Localization of DNA replicator sites near the nuclear membrane in plant cells

The replication domain model: regulating replicon firing in the context of large-scale chromosome architecture

The "Replicon Theory" of Jacob, Brenner, and Cuzin has reliably served as the paradigm for regulating the sites where individual replicons initiate replication. Concurrent with the replicon model was Taylor's demonstration that plant and animal chromosomes replicate segmentally in a defined temporal sequence, via cytologically defined units too large to be accounted for by a single replicon. Instead, there seemed to be a program to choreograph when chromosome units replicate during S phase, executed by initiation at clusters of individual replicons within each segment. Here, we summarize recent molecular evidence for the existence of such units, now known as "replication domains", and discuss how the organization of large chromosomes into structural units has added additional layers of regulation to the original replicon model.

The spatio-temporal organization of DNA replication sites is identical in primary, immortalized and transformed mammalian cells

We investigated the organization of DNA replication sites in primary (young or presenescent), immortalized and transformed mammalian cells. Four different methods were used to visualize replication sites: in vivo pulse-labeling with 5-bromo-2'-deoxyuridine (BrdU), followed by either acid depurination, or incubation in nuclease cocktail to expose single-stranded BrdU-substituted DNA regions for immunolabeling; biotin-dUTP labeling of nascent DNA by run-on replication within intact nuclei and staining with fluorescent streptavidin; and, finally, immunolabeling of the replication fork proteins PCNA and RPA. All methods produced identical results, demonstrating no fundamental differences in the spatio-temporal organization of replication patterns between primary, immortal or transformed mammalian cells. In addition, we did not detect a spatial coincidence between the early firing replicons and nuclear lamin proteins, the retinoblastoma protein or the nucleolus in primary human and rodent cells. The retinoblastoma protein does not colocalize in vivo with members of the Mcm family of proteins (Mcm2, 3 and 7) at any point of the cell cycle and neither in the chromatin-bound nor in the soluble nucleoplasmic fraction. These results argue against a direct role for the retinoblastoma or nuclear lamin proteins in mammalian DNA synthesis under normal physiological conditions.

Spatial and temporal dynamics of DNA replication sites in mammalian cells

Fluorescence microscopic analysis of newly replicated DNA has revealed discrete granular sites of replication (RS). The average size and number of replication sites from early to mid S-phase suggest that each RS contains numerous replicons clustered together. We are using fluorescence laser scanning confocal microscopy in conjunction with multidimensional image analysis to gain more precise information about RS and their spatial-temporal dynamics. Using a newly improved imaging segmentation program, we report an average of approximately 1,100 RS after a 5-min pulse labeling of 3T3 mouse fibroblast cells in early S-phase. Pulse-chase-pulse double labeling experiments reveal that RS take approximately 45 min to complete replication. Appropriate calculations suggest that each RS contains an average of 1 mbp of DNA or approximately 6 average-sized replicons. Double pulse-double chase experiments demonstrate that the DNA sequences replicated at individual RS are precisely maintained temporally and spatially as the cell progresses through the cell cycle and into subsequent generations. By labeling replicated DNA at the G1/S borders for two consecutive cell generations, we show that the DNA synthesized at early S-phase is replicated at the same time and sites in the next round of replication.

Replicon clusters may form structurally stable complexes of chromatin and chromosomes

Nuclear DNA replication was monitored 'in situ' in pea nuclei with the bromodeoxyuridine antibody technique. The labelling appeared to be restricted to a number of finely distinct spots. The labelling was followed through three subsequent cell cycles in meristematic and differentiating pea root cells. The results show that the spots as seen just after the labelling persist distinctly over the mitotic chromosomes as well as in the nuclei of the following cell cycles up to 44 hours after the pulse. Moreover, they are also present in the nuclei of differentiating cells. The spots over the mitotic chromosomes in specific cases give rise to a dynamic banding. Nuclei of the second and third cycle show absence of labelling in specific zones, owing to the segregation of the labelled strands of chromosomal DNA. The maintenance of the spotted appearance of the replication clusters through all stages of the three subsequent cell cycles may be an indication in favour of the hypothesis that such clusters represent structurally stable replicon complexes held together by the nuclear matrix and the chromosome scaffold.

Aphidicolin as synchronizing agent in root tip meristems of Haplopappus gracilis

Aphidicolin, a reversible inhibitor of nuclear DNA replication, was tested as syncrhonizing agent in root tip meristems of Haplopappus gracilis. Embryos (i.e. decoated seeds) or 3-day-old seedlings were used to this purpose. After a 24 h treatment with the drug, a high level of synchrony was observed in both experimental materials for two cell cycles as assessed by determining the accumulation of cells in the S and M phases of the cycle. Highest synchronization was obtained with germinating embryos, possibly owing to a low degree of synchrony already existing in this system.

The role of membrane damage in radiation-induced cell death

Radiation-induced cell death is probably mediated primarily through deposition of energy, in single events, in a few vital macromolecules, or targets, the integrity of which is indispensable for proliferation. The genome is customarily regarded as the main target, but several lines of evidence support the inference that there are important consequences of events in nuclear membranes in eukaryotes, and plasma membrane in bacteria. The identification of a target depends to some extent on parallelism between modifications of biological damage to putative targets and to the cell as a whole. An important modifying procedure is removal of oxygen from the irradiated system. The presence of oxygen almost always sensitizes cells, but when model systems with biological function are irradiated extra-cellularly a high degree of sensitization by oxygen has been observed only with those in which membrane function is important. This makes sense because the lipid content of membranes renders them readily peroxidizable. When the quality of the radiation is changed, its effectiveness changes in opposite directions for subcellular model targets and for cells. This could be accounted for if interactions between lesions in membranes and in attached DNA play a substantial role in cellular radiation effects.