34 DNA Replication in Physarum

Abstract

The natural synchrony of the nuclear cycle within the plasmodium of the myxomycete Physarum polycephalum provides an opportunity to study, without the need for any cell treatment, the complex and transient patterns of replication intermediates generated during chromosomal replication. In this review, we focus on the parameters of replication kinetics from the synthesis of Okazaki fragments to the chromosome-sized progeny molecules and summarize data suggesting that their appearance at some specific loci is precisely programmed and intimately linked to the transcriptional activity of the cell. STRUCTURE OF S PHASE The Plasmodium In one phase of their life cycle, the myxomycetes develop a particular multinucleated cell type, the plasmodium, that results from nuclear divisions in the absence of cell division (Rusch 1980). Deposited on a filter paper, small plasmodia cultivated in shaken liquid cultures fuse spontaneously into a single cell (5–10 cm in diameter) that contains more than 10 8 diploid nuclei (Fig. 1A). Being in a common cytoplasm, these nuclei divide synchronously every 10 hours. Their division is characterized by the persistence of the nuclear envelope throughout the mitotic process. This “closed” mitosis should not be seen as a loss of capability for cell division, but rather as the result of a developmental process, since in the amoebal, haploid vegetative phase of Physarum, the uninucleated cells divide by a conventional mitosis in which the nuclear envelope disintegrates in prophase. Thus, the Physarum genome encodes different cell types, with the plasmodial stage particularly suitable for DNA replication studies. The Synchronous Nuclear...