Single cell studies of the cell cycle and some models.

Analysis of growth and division often involves measurements made on cell populations, which tend to average data. The value of single cell analysis needs to be appreciated, and models based on findings from single cells should be taken into greater consideration in our understanding of the way in which cell size and division are co-ordinated. Examples are given of some single cell analyses in mammalian cells, yeast and other microorganisms.

Size control in growing yeast and mammalian cells.

Background: In a recent publication it was claimed that cultured mammalian cells, in contrast to yeasts, maintain a constant size distribution in the population without a size checkpoint. This inference may be challengeable.

Results: (1) It is argued that "weak" size control implies the existence of a checkpoint, and unfortunately the technique used by Conlon and Raff might obscure such a weak mechanism.

Growth during the cell cycle.

During the cell cycle, major bulk parameters such as volume, dry mass, total protein, and total RNA double and such growth is a fundamental property of the cell cycle. The patterns of growth in volume and total protein or RNA provide an "envelope" that contains and may restrict the gear wheels. The main parameters of cell cycle growth were established in the earlier work when people moved from this field to the reductionist approaches of molecular biology, but very little is known on the patterns of metabolism.

Mitotic control in the absence of cdc25 mitotic inducer in fission yeast.

Fission yeast cells tolerate the total absence of the cdc25 mitotic inducer in two cases, either in cdc2-3w or in wee1 genetic backgrounds. In the cdc2-3w cdc25Delta double mutant, the rate-limiting step leading to mitosis is reaching a critical size. However, the size control of this mutant operates in late G2, which is different from wild-type (WT) cells.