减数分裂染色体的行为及其分子基础

减数分裂是有性生殖生物配子产生的必需过程.在细胞进入减数分裂前,其染色体复制1次,但启动分裂后,细胞进行二次分裂,从而产生染色体数目减半的配子.减数分裂Ⅰ前期同源染色体的配对、联会、重组以及减数分裂Ⅰ后期同源染色体的分离是减数分裂的基本特征,而这些减数分裂特异事件的按时、依序发生则有赖于减数分裂Ⅰ前期程序性D N A双链断裂(D S B)的产生和以同源染色体为模板进行的同源重组修复.本文将对减数分裂特别是减数分裂Ⅰ前期染色体的行为进行简要综述,并就其分子基础和机制进行分析讨论.     

泛素化修饰在减数分裂重组修复中的作用

程序性和非程序性DNA双链断裂起始于生理条件下的需求(如减数分裂重组)和外界的刺激(如离子辐射等).DNA的双链断裂会严重影响基因组的稳定性,因而需要恰当的处理并以一种可调控的方式加以修复.近期研究表明,蛋白质的泛素化修饰在DNA损伤反应以及减数分裂重组修复过程中发挥了重要作用.本文拟综述参与在同源重组依赖的DNA双链断裂修复过程中与泛素化相关的蛋白质以及一些蛋白质复合体在此过程中的作用及功能.     

联会复合体的组成、功能及遗传控制

在多数有性生殖生物中, 减数分裂第一次分裂前期同源染色体间会形成一种复杂的超级蛋白结构--联会复合体(Synaptonemal complex, SC)。该结构与同源染色体间的配对、联会、交换、分离等过程密切相关。若其出现异常, 将可导致性母细胞大量凋亡, 宏观上即表现为生物个体不育。近年来, 该结构已成为减数分裂研究领域的一个热点, 但其控制机理至今所知还十分有限。文章对联会复合体的组成、功能及其遗传控制等情况进行概述, 并对其未来的研究进行探讨和展望。     

联会复合体的组成、功能及遗传控制

在多数有性生殖生物中,减数分裂第一次分裂前期同源染色体间会形成一种复杂的超级蛋白结构——联会复合体(Synaptonemal complex,SC)。该结构与同源染色体间的配对、联会、交换、分离等过程密切相关。若其出现异常,将可导致性母细胞大量凋亡,宏观上即表现为生物个体不育。近年来,该结构已成为减数分裂研究领域的一个热点,但其控制机理至今所知还十分有限。文章对联会复合体的组成、功能及其遗传控制等情况进行概述,并对其未来的研究进行探讨和展望。     

对减数分裂的新理解

减数分裂历来被认为是：同源染色体联会－重组－分离。染色体配对是其中最早的事件,配对又叫联会,联会由联会复合体（ＳＣ）引起或促进。联会复合体又是减数分裂重组所必需的。重组引起细胞学上可见的交叉,能够确保同源染色体分离。这些经典观点在９０年代受到了严重挑战,对减数分裂的许多新理解正在取而代之。按照新观点,减数分裂的过程可以用下图表示。１　同源性搜索是减数分裂的第一步减数分裂最早的事件不是同源染色体的配对,其前在细线期还发生了同源性搜索。它是在全染色体组内识别染色体上同源性位点的过程。搜索不仅仅限于染…     

减数分裂型黏连蛋白RAD21L的作用机制

减数分裂是在有性生殖过程中高度专业化的真核细胞分裂。在减数分裂过程中,DNA复制一次,细胞连续分裂两次,子细胞染色体数目减半。在减数第一次分裂过程中为确保同源染色体正确分离,必须通过同源染色体配对、联会及重组等减数分裂特异性染色体运动。如果其中任一运动发生异常会导致先天性疾病或不孕不育症。因此,了解这些减数分裂型染色体的运动机制极为重要。该综述重点探讨了减数分裂型黏连蛋白RAD21L的特殊作用及其在哺乳动物减数分裂过程中对染色体运动的调控机制。     

溯源联会

联会被认为是减数分裂过程中染色体的特殊行为之一,是高中生物学教学中的一个难点。同源重组最初的功能可能只是修复DNA损伤,后来得到发展并被"借用"到减数分裂重组中,确保减数分裂过程中同源染色体的正确配对,保持基因组的完整性。追根溯源,是DNA分子的同源重组导致了联会。     

联会复合体——原发无精症发病中的重要角色

联会复合体(synaptonemal complex,SC)是一种减数分裂特异性超分子蛋白质结构,与减数分裂I(改罗文)中同源染色体的凝缩、配对、重组和分离密切相关。近年来,联会复合体的研究取得了一系列重要的进展,包括在其组成成分和功能上的一些新发现。在小鼠不育模型中联会复合体及其编码基因的异常可引起精子发生障碍。更重要的是,联会复合体编码基因之一SCP3单个碱基缺失导致的无精症已在人类原发不育患者中得到证实。对联会复合体基因SCP1的进一步研究也正在进行之中。      

嗜热四膜虫SPO11基因缺陷型细胞株的基因表达变化分析

有性生殖的关键过程是通过减数分裂产生生殖细胞,而减数分裂的一个重要环节是进行基于DNA双链断裂的同源染色体重组。在同源染色体重组过程中,SPO11蛋白催化产生DNA双链断裂,从而起始同源染色体的重组。因此,研究SPO11基因缺失在减数分裂过程中所引起的基因表达变化有助于在转录组水平上了解该基因的作用。本研究通过对嗜热四膜虫(Tetrahymena thermophila)野生型和SPO11敲除细胞株在接合生殖时期2 h、3 h、4 h、5 h四个时间点的转录组进行高通量测序。通过差异表达基因分析和功能富集分析,发现SPO11基因敲除之后嗜热四膜虫在接合生殖时期2 h时,与DNA代谢过程和DNA复制相关基因的表达发生变化,推测SPO11基因敲除导致的减数分裂过程异常可能与DNA代谢过程和DNA复制相关。     

“减数分裂”教学的重要性及建议

(一)减数分裂与细胞学、遗传学的关系现行《生物》课本中“减数分裂与生殖细胞的成熟”一节教材是全书中重要的一节,减数分裂是一种特殊的有丝分裂。在减数分裂过程中细胞经过连续两次分裂,而染色体只复制一次,结果使性细胞中染色体数目减半。性细胞再经受精作用形成合子,合子中染色体数目又恢复到亲代体细胞中染色体数目,从而使亲子代细胞中的遗传物质保持相对稳定。减数分裂的前期I,细胞中的染色体发生了一系列特殊的变化——同源染色体联会、交叉互换和分离。每一对同源染色体中的两条染色体彼此分离,以后随机地分配到二个子细胞中去;异源染色体     

Double-stranded DNA breaks and gene functions in recombination and meiosis

Meiotic prophase I is a long and complex phase. Homologous recombination is an important process that occurs between homologous chromosomes during meiotic prophase I. Formation of chiasmata, which hold homologous chromosomes together until the metaphase I to anaphase I transition, is critical for proper chromosome segregation. Recent studies have suggested that the SPO 11 proteins have conserved functions in a number of organisms in generating sites of double-stranded DNA breaks （DSBs） that are thought to be the starting points of homologous recombination. Processing of these sites of DSBs requires the function of RecA homologs, such as RAD5 1, DMC 1, and others, as suggested by mutant studies; thus the failure to repair these meiotic DSBs results in abnormal chromosomal alternations, leading to disrupted meiosis. Recent discoveries on the functions of these RecA homologs have improved the understanding of the mechanisms underlying meiotic homologous recombination.     

Temporal comparison of recombination and synaptonemal complex formation during meiosis in S. cerevisiae

In synchronous cultures of S. cerevisiae undergoing meiosis, an early event in the meiotic recombination pathway, site-specific double strand breaks (DSBs), occurs early in prophase, in some instances well before tripartite synaptonemal complex (SC) begins to form. This observation, together with previous results, supports the view that events involving DSBs are required for SC formation. We discuss the possibility that the mitotic pathway for recombinational repair of DSBs served as the primordial mechanism for connecting homologous chromosomes during the evolution of meiosis. DSBs disappear during the period when tripartite SC structure is forming and elongating (zygotene); presumably, they are converted to another type of recombination intermediate. Neither DSBs nor mature recombinant molecules are present when SCs are full length (pachytene). Mature reciprocally recombinant molecules arise at the end of or just after pachytene. We suggest that the SC might coordinate recombinant maturation with other events of meiosis.     

CRA-1 uncovers a double-strand break-dependent pathway promoting the assembly of central region proteins on chromosome axes during C. elegans meiosis

The synaptonemal complex (SC), a tripartite proteinaceous structure that forms between homologous chromosomes during meiosis, is crucial for faithful chromosome segregation. Here we identify CRA-1, a novel and conserved protein that is required for the assembly of the central region of the SC during C. elegans meiosis. In the absence of CRA-1, central region components fail to extensively localize onto chromosomes at early prophase and instead mostly surround the chromatin at this stage. Later in prophase, central region proteins polymerize along chromosome axes, but for the most part fail to connect the axes of paired homologous chromosomes. This defect results in an inability to stabilize homologous pairing interactions, altered double-strand break (DSB) repair progression, and a lack of chiasmata. Surprisingly, DSB formation and repair are required to promote the polymerization of the central region components along meiotic chromosome axes in cra-1 mutants. In the absence of both CRA-1 and any one of the C. elegans homologs of SPO11, MRE11, RAD51, or MSH5, the polymerization observed along chromosome axes is perturbed, resulting in the formation of aggregates of the SC central region proteins. While radiation-induced DSBs rescue this polymerization in cra-1; spo-11 mutants, they fail to do so in cra-1; mre-11, cra-1; rad-51, and cra-1; msh-5 mutants. Taken together, our studies place CRA-1 as a key component in promoting the assembly of a tripartite SC structure. Moreover, they reveal a scenario in which DSB formation and repair can drive the polymerization of SC components along chromosome axes in C. elegans.     

Fundamental cell cycle kinases collaborate to ensure timely destruction of the synaptonemal complex during meiosis

The synaptonemal complex (SC) is a proteinaceous macromolecular assembly that forms during meiotic prophase I and mediates adhesion of paired homologous chromosomes along their entire lengths. Although prompt disassembly of the SC during exit from prophase I is a landmark event of meiosis, the underlying mechanism regulating SC destruction has remained elusive. Here, we show that DDK (Dbf4‐dependent Cdc7 kinase) is central to SC destruction. Upon exit from prophase I, Dbf4, the regulatory subunit of DDK, directly associates with and is phosphorylated by the Polo‐like kinase Cdc5. In parallel, upregulated CDK1 activity also targets Dbf4. An enhanced Dbf4‐Cdc5 interaction pronounced phosphorylation of Dbf4 and accelerated SC destruction, while reduced/abolished Dbf4 phosphorylation hampered destruction of SC proteins. SC destruction relieved meiotic inhibition of the ubiquitous recombinase Rad51, suggesting that the mitotic recombination machinery is reactivated following prophase I exit to repair any persisting meiotic DNA double‐strand breaks. Taken together, we propose that the concerted action of DDK, Polo‐like kinase, and CDK1 promotes efficient SC destruction at the end of prophase I to ensure faithful inheritance of the genome.     

Mnd1 is required for meiotic interhomolog repair

BACKGROUND: While double-strand break (DSB) repair is vital to the survival of cells during both meiosis and mitosis, the preferred mechanism of repair differs drastically between the two types of cell cycle. Thus, during meiosis, it is the homologous chromosome rather than the sister chromatid that is used as a repair template. RESULTS: Cells attempting to undergo meiosis in the absence of Mnd1 arrest in prophase I due to the activation of the Mec1 DNA-damage checkpoint accumulating hyperresected DSBs and aberrant synapsis. Sporulation of mnd1Delta strains can be restored by deleting RED1 or HOP1, which permits repair of DSBs by using the sister chromatid as a repair template. Mnd1 localizes to chromatin as foci independently of DSB formation, axial element (AE) formation, and synaptonemal complex (SC) formation and does not colocalize with Rad51. Mnd1 does not preferentially associate with hotspots of recombination. CONCLUSIONS: Our results suggest that Mnd1 acts specifically to promote DSB repair by using the homologous chromosome as a repair template. The presence of Rec8, Red1, or Hop1 renders Mnd1 indispensable for DNA repair, presumably through the establishment of interhomolog (IH) bias. Localization studies suggest that Mnd1 carries out this function without being specifically recruited to the sites of DNA repair. We propose a model in which Mnd1 facilitates chromatin accessibility, which is required to allow strand invasion in meiotic chromatin.     

The CSN/COP9 Signalosome Regulates Synaptonemal Complex Assembly during Meiotic Prophase I of Caenorhabditis elegans

The synaptonemal complex (SC) is a conserved protein structure that holds homologous chromosome pairs together throughout much of meiotic prophase I. It is essential for the formation of crossovers, which are required for the proper segregation of chromosomes into gametes. The assembly of the SC is likely to be regulated by post-translational modifications. The CSN/COP9 signalosome has been shown to act in many pathways, mainly via the ubiquitin degradation/proteasome pathway. Here we examine the role of the CSN/COP9 signalosome in SC assembly in the model organism C. elegans. Our work shows that mutants in three subunits of the CSN/COP9 signalosome fail to properly assemble the SC. In these mutants, SC proteins aggregate, leading to a decrease in proper pairing between homologous chromosomes. The reduction in homolog pairing also results in an accumulation of recombination intermediates and defects in repair of meiotic DSBs to form the designated crossovers. The effect of the CSN/COP9 signalosome mutants on synapsis and crossover formation is due to increased neddylation, as reducing neddylation in these mutants can partially suppress their phenotypes. We also find a marked increase in apoptosis in csn mutants that specifically eliminates nuclei with aggregated SC proteins. csn mutants exhibit defects in germline proliferation, and an almost complete pachytene arrest due to an inability to activate the MAPK pathway. The work described here supports a previously unknown role for the CSN/COP9 signalosome in chromosome behavior during meiotic prophase I.     

ATM controls meiotic DNA double-strand break formation and recombination and affects synaptonemal complex organization in plants

Meiosis is a specialized cell division that gives rise to genetically distinct gametic cells. Meiosis relies on the tightly controlled formation of DNA double-strand breaks (DSBs) and their repair via homologous recombination for correct chromosome segregation. Like all forms of DNA damage, meiotic DSBs are potentially harmful and their formation activates an elaborate response to inhibit excessive DNA break formation and ensure successful repair. Previous studies established the protein kinase ATM as a DSB sensor and meiotic regulator in several organisms. Here we show that Arabidopsis ATM acts at multiple steps during DSB formation and processing, as well as crossover (CO) formation and synaptonemal complex (SC) organization, all vital for the successful completion of meiosis. We developed a single-molecule approach to quantify meiotic breaks and determined that ATM is essential to limit the number of meiotic DSBs. Local and genome-wide recombination screens showed that ATM restricts the number of interference-insensitive COs, while super-resolution STED nanoscopy of meiotic chromosomes revealed that the kinase affects chromatin loop size and SC length and width. Our study extends our understanding of how ATM functions during plant meiosis and establishes it as an integral factor of the meiotic program.

Arabidopsis ATM acts at multiple steps during DSB formation and processing, as well as crossover formation and synaptonemal complex organization, all vital for the successful completion of meiosis.     

Synaptonemal complex-dependent centromeric clustering and the initiation of synapsis in Drosophila oocytes

The pairing of homologous chromosomes and the intimate synapsis of the paired homologs by the synaptonemal complex (SC) are essential for subsequent meiotic processes including recombination and chromosome segregation. Here we show that the centromere clustering plays an important role in initiating homolog synapsis during meiosis in Drosophila females. Although centromeres are not clustered prior to the onset of meiosis, all four pairs of centromeres are actively clustered into one or two masses during early meiotic prophase. Within the 16-cell cyst, centromeric clustering appears to define the first step in the initiation of synapsis. Clustering is restricted to the nuclei that form the SC and is dependent on all known SC proteins. Surprisingly, both centromeric clusters and the SC components associated with them persist long after the disassembly of the euchromatic SC at the end of pachytene. The initiation of homologous recombination through the formation of programmed double-strand breaks (DSBs) is not required for either the formation or the maintenance of the centromeric clusters. Our data support a view in which the SC-mediated clustering at the centromeres is the initiating event for meiotic synapsis.     

A new thermosensitive smc-3 allele reveals involvement of cohesin in homologous recombination in C. elegans

The cohesin complex is required for the cohesion of sister chromatids and for correct segregation during mitosis and meiosis. Crossover recombination, together with cohesion, is essential for the disjunction of homologous chromosomes during the first meiotic division. Cohesin has been implicated in facilitating recombinational repair of DNA lesions via the sister chromatid. Here, we made use of a new temperature-sensitive mutation in the Caenorhabditis elegans SMC-3 protein to study the role of cohesin in the repair of DNA double-strand breaks (DSBs) and hence in meiotic crossing over. We report that attenuation of cohesin was associated with extensive SPO-11-dependent chromosome fragmentation, which is representative of unrepaired DSBs. We also found that attenuated cohesin likely increased the number of DSBs and eliminated the need of MRE-11 and RAD-50 for DSB formation in C. elegans, which suggests a role for the MRN complex in making cohesin-loaded chromatin susceptible to meiotic DSBs. Notably, in spite of largely intact sister chromatid cohesion, backup DSB repair via the sister chromatid was mostly impaired. We also found that weakened cohesins affected mitotic repair of DSBs by homologous recombination, whereas NHEJ repair was not affected. Our data suggest that recombinational DNA repair makes higher demands on cohesins than does chromosome segregation.