old dilemma. Plant Reprod. Bhatt, A. Plant meiosis: the means to 1N. Trends Plant Sci. Boateng, K. SWI1 is required for meiotic chromosome remodeling events. Borgne, A. CrossRef Full Text. Bowles, J. Retinoid signaling determines germ cell fate in mice.

Science , — Retinoic acid, meiosis and germ cell fate in mammals. Development , — Bulankova, P. Identification of Arabidopsis meiotic cyclins reveals functional diversification among plant cyclin genes. PLoS Genet. Carman, J. Asynchronous expression of duplicate genes in angiosperms may cause apomixis, bispory, tetraspory, and polyembryony.

Che, L. OsAM1 is required for leptotene-zygotene transition in rice. Cell Res. Chu, S. They also give rise to gametes in the human body, but plant spores in plants. Meiosis occurs in the sex cells, so the sperm and egg cells in the human body, to create even more of themselves.

Where does meiosis occur in the human body? Biology The Eukaryotic Cell Meiosis. Nam D. Apr 26, Sex cells. In females, oogenesis and meiosis begin while the individual is still in the womb.

The primary oocytes, analogous to the spermatocyte in the male, undergo meiosis I up to diplonema in the womb , and then their progress is arrested. Once the female reaches puberty, small clutches of these arrested oocytes will proceed up to metaphase II and await fertilization so that they may complete the entire meiotic process; however, one oocyte will only produce one egg instead of four like the sperm.

This can be explained by the placement of the metaphase plate in the dividing female germ cell. Instead of lying across the middle of the cell like in spermatogenesis, the metaphase plate is tucked in the margin of the dividing cell, although equal distribution of the genetic material still occurs. This results in a grossly unequal distribution of the cytoplasm and associated organelles once the cell undergoes cytokinesis. This first division produces a large cell and a small cell.

The large cell, the secondary oocyte , contains the vast majority of the cytoplasm of the parent cell, and holds half of the genetic material of that cell as well. The small cell, called the first polar body, contains almost no cytoplasm, but still sequesters the other half of the genetic material. This process repeats in meiosis II, giving rise to the egg and to an additional polar body. These differences in meiosis reflect the roles of each of the sex cells.

Sperm must be agile and highly motile in order to have the opportunity to fertilize the egg—and this is their sole purpose. For this reason, they hardly carry any cellular organelles excluding packs of mitochondria which fuel their rapid motion , mostly just DNA. For this reason, only a single, well-fortified egg is produced by each round of meiosis. Meiosis is a process that is conserved, in one form or another, across all sexually-reproducing organisms.

This means that the process appears to drive reproductive abilities in a variety of organisms and points to the common evolutionary pathway for those organisms that reproduce sexually. It is vitally important for the maintenance of genetic integrity and enhancement of diversity.

Since humans are diploid 2N organisms, failure to halve the ploidy before fertilization can have disastrous effects. For this reason, only very select types of abnormal ploidy survive and do so with noticeable defects ; most combinations containing abnormal ploidy never make it into the world. The correct reduction of the number of chromosomes insures that once fertilization takes place, the correct amount of genetic material is established in the fertilized egg and, eventually, in the person resulting from it.

Meiosis in Humans By: Inbar Maayan. Keywords: Human development , Meiosis. Meiosis in Humans Meiosis, the process by which sexually reproducing organisms generate gametes sex cells , is an essential precondition for the normal formation of the embryo. Gilbert, Scott F. Sunderland, MA: Sinauer, Hochwagen, Andreas. Klug, William S. Cummings, Charlotte Spencer, and Michael A.