Genes That Drive Meiotic Chromosome Pairing Biology Essay


Specialized meiotic cell division is a cardinal procedure in sexual reproduction. Accurate chromosome coupling, synapsis and recombination during meiotic prophase guarantee proper chromosome segregation subsequently on, which is critical in cell division in order to avoid aneuploidy ( reviewed in Gerton and Hawley 2005 ) . The cardinal characteristic of miosis is two consecutive chromosome segregations: one homolog segregation ( maternal and paternal chromosomes ) , and another sister chromatid segregation, as in mitosis. Homolog synapsis along chromosomes length involves dynamic motions, which in their bend involve direct association of chromosome terminals ( reviewed in Koszul 2009 ) . The motion and homolog partner offing occur at the early phases of meiotic prophase, and are really of import events in miosis ( reviewed in Koszul 2009 ) .The chief event in meiotic prophase is homolog partner offing. The manner chromosomes brace is still non wholly understood, and it has been noticed that different beings engaged different characteristics to drive partner offing mechanism ( reviewed in McKee 2004 ; Dinging 2009 ) .

The early phases of the procedure are likely to affect interactions between telomeres and/or kinetochores of homologs, which at these phases are non associated with recombination procedures ( reviewed in Bhalla and Dernburg 2008 ) . The partner offing becomes obvious when G2 prophase recombination procedure takes topographic point ( reviewed in Koszul 2009 ) . However, in some beings meiotic homolog partner offing returns irrespective of whether recombination takes topographic point or non ( see Vazquez et Al. 2002 ) .It is deserving detecting that in many reappraisals ‘pairing ‘ is referred to as side-by-side alliance of homologs, which is clearly distinguishable from the synapsis of chromosomes in many beings, which, in its bend, is described as specific association of chromosomes in the synaptonemal composite ( SC ) ( reviewed in McKee 2004 ; Roeder 1997 ) .

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In beings like workss, animate beings and filiform Fungis, the motion starts from chromosome axes being brought together along their length, so that they could be closely associated through the SC that holds homologs near together. However, there are certain beings in which chromosome coupling is non accompanied by the formation of SC in meiotic prophase, e.g. Schizosccharamyces pomb. However, the close association of homologous chromosomes is necessary in order to avoid their battle with unrelated chromosomes ( Koszul 2009 ) .

These dynamic chromosome motions, partner offing and synapsis occur during early phases of meiotic prophase I: chromosome polarization and orientation during leptotene, chromosome coupling and synaptonemal composite ( SC ) formation during zygotene, and thickener of mated chromosomes and SC extension along the full length of the chromosomes during pachytene ( Koszul 2009 ) .At these phases of miosis another characteristic agreement of chromosomes takes topographic point which is called ‘bouquet ‘ formation, during which chromosomes ‘ telomeres are attached to the interior atomic envelope ( NE ) , so that chromosomes would look to be clustered together organizing a construction that resembles a corsage of flowers ( Cande et al. 2004 ; Scherthan 2001 ) . This phase of bouquet formation was observed in assortment of beings studied ( Scherthan 2001 ) , apart from Caenorhabditis elegans and Drosophila melanogaster, which showed to hold a different methods of homology hunt ( McKee 2004 ) .

Many research experiments suggested that the corsage promoted homologous chromosome coupling and synapsis.Some surveies suggested at that place had to be a gesture for the chromosomes to aline and to partner off, which would play a function of an pulling force for the homologous spouses, and besides a repeling force to forestall contacts between non-homologous chromosomes ( Dinging et al. 2009 ) . Two members of conserved sphere protein households of SUN and KASH were identified to be involved in a mechanism for the intranuclear chromosome gesture.

Sad1 and Kms1 form a protein composite on the atomic envelope and promote telomere motion ( Starr 2009 ) .Therefore, it is seen that a corsage formation is used by the beings as a mechanism for the chromosome alliance. Though, it is still non clear how homologs recognise each other. Dinging et Al. ( 2009 ) suggested that heterochromatin blocks could be involved in a specific ‘barcode ‘ formation for each chromosome, so that homologous spouses could place each other.

Another factor impacting homolog partner offing are meiotic DNA double-stranded interruptions ( DSBs ) , which are generated by type II topoisomerase-like enzyme Spo11 ( Keeney 1997 ) . It has been proposed that DNA homology hunt during DSB fix could besides let partner offing to happen in such manner ( Gerton and Hawley 2005 ) . However, it was found that spo11? mutations could still show coupling, even though there was a defect in the mechanism of DSB formation and DNA recombination ( see Weiner and Kleckner 1994 ) . Same worked for the other cistrons involved in DSB formation: mutants in RAD50, MEI4 and REC102 reduced pairing frequence, but did non extinguish partner offing wholly.In this reappraisal the cistrons that are involved in chromosomes motion and coupling are described for barm, workss and animate beings. Particular attending is paid to chromosome paring mechanism, seeking to place its cardinal characteristics.

Pairing in Yeast

Weiner and Kleckner ( 1994 ) analyzed chromosome coupling in spread barm karyon, where short parts were probed by FISH. Pairing was represented as physical interaction between two homologous chromosomes in the probed part or really near to it. The mean frequence of partner offing for the wild type or meiosis-specific mutant spo11? was ~0.45. It was seen that each brace of homologs exhibited independent of each other partner offing. They besides found that partner offing interaction frequence was ~190 interactions per barm genome of 12,500 kilobit. It was suggested that cistrons involved in DSB fix mechanisms could besides be campaigners for the maps in chromosome coupling, since homology hunt must be carried out in order to mend chromosomes with DSBs. Hence, the chosen campaigners were RAD50, RAD51 and RAD52.

It was found that void mutants of RAD50 and RAD51, and a partial omission of RAD52 caused decrease in the coupling degree. Furthermore, when cell progressed into miosis, chromosome coupling started to diminish, which, as was presumed, happened because of the break in meiotic DNA reproduction. Weiner and Kleckner have identified seven mutations that were specific to meiotic prophase and exhibited defects in chromosome coupling at different scopes from little to severe. These were hop1? , mer1? , dmc1? rad51? , dmc1? , rad50S, rad50? , spo11? . Spo11? appeared to be the most faulty 1. There was no SC ( synaptonemal composite ) detected in hop1? and mer1? mutations that exhibited wild-type chromosome coupling degrees, which suggested that chromosome coupling is independent of SC.

Besides, hop1? mutations showed really low degrees of miosis specific DSBs, which indicated that they were non required for single coupling interactions. However, the phenotypes of Spo11 and Rad50 showed that there was a nexus between partner offing and DSB fix mechanisms.Several chief decisions were made harmonizing to the consequences: foremost, chromosomes were paired in barm cells about to come in miosis ; 2nd, partner offing involved multiple interactions, one in approx. 65 kilobit, which was about the same as meiotic recombination frequence ; 3rd, partner offing disappeared during meiotic S stage ; Forth, partner offing interactions were restored during meiotic prophase phase independently of SC ; fifth, DSBs particular for miosis were non required to set up single coupling interactions ; sixth, mutations that were found to be faulty at different phases during meiotic recombination, were besides faulty in chromosome coupling.The ascertained multiple coupling interactions were guided by homologous acknowledgment at DNA degree.

Weiner and Kleckner suggested a individual tract theoretical account for chromosomal interactions based on the observations from the experiments. The tract involved constitution of partner offing interactions, breaks of those interactions during DNA reproduction, their reformation and stabilisation at the same sites, where they were converted into recombinational interactions, and was supported by the consequences from the experiments with the mutations.Scherthan et Al. ( 1994 ) , on the other manus, reported a high association degree of homologs at the monogenesis phase in budding barm Saccharomyces cerevisiae, nevertheless these associations were non the consequence of meiotic coupling because synaptonemal complex and its constructions were losing. It was concluded that signals resulted from the association of homologs were due to general bunch. In the condensed karyon a important homologue coupling was observed even for the meiotic mutations. Nevertheless, partner offing efficiency was much lower for the mutations comparing to the wild type. There was besides a clear suppression of SCs in all the mutations apart from rad50S, which despite the absence of SC showed some degree of homologous chromosome coupling, proposing that the procedure of homolog acknowledgment is non dependent on the presence of mature SC.

As it was thought, DSBs facilitate homology seeking during the early phases of miosis, but in rad50 and spo11 mutant homology acknowledgment still existed despite the really low sum of meiotic DSBs, bespeaking that DSBs are non the portion of the seeking mechanism.Rockmill and Roeder ( 1998 ) speculated that homolog coupling is promoted by telomere-mediated chromosome motions in S.cerevisiae. Their informations showed that there was a hold in miosis in disomic barm strains compared to the haploids, and that this hold was caused by the presence of homologous chromosomes.

The observations showed that barm cells that proceeded through miosis on clip were those where chromosomes were neglecting in homolog acknowledgment, whereas cells that were delayed in miosis were those with chromosomes traveling through the homolog partner offing phase. It was stated that homolog acknowledgment was independent of chromosome recombination and synapsis, since mutations tested still underwent homolog coupling, even though in some of them ( spo11, mer2, rec104 ) meiosis-specific DSBs originating meiotic recombination were avoided, and the formation of SC failed ( spo11, mer2 ) . The information besides indicated that telomeres promoted coupling of homologs in disomic strains, since it was demonstrated that in 25 % of the instances homologs paired in disomic strains transporting additive chromosomes, and they paired merely in 8 % when one of the chromosomes was round.

It was shown that two homologs will recognize each other if they are in close propinquity, even if the telomeres are absent as in the instance of round chromosomes, bespeaking that telomeres are non required in this instance. Furthermore, it was found that meiosis-specific Ndj1 protein that was found to be localized specifically to the terminals of meiotic chromosomes, was advancing homolog acknowledgment so doing the hold in monogenesis in disomes, and besides advancing partner offing by interceding telomere-dependent motions of the chromosomes. It was suggested that telomeres promoted homolog acknowledgment by constellating together in a corsage on the atomic membrane.In add-on, Csm4 was identified as a protein required for meiotic telomere kineticss and dependant on Ndj1-mediated telomere/NE association ( Wanat 2008 ) . Telomeres tend to co-localize in a corsage at zygotene phase with less frequence in csm4? mutations.

S.cerevisiae hop2 mutations showed high degree of SC formation between non-homologous chromosomes ( Leu 1998 ) . This was thought to be resulted from the defect in partner offing mechanism. Further surveies discovered MND1 cistron, which when overexpressed could stamp down hop2 faulty phenotype ( Tsubouchi and Roeder 2002 ) .

Mnd1 and Hop2 proteins were demonstrated to organize a complex and work together to ease homologous chromosome coupling and meiotic DSB fix, since both of them were found to resemble Rad51 and Dmc1 proteins, known as cardinal participants in the fix of DSBs ( Tsubouchi and Roeder 2002 ) .Surveies of homolog partner offing in fission barm Schizosaccharomyces pombe revealed Meu13p protein that was required for proper homologous chromosome coupling and recombination ( Nabeshima 2001 ) . S.cerevisiae Spo11 homologue Rec12p, which is known to be involved in recombination procedure in miosis, was besides found to intercede partner offing procedure in S.pombe.

However, Rec12p was non indispensable for proper operation of Meu13p, since meu13p mutations showed to hold partner offing defect irrespective whether rec12p was absent or non, proposing that partner offing is non dependent on recombination in S.pombe.

Pairing in Plants

In 1957 Okamoto discovered a individual venue called Ph1, which was believed to command chromosome coupling and recombination in wheat by impacting the specificity of centromere associations that initiate homologous coupling ( Moore 2010 ) . Luo et Al. ( 1996 ) in their survey showed that Ph1 was alternatively treating homology along the chromosome lengths. It was besides reported that Ph1 eliminated recombination between homeologous chromosome braces, and even in homeologous sections where kinetochores and both telomeres were homologous, whereas in the absence of Ph1 recombination could happen decently in homologous and related chromosome braces ( Luo et al. 1996 ) . These observations suggested the function of Ph1 in recombination instead than in partner offing mechanism.

Mikhailova et Al. ( 1998 ) on the other manus suggested that Ph1 was straight involved in chromosome coupling, since it was observed that the omission of ph1b allelomorph resulted in break of premiotic chromosome association. The new function of Ph1 was besides emphasised: chromosome condensation appeared to be disturbed in ph1bph1b mutations, which, as was suggested by the writers, could be related to the disturbed homeologous chromosome coupling, perchance by forestalling the ordered motion of chromosomes and so increasing the chance of homeologous hits ( Mikhailova et al. 1998 ) . However, the consequences from Corredor et Al. ( 2007 ) surveies indicated that in no manner Ph1 venue was responsible for commanding bivalent partner offing through the kinetochores in wheat, since when the fundamental law of kinetochores was modified in homozygotes and heterozygotes it did non hold any consequence on miosis I chiasmata associations of chromosomes neither in wild-type nor in Ph1 mutations.

Other surveies by Boden et Al. ( 2008 ) showed that ASY1 homologue TaASY1 in bread wheat, which is required for chromosome synapsis and publicity of homologous chromosome partner offing during miosis I, is affected by Ph1 that regulates the degrees of TaASY1 look during miosis. The absence of Ph1 besides affected localization of function of TaASY1 ( Boden 2008 ) . Their findings suggested that ASY1 was involved in Ph1-dependent control of homologue coupling.Pawlowski et Al. ( 2004 ) identified hapless homologous synapsis1 ( phs1 ) cistron in corn that was modulating homologous chromosome coupling and was required to forestall synapsis between non-homologous chromosomes. A really low degree of synapsis was observed for phs1 mutations at zygonema phase of miosis I prophase, and those constructions that synapsed showed improper alliance.

Maize besides expresses telomere corsage formation, which is known to advance chromosome coupling, and was found to be small affected in phs1 mutations ( Pawlowski 2004 ) . phs1 mutations besides expressed a defect in recombination that resulted in the decrease in the figure of RAD51 focal point. These mutant phenotypes suggested that the phs1 cistron could be involved in the mechanism of homology hunt and the procedure of organizing coupling, recombination and synapsis.Buding yeast protein Hop2 has a similar function to Phs1 protein in corn ( Pawlowski 2004 ; Leu 1998 ) .Surveies in rice identified PAIR3 which localised to the nucleus of the chromosomes looking as focal point in preleptotene ( shown by immunological experiments ) , and showed to be indispensable for telomere bunch and corsage formation, homologous chromosome coupling, SC assembly and normal chromosome recombination during miosis ( Wang 2011 ; Yuan 2009 ) .

Furthermore, PAIR3 appeared to be required for proper chromosome localisation of PAIR2, which is associated with chromosome axes and its mutant causes riddance of homologous sliver and synapsis ( Nonomura 2004 ) .As was already mentioned ( see debut ) , telomere constellating in a corsage agreement facilitates early phases of homologous chromosome coupling in many beings. However, in Arabidopsis thaliana telomeres do non organize a corsage, but alternatively a nucleolus-associated bunch is formed in early leptotene ( Armstrong 2001 ) .

GFP-labelling experiments in Arabidopsis diploid guard cells indicated to the important degree of coupling, but showed no part to meiotic coupling in centromere associations in bodily cells ( Kato and Lam 2003 ) .In male Drosophila coupling occurs in the absence of recombination.In C.elegans homologous chromosome acknowledgment, partner offing and synapsis occur independently of DSB formation and recombination.

Similar mechanisms may drive the meiotic telomere motions during prophase I for the assortment of species ( reviewed in Osman et Al. 2011 ) .A function of cohesins in chromosome coupling was revealed in Coprinus cinereus, which involved Rad9 protein moving in Mre11-dependent fix of DNA ( Cummings 2002 ) . It was found that the coherence was lost in rad9-1 mutation karyon which besides failed at homolog partner offing. Furthermore, msh5-22 mutant, ensuing in the defect in premeiotic DNA reproduction and failure to do sister chromatids, seemed to partly stamp down the rad9-1 mutation phenotype in homolog coupling, still exhibiting rather noticeable lessening in the degree of coupling, proposing that partner offing defects in rad9-1 mutation could be affected by hapless interactions of sister chromatids ( Cummingns 2002 ) .Other surveies described Mer3, Msh4 and Mlh1 recombination proteins indispensable for normal meiotic chromosome coupling in filiform fungus Sordaria macrospora ( Storlazzi 2002 ) . Mer3 is involved in stabilising recombinational interactions, and Msh4 and Mlh1 are moving as DNA mismatch fix proteins ( Nakagawa 2002 ; Snowden 2004 ; Argueso 2003 ) . All three mutations: mer3? , msh4? and mlh1? , showed to hold defects in coupling and synapsis, every bit good as in corsage formation ( Storlazzi 2002 ) .

Pairing in Animals

In S.cerevisiae recombination is required for SC formation and chromosome segregation, whereas if there is a mutant impacting SC formation, it does non forestall recombination ( Vazquez 2002 ) . In contrast, in Drosopphila and C.elegans the formation of SC and chromosome segregation occur even in the absence of recombination.In the survey by Vazquez et Al. ( 2002 ) partner offing in male Drosophila was analysed. They have concluded that Drosophila chromosomes were already paired when they entered miosis and that the degree of chromosome coupling was non dependent upon synapsis and recombination ( reviewed in McKee 2004 ) .

It was found that homologous coupling was taking topographic point in spermatogonial karyon during interphase, and that homologs remained paired in G1 and S stages. The observations suggested that in immature spermatocytes partner offing occurred purely at euchromatic parts and it was non-specific at heterochromatic parts ( Vazquez 2002 ) . In add-on, earlier surveies utilizing fluorescent in situ hybridisation ( FISH ) revealed that partner offing between Drosophila chromosomes was achieved at separated single venue, instead than through uninterrupted interaction of the sites along the length of the chromosomes ( Fung 1998 ) . Tomkiel et Al.

( 2001 ) discovered a Teflon ( tef ) cistron which was suggested to be responsible for set uping and/or modulating coupling of all autosomal bivalents in miosis I in male Drosophila. Mutant in tef did non impact univalent chromosome transmittal in miosis, which indicated to its peculiar function in pairing-dependent procedures. Furthermore, it was speculated that tef defect was likely to impact care of homolog partner offing instead than the induction, since no difference was noticed between wild-type and mutant spermatocytes until late prophase ( Tomkiel 2001 ) .Another protein called SUN1 was described to be associated with telomeres during prophase in miosis I. Consequences from the surveies in mice showed that the break of Sun1 cistron led to a bar of telomere fond regard to the atomic envelop ( NE ) , followed by the bar of meiotic homolog coupling and meiotic synapsis ( Dinging 2007 ) . Sun1-/- mutations showed dramatic lessening in coupling degrees for chromosomes 8 and 19 analyzed in the survey. Furthermore, because of the damage of telomere fond regard to the NE and failure to organize a corsage, the work of partner offing and synapsis mechanisms was inhibited, but could still be proceeded if the formation of DSBs was initiated, as was shown in the survey by Dinging et Al. ( 2007 ) .

This demonstrated the of import function of telomere fond regard to the NE in partner offing and synapsis mechanisms for homologous chromosomes in mammals.Mammalian SUN1 and SUN2 belong to the SUN-domain protein household, which is conserved from barm to mammals ( Hodzic 2004 ) . Both proteins are identified as homologs of UNC-84 protein in C.elegans required for atomic migration and placement ( Hodzic 2004 ; Schmitt 2007 ; Dinging 2007 ) . As Sun1, Sun2 was besides found to be associated with NE fond regard sites of meiotic chromosome telomeres ( Schmitt 2007 ) .

Hence, because of the Sun2 engagement in telomere bunch, which, as was antecedently established, affects homologous chromosome partner offing during miosis, it could be speculated that Sun2 besides contributes to the operation of the coupling mechanism.SUN1 in C.elegans Acts of the Apostless in a complex with matefin at leptotene and zygotene phases of miosis I. SUN1/matefin protein was proposed to travel chromosome ends conveying them together, so they could so scuffle and could happen the right spouse for synapsis ( Baudrimont 2010 ) . Phosphorylation of SUN1 was said to modulate homolog partner offing and recombination in C.elegans ( Baudrimont 2010 ) .S.pombe Sad1p and mammalian Sun2 were suggested to hold a common telomere constellating and bouquet formation mechanism which is conserved in eucaryotes ( Schmitt 2007 ) .

Earlier surveies of Caenorhabditis elegans proposed the being of homolog acknowledgment parts ( HRR ) at one terminal of the chromosome, which seemingly defined proper coupling and recombination of homologs ( McKim 1993 ) . It was noticed that these parts had to be positioned in Commonwealth of Independent States for meiotic homologous chromosome recombination and segregation ( Phillips 2009 ) . Mutant analysis utilizing duplicates and translocations revealed unc-54 part on chromosome I, which when disrupted caused failure in homolog coupling ( McKim 1993 ) . It was noticed that homologs merely mated and recombined if the HRR was present ( reviewed in McKim 2007 ) .DSBs are non required for the induction of synapsis in C.elegans and Drosophila ( McKim 2007 ) .Homolog acknowledgment parts, besides called coupling Centres ( Personal computer ) , were found to be required for induction of synapsis and stabilization of coupling, but either mechanism could work without the presence of one another ( MacQueen 2005 ) .

However, different surveies showed that these meiotic events, synapsis and coupling, did non needfully necessitate Personal computers to be able to take topographic point, but still synapsis failed when the Personal computer was deleted ( MacQueen 2005 ) .A cistron called him-8 was discovered to encode zinc-finger protein C2H2 that localised to a Personal computer on X chromosome in C.elegans ( Phillips 2005 ) . Mutant in the cistron resulted in the defect in stabilisation of homolog coupling and induction of synapsis, hence, proposing its function in both mechanisms.HIM-8 paralogs ZIM-1,2 and 3 were besides found to be localised to Personal computers during early phases of miosis, and to be required for meiotic coupling and synapsis ( Phillips 2006 ; reviewed in McKim 2007 ) . All four are known to be associated with atomic envelop, which indicates their function in a similar to a corsage formation mechanism ( Phillips 2006 ; reviewed in Hawley and Gilliland 2009 ) .

Furthermore, HIM-8/ZIM proteins work together to guarantee proper binding of each of these proteins to a specific short sequence sphere on the every chromosome ( Phillips 2009 ) . The short sequences were found in partner offing Centres of the chromosomes, and were suggested to be specifically required for the acknowledgment by HIM-8/ZIM proteins, and proper subsequent homolog coupling, synapsis, recombination and segregation ( Phillips 2009 ) .


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