Chromosome Territories

Chromosome territories (CTs) constitute a major feature of nuclear architecture. In a brief statement, the possible contribution of nuclear architecture studies to the field of epigenomics is considered, followed by a historical account of the CT concept and the final compelling experimental evidence of a territorial organization of chromosomes in all eukaryotes studied to date. Present knowledge of nonrandom CT arrangements, of the internal CT architecture, and of structural interactions with other CTs is provided as well as the dynamics of CT arrangements during cell cycle and postmitotic terminal differentiation. The article concludes with a discussion of open questions and new experimental strategies to answer them.Impressive progress has been achieved during the last decade with regard to the functional implications of DNA methylation, histone modifications, and chromatin remodeling events for gene regulation (Fuks 2005; Kouzarides 2007; Maier et al. 2008; Jiang and Pugh 2009). It has, however, also become obvious that decoding the chromatin language does not suffice to fully understand the ways in which the diploid genome contributes to the formation of the different epigenomes present in the various cell types of a multicellular organism.Different epigenomes and their functional implications also depend on differences in higher‐order chromatin organization and nuclear architecture at large. Epigenomic research aims for an integrated understanding of the structural and functional aspects of epigenetics with nuclear architecture during the differentiation of toti- or pluripotent cells to functionally distinct cell types.The territorial organization of chromosomes in interphase (chromosome territories, CTs) constitutes a basic feature of nuclear architecture. This article starts with a brief historical account of the CT concept and the compelling experimental evidence in favor of a territorial organization of chromosomes in all eukaryotes studied to date. A survey of what is presently known about nonrandom arrangements of CTs, about changes of such arrangements in cycling cells as a result of internal or external influences and about the internal architecture of CTs and their structural interactions with each other is provided. The article concludes with a discussion of open questions on CT organization and new experimental strategies to answer them.     

染色体浓缩素

染色体的形成是细胞周期的重要事件,然而有关染色体构筑动力学的分子机制仍未阐明。近年来对染色体浓缩素的分离与研究,为认识DNA浓缩和染色体构建机制提供了重要的线索,是细胞生物学研究领域的里程碑。现对浓缩素的发现过程,浓缩素在有丝分裂和减数分裂中的作用,浓缩素与黏着素的关系,浓缩素参与基因调节等方面进行综述,为相关领域的研究者提供参考。     

Chromosome segregation

The ability to visualise specific genes and proteins within bacterial cells is revolutionising knowledge of chromosome segregation. The essential elements appear to be the driving force behind DNA replication, which occurs at fixed cellular positions, the condensation of newly replicated DNA by a chromosome condensation machine located at the cell 1/4 and 3/4 positions, and molecular machines that act at midcell to allow chromosome separation after replication and movement of the sister chromosomes away from the division septum prior to cell division. This review attempts to provide a perspective on current views of the bacterial chromosome segregation mechanism and how it relates to other cellular processes.     

Chromosome micromanipulation

The attachment of individual chromosomes to the spindle has been studied by micromanipulation in functionally normal grasshopper spermatocytes. Prometaphase to anaphase I chromosomes can be repeatedly stretched with a microneedle without much increase in the distance between the kinetochores and the poles. Individual chromosomes can, however, be displaced laterally (prometaphase-anaphase) or toward the pole (anaphase) without loss of spindle attachment and without greatly disturbing other chromosomes. It is concluded that chromosomes are firmly and individually attached to the spindle by chromosomal spindle fibers which are capable of bearing any normal mitotic load, including the stretching of dikinetic (dicentric) chromosomes in anaphase. Prolonged or severe manipulation can produce a small — three or four micron — increase in the kinetochore-to-pole distance. Anaphase motion continues normally in spite of lateral or poleward displacements or of small increases in the kinetochore-to-pole distance. In late anaphase, chromosomes can be displaced to the opposite pole. An unusual, rapid motion back toward the original pole follows such displacements, but repeated displacements eventually result in non-disjunction. No evidence for firm interzonal connections between anaphase chromosomes was obtained. Prometaphase and metaphase bivalents can be detached from the spindle by manipulations other than bivalent stretching, but half-bivalents in anaphase are never detached by these manipulations.This investigation was supported in part by research grants GM-8480 and GM-13745 from the Division of General Medical Sciences, United States Public Health Service.     

Chromosome micromanipulation

Two types of unusual motion within the spindle have heen studied in a grasshopper (Melanoplus differentialis) spermatocyte. The first is the motion of granules placed by micromanipulation within the normally granule-free spindle. The most specific motions are poleward, approximate the speed of the chromosomes in anaphase, and occur in the area between the kinetochores and the nearer pole during both metaphase and anaphase. Exactly the same transport properties were earlier observed by Bajer inHaemanthus endosperm spindles. The absence of significant motion in the interzone between the separating chromosomes at anaphase has been unequivocally demonstrated inMelanoplus spermatocytes. Thus very specific motion of non-kinetochoric materials is probably a general spindle capability which would much restrict admissible models of mitotic force production,if the same forces move both granules and chromosomes. The second unusual motion is seen following chromosome detachment from the spindle by micromanipulation during anaphase. These tend to move toNearer pole rather than to the pole the chromosome's kinetochoresFace. The latter preference was earlier demonstrated after detachment during prometaphase or metaphase and has been confirmed without exception in the present studies. The apparent preference for motion to the nearer pole in anaphase provides the first evidence for poleward forces within each half-spindle which cannot be entirely specified by the chromosomal spindle fibers. Almost certainly these would be the usual forces responsible for chromosome motion since they act specifically at the kinetochores of detached chromosomes. This evidence requires interpretation, however because additional factors influence chromosome motion following detachment at anaphase. On thesimplest interpretation, certain current models of mitosis clearly are not satisfactory and others are favored.     

Chromosome orientation

Precise chromosome segregation during cell division results from the attachment of chromosomes to microtubules emanating from both poles of the spindle apparatus. The molecular machinery involved in establishing and maintaining properly oriented microtubule attachments remains murky. Some clarity is now emerging with the identification of Bod1 (Biorientation Defective 1), a protein that promotes chromosome biorientation by unleashing chromosomes from improperly oriented microtubule attachments.     

Chromosome evolution

The idea of evolution as a principle for the origin of biodiversity fits all phenomena of life, including the carriers of nuclear inheritance, the chromosomes. Insights into the evolutionary mechanisms that contribute to the shape, size, composition, number and redundancy of chromosomes elucidate the high plasticity of nuclear genomes at the chromosomal level, and the potential for genome modification in the course of breeding processes. Aspects of chromosome fusion, as exemplified by karyotype evolution of relatives of Arabidopsis, have recently received special attention.     

人工染色体

着丝粒、端粒和复制起点是保持染色体在有丝分裂时稳定性的重要组分,利用酵母染色体的这些组分,1983年人们首次成功地构建了酵母人工染色体（YAC）.此后,生物学家对于构建以人类为代表的哺乳动物的人工染色体产生了极大的兴趣,并于1997年成功地构建了第一条人类人工染色体（HAC）.人工染色体具有极其重要的理论和实际意义,不仅可以作为研究必需组分的基础,还为基因治疗开辟了一条新的途径.目前,人们正致力于人工染色体的实际应用的研究,并期望在包括绿色植物的更多物种中构建人工染色体,前景十分广阔.