Review Cell Division from a Genetic Perspective
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REVIEW CELL DIVISION FROM A GENETIC PERSPECTIVE LELAND H. HARTWELL From the Department of Genetics, University of Washington, Seattle, Washington 98195 Recently, a number of laboratories have begun to incubation at the restrictive condition for that study mutant cells that are defective in specific mutation, whereas mutants with defects in one of stages of the eukaryotic cell cycle. The long-range the continuously required functions will arrest at goals of this work are to identify the genes that the restrictive temperature with cells at a variety code for division-related proteins, to define the of positions in the cell cycle. roles that these gene products play and to investi- Classes of mutants may be distinguished from gate the hierarchies of order that assure their one another and the roles of their products delim- coordinated activity. It is my intent in this brief ited by determining the stage-specific event at review to discuss the strategies employed in this which they arrest. It is convenient to have a genetic approach and to enumerate some of the designation for the first landmark of the cell cycle new conclusions that have come to light. A recent that is blocked in a particular mutant, and I shall review on the genetics of meiosis (2) complements call it the diagnostic landmark for that mutant. this review on mitosis. Mutants of Saccharomyces cerevisiae have been identified that have diagnostic landmarks at spin- MUTANTS dle pole body (SPB) duplication, SPB separation, Mutations that inactivate gene products essential initiation of DNA synthesis, DNA replication, for division would be lethal. Cell cycle mutants each of two stages of nuclear division, cytokinesis, must be therefore either temperature-sensitive or and bud emergence (8, 9, 25). In Schizosaccharo- suppressor-sensitive conditional mutants in order myces pombe, diagnostic landmarks have been to be useful for study. Most reports to date have localized to DNA replication, nuclear division, utilized the former, but the availability of temper- and two stages of cell plate formation (42). Mu- ature-sensitive nonsense suppressors (46) should tants with diagnostic landmarks at DNA replica- permit study of the latter as well. tion (44), at least two stages of nuclear division Although any mutation that blocks cell division (38, 44), and septation (38) have been detected could be considered a cell-cycle mutation, the in Aspergillus nidulans; some of the nuclear divi- term will be reserved here for mutations that lead sion mutants arrest with condensed chromosomes to defects in, or failure of, a stage-specific event characteristic of a metaphase block (39, 44). (or landmark) of the cell cycle, such as DNA Tetrahymena pyriformis mutants with diagnostic replication, or nuclear division, events which nor- landmarks at two stages of cell division, formation mally occur but once each cycle. This definition of of the fisson zone and constriction of the fission a cell cycle mutant excludes mutations in genes zone have been found (17). The work with Usti- whose products control the continuous processes lago maydis mutants has concentrated upon le- of growth and metabolism and permits the stage- sions affecting DNA replication and recombina- specific mutations to be distinguished empirically: tion (58); a mutant with a thermolabile DNA an asynchronous population of mutant cells will polymerase has been described (33). In both become arrested at one cell cycle landmark after Physarum polycephalum (19, 61) and Chlamydo- J. CELL BIOLOGY ~) The Rockefeller University Press 0021-9525/7810601-062751.00 627 monas reinhardtii (30), mutants have been iso- bination, mutation, or deletion, because each type lated and studied that may be cell cycle mutants, of aberration produces a unique pattern of spots. but their characterization has not progressed far Mutations at 13 loci, originally isolated for their enough to identify the diagnostic landmarks. A effects upon meiosis, have been examined. Six of number of mutants have been described for ver- them produce mitotic chromosome instability, and tebrate animal cells (reviewed in reference 4); at least three of these are also abnormally sensitive their properties suggest diagnostic landmarks in to mutagenic agents, suggesting defects in DNA G1 (7, 12, 35, 49, 50, 51, 56), at DNA synthesis repair. Four influence the frequency of chromo- (50, 52, 54), at mitosis (60), or at cytokinesis some nondisjunction and/or chromosome loss. (28), although not all have been rigorously dem- The mutants studied to date represent only onstrated to achieve synchronous arrest from an initial forays into the genetics of cell division, yet initially asynchronous culture. they demonstrate that stage-specific mutants can Mutants with specific effects on mitosis are also be found for each landmark. Hundreds of cell being studied systematically and elegantly in Dro- cycle genes will probably be found in any species, sophila melanogaster (reference 1; B. Baker, A. if and when systematic and exhaustive searches Carpenter, and P. Ripoll, personal communica- are conducted. tion). Although most of the mutations that have been studied to date appear primarily to affect ORDER genetic recombination and/or DNA repair, the One of the impressive characteristics of cells is the Drosophila studies merit discussion because the reproducibility with which they achieve the com- methodology is applicable to a large variety of plicated task of division. Although potentially mitotic lesions and because the information al- lethal pitfalls exist at every stage of the cell cycle, ready attained is relevant to several topics consid- cells accomplish with only rare mistakes the faith- ered below. The types of mutations utilized in the ful replication of each gene, packaging of every Drosophila studies and, consequently, the experi- chromosome, disjunction of sister chromatids, and mental rationales used to extract information from distribution of nuclei and other organelles to them are distinctly different from those of the daughter cells. The numerous biochemical and other organisms reviewed here. Whereas other morphological events in each of these processes investigators have concentrated specifically upon are undoubtedly rigorously ordered with respect mutants displaying an absolute block at some to one another. Studies of mutants are revealing stage of the mitotic cycle, the Drosophila work some of the underlying events responsible for this employs mutants having less deleterious mitotic order. defects which do not prevent the extensive cell division that is essential for the construction of an Temporal Order adult fly.1 In order to study such mutants, fly Conditional mutants lend themselves to an in- embryos homozygous for the mitotic mutation in vestigation of the temporal order of events be- question and heterozygous for various recessive, cause a single gene product can be inactivated at cell autonomous genes that affect body color or any time during the division cycle simply by bristle shape are constructed by appropriate ge- shifting cells from the permissive to the restrictive netic crosses. The influence of the mutation upon condition. With temperature-sensitive mutants mitosis is revealed through analysis of the number, one can determine the point (termed the execu- size, and distribution of patches of mutant tissue tion or transition point) in the cell cycle at which on the surface of the fly, patches which result from the temperature-sensitive event has been com- abnormalities in chromosome behavior during de- pleted since, before this point, cells are incapable velopment of the integument. It is possible to of dividing upon a shift to restrictive temperature, detect and distinguish by these measurements but after this point they are capable of division mitotic chromosome loss, nondisjunction, recom- (Fig. 1). It should, however, be noted that in the It should be noted that strategies exist for studying the absence of additional biochemical information effects on development of mutations that completely about the defective gene product, the molecular arrest cell division (48), although none of the mutants basis of the execution point is ambiguous because studied to date is known to produce a stage-specific proteins can be thermolabile in at least two differ- arrest. ent ways: for synthesis or for function. If the 628 THE JOURNAL OF CELL BIOLOGY" VOLUME 77, 1978 DNA replication upon a shift to the restrictive temperature; both mutants have execution points near the end of the DNA synthetic period (20, 26). The coincident occurrence of execution point and diagnostic landmark, found for these and many other mutants of S. cerevisiae (23), also obtains for 10 of 13 cdc genes of Schizosaccharo- myces pombe (42), for 7 of 9 mutants in Aspergil- lus nidulans (45), and for all 3 of the fully penetrant mutants of Tetrahyrnena pyriforrnis (J. Frankel, personal communication). This result implies that the thermolabile gene product in each of these mutants is synthesized or functions at the time of occurrence of the diagnostic landmark and suggests that the gene product in question plays a role intrinsic to this stage-specific event. In such cases, a biochemical analysis of the mutants for enzymes thought to be involved in this landmark FmuR~ 1 Determination by time-lapse photomicros- is in order. Indeed, the cdc 21 S. cerevisiae mutant copy of the execution point in a temperature-sensitive mentioned above has been shown to be defective cdc mutant of the budding yeast, Saccharomyces