![Bivalent](https://upload.wikimedia.org/wikipedia/commons/b/ba/Bivalent.png)
A bivalent is one pair of chromosomes (sister chromatids) in a tetrad. A tetrad is the association of a pair of
homologous chromosome
A couple of homologous chromosomes, or homologs, are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during fertilization. Homologs have the same genes in the same loci where they provide points alon ...
s (4 sister chromatids) physically held together by at least one
DNA crossover. This physical attachment allows for alignment and segregation of the homologous chromosomes in the first
meiotic
Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately res ...
division. In most organisms, each replicated chromosome (composed of two identical sisters
chromatid
A chromatid (Greek ''khrōmat-'' 'color' + ''-id'') is one half of a duplicated chromosome. Before replication, one chromosome is composed of one DNA molecule. In replication, the DNA molecule is copied, and the two molecules are known as chro ...
) elicits formation of DNA double-strand breaks during the leptotene phase. These breaks are repaired by
homologous recombination
Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in cellular organisms but may ...
, that uses the homologous chromosome as a template for repair. The search for the homologous target, helped by numerous proteins collectively referred as the
synaptonemal complex, cause the two homologs to pair, between the leptotene and the pachytene phases of meiosis I
Contents
*
1 Formation
*
2 Structure
*
3 Function
*
4 References
Formation
The formation of a bivalent occurs during the first division of
meiosis
Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately resu ...
(in the
Zygotene
Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately res ...
stage of meiotic prophase 1). In most organisms, each replicated chromosome (composed of two identical sister
chromatid
A chromatid (Greek ''khrōmat-'' 'color' + ''-id'') is one half of a duplicated chromosome. Before replication, one chromosome is composed of one DNA molecule. In replication, the DNA molecule is copied, and the two molecules are known as chro ...
s
) elicits formation of DNA double-strand breaks during the leptotene phase. These breaks are repaired by
homologous recombination
Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in cellular organisms but may ...
, that uses the homologous chromosome as a template for repair. The search for the homologous target, helped by numerous proteins collectively referred as the
synaptonemal complex, cause the two homologs to pair, between the leptotene and the pachytene phases of meiosis I.
Resolution of the DNA recombination intermediate into a
crossover
Crossover may refer to:
Entertainment
Albums and songs
* ''Cross Over'' (Dan Peek album)
* ''Crossover'' (Dirty Rotten Imbeciles album), 1987
* ''Crossover'' (Intrigue album)
* ''Crossover'' (Hitomi Shimatani album)
* ''Crossover'' (Yoshino ...
exchanges DNA segments between the two homologous chromosomes at a site called a
chiasma (plural: chiasmata). This physical strand exchange and the cohesion between the sister chromatids along each chromosome ensure robust pairing of the homologs in diplotene phase. The structure, visible by microscopy, is called a bivalent.
Resolution of the DNA recombination intermediate into a
crossover
Crossover may refer to:
Entertainment
Albums and songs
* ''Cross Over'' (Dan Peek album)
* ''Crossover'' (Dirty Rotten Imbeciles album), 1987
* ''Crossover'' (Intrigue album)
* ''Crossover'' (Hitomi Shimatani album)
* ''Crossover'' (Yoshino ...
exchanges DNA segments between the two homologous chromosomes at a site called a
chiasma (plural: chiasmata) . This physical strand exchange and the cohesion between the sister chromatids along each chromosome ensure robust pairing of the homologs in diplotene phase. The structure, visible by microscopy, is called a bivalent. An intricate molecular machinery is at the core of gene expression regulation in every cell. During the initial stages of organismal development, the coordinated activation of diverse transcriptional programs is crucial and must be carefully executed to shape every organ and tissue. Bivalent which promoters and poised enhancers are regulatory regions decorated with histone marks that are associated with both positive and negative transcriptional outcomes. Finally, we highlight the potential link between bivalency and cancer which could drive biomedical research in disease etiology and treatment.In eukaryotes gene machinery
The information of a one gene should be the different in executive way in the cell types to achieve main program in this diversity. Chromatin is carrier of the instructions and also the DNA surrounded by the histones shows impact of the nucleosome which we can see this is the basic unit. The packed gives information for regulation nucleosome of physical barrier they show impact on the chromatin remodelers parts N- terminal parts of histone particle, histone tails, covalent post-translational modifies and also creates an epigenetics of
CGand
RXGplays an initial role these mutations caused in groups from transformation in Drosophila shows a clearcut information
Structure
A bivalent is the association of two replicated homologous chromosomes having exchanged DNA strand in at least one site called chiasmata. Each bivalent contains a minimum of one chiasma and rarely more than three. This limited number (much lower than the number of initiated DNA breaks) is due to
crossover interference, a poorly understood phenomenon that limits the number of resolution of repair events into crossover in the vicinity of another pre-existing crossover outcome, thereby limiting the total number of crossovers per homologs pair.
A bivalent is the association of two replicated homologous chromosomes having exchanged DNA strand in at least one site called chiasmata. Each bivalent contains a minimum of one chiasma and rarely more than three. This limited number (much lower than the number of initiated DNA breaks) is due to
crossover interference, a poorly understood phenomenon that limits the number of resolutions of repair events into crossover in the vicinity of another pre-existing crossover outcome, thereby limiting the total number of crossovers per homologs pair. Bivalent gene is a gene marked with both H3K4me3 and H3K27me3 epigenetic modification in the same area of this kind and is proposed to play a pivotal role related to pluripotency in embryonic stem (ES) cells. Bivalent promoters marked with both H3K27me3 and H3K4me3 histone modifications are characteristic of poised promoters in embryonic stem (ES) cells. The model of poised promoters postulates that bivalent chromatin in ES cells is resolved to Mono valency upon differentiation. With the availability of single-cell RNA sequencing (scRNA-seq) data, subsequent switches in transcriptional state at bivalent promoters can be studied more closely.
Function
At the meiotic metaphase I, the
cytoskeleton
The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is compos ...
puts the bivalents under tension by pulling each homolog in opposite direction (contrary to mitotic division where the forces are exerted on each chromatid). The anchorage of the cytoskeleton to the chromosomes takes place at the
centromere
The centromere links a pair of sister chromatids together during cell division. This constricted region of chromosome connects the sister chromatids, creating a short arm (p) and a long arm (q) on the chromatids. During mitosis, spindle fibers ...
thanks to a protein complex called
kinetochore
A kinetochore (, ) is a disc-shaped protein structure associated with duplicated chromatids in eukaryotic cells where the spindle fibers attach during cell division to pull sister chromatids apart. The kinetochore assembles on the centromere and ...
. This tension results in the alignment of the bivalent at the center of the cell, the chiasmata and the distal cohesion of the sister chromatids being the anchor point sustaining the force exerted on the whole structure. Impressively, human female primary
oocytes
An oocyte (, ), oöcyte, or ovocyte is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female g ...
remains in this tension state for decades (from the establishment of the oocyte in metaphase I during embryonic development, to the
ovulation
Ovulation is the release of eggs from the ovaries. In women, this event occurs when the ovarian follicles rupture and release the secondary oocyte ovarian cells. After ovulation, during the luteal phase, the egg will be available to be fertilized ...
event in adulthood that resume the meiotic division), highlighting the robustness of the chiasma and the cohesion that hold the bivalents together. The cell transcription regulates of developmental genes We develop an approach for capturing genes undergoing transcriptional switching by detecting 'bimodal' gene expression patterns from scRNA-seq data. We integrate the identification of bimodal genes in ES cell differentiation with analysis of chromatin state and for kind of then identify clear cell-state dependent patterns of bimodal, bivalent genes. We show that binarization of bimodal genes can be used to identify differentially expressed genes from fractional ON/OFF proportions. In time series data from differentiating cells, we build a pseudo time approximation and use a hidden Markov model to infer gene activity switching pseudo times, which we use to infer a regulatory network. We identify pathways of switching during differentiation, novel details of those pathway, and transcription factor coordination with downstream targets.
Conclusions: Genes with expression levels too low to be informative in conventional scRNA analysis can be used to infer transcriptional switching networks that connect transcriptional activity to chromatin state. in with analysis of chromatin state and for kind of then identify clear cell-state dependent patterns of bimodal, bivalent genes. We show that binarization of bimodal genes can be used to identify differentially expressed genes from fractional ON/OFF proportions. In time series data from differentiating cells, we build a pseudo time approximation and use a hidden Markov model to infer gene activity switching pseudo times, which we use to infer a regulatory network. We identify pathways of switching during differentiation, novel details of those pathway, and transcription factor coordination with downstream targets. This offers a novel and productive means of inferring regulatory networks from scRNA-seq data.
Keywords: Bimodality; Bivalency; Chromatin state; Embryonic stem cells; Genome regulatory network; Hidden Markov model; Pseudo time; scRNA-seq.
References
{{ReflistBlanco E., Gonzalez Ramirez M., Alcaine-colet , A., Aranda the bivalent genome ; characterization structure trends in genetics
. Thomson JA, itskovitz-Eldor J,Shapiro SS, et al.
. Santos-Rosa H, Schneider R, Beernstein BE, et al. Methylation of histone H3 K4
. Ringrose L, ehret H, paro R. District contribution of histones H3 lysine 9 and 27
Chromosomes
Cell cycle