No abstract

A re-examination of the mode of vegetative nuclear division in Neurospora craesa was facilitated by the availability of the mutant "clock" which produces definite growth bands. Since the growth rhythm is correlated with nuclear divisions, stained mycelial mats of this mutant prepared at intervals from the beginning of a growth period provided a sequence of stages of division. In a 28-hour period the following broad features of nuclear behavior were observed: In the early part of the period during rapid mycelial growth, dividing elongated nuclei predominated. At the end of the period the mycelium contained mostly rounded resting nuclei. In the middle of a growth period nuclear forms of various degrees of annularity occurred along with elongated and rounded nuclei. Elongated and rounded nuclei completed division cycles without change in form, although the corresponding stages of the two types were similar. Elongated nuclei assumed a spiral form at the beginning of division. As division proceeded, relaxation of the nuclear gyres was accompanied by a visible duplication of the chromatin thread and the appearance of chromomere-like bodies on the daughter threads. One of the ehromomere-like bodies became displaced and was interpreted to be a chromosome or a segment of a chromosome that acts as a mitotic center. All the chromosomes were found to be interconnected and to act as a unit throughout the division cycle. Only after the separation of the daughter chromatin threads could seven chromosomes be counted. Electron microscopic studies complemented the observations with the light microscope. On the basis of the evidence it was concluded that the vegetative nuclear division in Neurospora differs from the classical mitotic pattern in the following respects: (1) absence of visible centrioles, (2) the presence of interconnected chromosomes, (3) the comparatively late appearance of countable chromosomes, and (4) the frequent presence of interzonal connections between separating chromatin threads.

A re-examination of the mode of vegetative nuclear division in Neurospora craesa was facilitated by the availability of the mutant "clock" which produces definite growth bands. Since the growth rhythm is correlated with nuclear divisions, stained mycelial mats of this mutant prepared at intervals from the beginning of a growth period provided a sequence of stages of division. In a 28-hour period the following broad features of nuclear behavior were observed: In the early part of the period during rapid mycelial growth, dividing elongated nuclei predominated. At the end of the period the mycelium contained mostly rounded resting nuclei. In the middle of a growth period nuclear forms of various degrees of annularity occurred along with elongated and rounded nuclei. Elongated and rounded nuclei completed division cycles without change in form, although the corresponding stages of the two types were similar. Elongated nuclei assumed a spiral form at the beginning of division. As division proceeded, relaxation of the nuclear gyres was accompanied by a visible duplication of the chromatin thread and the appearance of chromomere-like bodies on the daughter threads. One of the ehromomere-like bodies became displaced and was interpreted to be a chromosome or a segment of a chromosome that acts as a mitotic center. All the chromosomes were found to be interconnected and to act as a unit throughout the division cycle. Only after the separation of the daughter chromatin threads could seven chromosomes be counted. Electron microscopic studies complemented the observations with the light microscope. On the basis of the evidence it was concluded that the vegetative nuclear division in Neurospora differs from the classical mitotic pattern in the following respects: (1) absence of visible centrioles, (2) the presence of interconnected chromosomes, (3) the comparatively late appearance of countable chromosomes, and (4) the frequent presence of interzonal connections between separating chromatin threads.

No abstract