Interphase chromosome order: A proposal

A strategy for detecting chromosome-specific rearrangements in rye

To obtain translocations involving specific chromosomes in rye, a line in which chromosome 1R has large C-bands on its two telomeres but which lacks C-bands (or has very small ones) on the telomeres of the remaining chromosomes was used. About 6% of the plants produced using pollen from irradiated (1.2 krad (1 rad = 10 mGy)) spikes of this line possessed structural changes involving the labeled chromosome. These aberrations included translocations, ring chromosomes, isochromosomes, and telocentrics. It is concluded (i) that all nonlabeled chromosomes have the same probability of participating in reciprocal translocations with the labeled chromosome, 1R, and (ii) that most induced reciprocal translocations involved exchanges of chromosome segments of approximately equal length. The use of lines having the appropriate combination of telomeric C-bands improves the efficiency of obtaining reciprocal translocations involving specific chromosomes that could be used in the construction of detailed physical maps.

Telomeres cluster de novo before the initiation of synapsis: a three-dimensional spatial analysis of telomere positions before and during meiotic prophase

We have analyzed the progressive changes in the spatial distribution of telomeres during meiosis using three-dimensional, high resolution fluorescence microscopy. Fixed meiotic cells of maize (Zea mays L.) were subjected to in situ hybridization under conditions that preserved chromosome structure, allowing identification of stage-dependent changes in telomere arrangements. We found that nuclei at the last somatic prophase before meiosis exhibit a nonrandom, polarized chromosome organization resulting in a loose grouping of telomeres. Quantitative measurements on the spatial arrangements of telomeres revealed that, as cells passed through premeiotic interphase and into leptotene, there was an increase in the frequency of large telomere-to-telomere distances and a decrease in the bias toward peripheral localization of telomeres. By leptotene, there was no obvious evidence of telomere grouping, and the large, singular nucleolus was internally located, nearly concentric with the nucleus. At the end of leptotene, telomeres clustered de novo at the nuclear periphery, coincident with a displacement of the nucleolus to one side. The telomere cluster persisted throughout zygotene and into early pachytene. The nucleolus was adjacent to the cluster at zygotene. At the pachytene stage, telomeres rearranged again by dispersing throughout the nuclear periphery. The stage-dependent changes in telomere arrangements are suggestive of specific, active telomere-associated motility processes with meiotic functions. Thus, the formation of the cluster itself is an early event in the nuclear reorganizations associated with meiosis and may reflect a control point in the initiation of synapsis or crossing over.

The spatial localization of 18 S rRNA genes, in relation to the descent of the cells, in the root cortex of Petunia hybrida

The 3-D localization of transcription inactive 18 S rRNA genes was studied in interphase nuclei of Petunia hybrida root tip cells. To enable a cell type (i.e. cortex)-specific study in which also the orientation and descent of the cells could be taken into account, a method was developed to preserve the spatial organization of the root meristem. The ribosomal genes were detected by fluorescence in situ hybridization using a biotinylated cDNA probe. 3-D images of 81 nuclei, obtained by confocal scanning laser microscopy, were processed with newly developed computer software. 3-D nucleolar and nuclear dimensions, and the localization of the FISH-spots, were recorded interactively. We compared the absolute and relative position of the genes within and between files of cells of the cortex region of several roots, taking into account the genealogical relationship of the cells. Statistical analysis showed that both the relative and absolute positions of the inactive genes were random, also in more closely related cells within a file of cells. A ‘relict telophase orientation’ of the genes (i.e. the position of the genes in the daughter cells are mirror images of each other) could only be observed in the G0/1 phase of ‘true’ daughter cells; the orientation was not preserved throughout the next cell cycle.

The structure of a primitive kinetochore

The isolation of yeast centromeres has provided the opportunity to describe the molecular structure of chromosome attachments to the mitotic spindle. Nucleolytic probes of chromatin structure and construction of conditional mutants in centromere function have been used to study the regulation and assembly of centromeres throughout the cell cycle in Saccharomyces cerevisiae.

Isolation of a higher eukaryotic telomere from Arabidopsis thaliana

We have developed a method for constructing genomic libraries enriched for telomeric DNA sequences, enabling the isolation of telomeres from higher eukaryotic organisms with large chromosomes. The method was used to clone telomeric DNA sequences from the flowering plant Arabidopsis thaliana. A. thaliana telomeres are composed primarily of tandemly repeated blocks of the sequence 5'-CCCTAAA-3' and are heterogeneous in size. Genomic sequences that cross-hybridize at high stringency with A. thaliana telomeric repeats are present in other higher plants. In Zea mays (corn), these cross-hybridizing sequences are located at the telomeres. In addition, the A. thaliana telomeric repeats cross-hybridize at low stringency to genomic sequences located at the telomeres of human chromosomes.