Studies on DNA replication during synchronized vegetative growth and gametic differentiation in Chlamydomonas reinhardtii

Abstract

DNA synthesis occurs just prior to nuclear division in synchronized vegetative cultures of Chlamydomonas reinhardtii. The above timing of DNA replication is also observed during gametic differentiation which involves the division of a vegetative cell into four gametes in nitrogen-free medium. When vegetative cells divide to produce four vegetative daughter cells within a single cell cycle, a round of DNA replication appears to precede each of the two consecutive mitotic events. If synchronized vegetative cultures are grown under conditions of limiting light intensity, two rounds of DNA replication may occur while only one nuclear division follows. The vegetative cells produced by such a division contain twice the normal vegetative complement of DNA within a single nucleus. These cells divide without DNA synthesis about 24 h later yielding two cells with the normal vegetative amount of DNA. 15N transfer experiments [14, 17]were performed in order to corroborate the chemical determinations of DNA synthesis and to establish conclusively the exact number of replications of the cellular genome during vegetative growth, gametic differentiation and gametic dedifferentiation. In all cases, DNA replication was by a semiconservative mechanism as previously observed by Sueoka [17]. No appreciable synthesis and accumulation of “M-band” DNA, characteristic of zygote maturation, was observed in these other phases of the Chlamydomonas reinhardtii life cycle. Furthermore, the DNA content per cell never reached the lower limit observed in zoospores produced after meiosis which contain one half the amount of DNA present in gametes and vegetative cells. The results of the present study are discussed in terms of the Chlamydomonas life cycle with particular reference to previous studies of DNA replication during meiosis in this strain of Chlamydomonas.