Update on Telomeres The Polar Arrangement of Telomeres in Interphase and Meiosis. Rabl Organization and the Bouquet

It has long been appreciated that chromosomes do not lay passively in the nucleus, but are dynamically reorganized to suit the cell’s needs. Chromosomes condense into compact bodies for cell division, decondense in interphase to allow gene expression, and pair along their lengths during meiosis in preparation for the reductional division. Chromosome behavior can be broadly classified into two types: autonomous and nonautonomous. Autonomous behavior is seen at the chromosome level and below, and involves only the chromosomes themselves, as in the examples given above. Nonautonomous behavior involves physical interactions between the chromosomes and the rest of the cell and results in global changes in the organization of the chromosomes. The best known example of nonautonomous chromosome behavior may be the orchestration of chromosome movement at mitosis through interaction with the spindle microtubules. However, in recent years the study of chromosome behavior in interphase and meiotic prophase has revealed many roles for global, nonautonomous reorganization of chromosomes within the nucleus. Mitotic chromosomes rely on centromere-based organization, whereas interphase and meiotic prophase cells appear to have chosen telomeres as the sites of interaction with the rest of the cell. Many descriptive studies have revealed the special positioning of telomeres in cells at certain times. In particular, during both interphase and meiotic prophase, telomeres are often located at the nuclear periphery in limited regions or clusters. Plants have played an important role in many of these studies because of their superior cytological features. In each case, the telomere organization can be thought of as imposing order on the chromosomes. This review will discuss the mechanisms by which telomeres are positioned and maintained, and the possible functions of the polar arrangement of the telomeres. TELOMERE STRUCTURE