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Riddle DL, Blumenthal T, Meyer BJ, et al., editors. C. elegans II. 2nd edition. Cold Spring Harbor (NY): Cold Spring Harbor Laboratory Press; 1997.

Cover of C. elegans II

C. elegans II. 2nd edition.

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Section IIntroduction

The mitotic chromosomes of Caenorhabditis elegans, and those of certain other organisms, including some plants, protozoa, insects, and other nematodes, are remarkable for having a well-differentiated kinetochore that extends along the entire poleward face of the metaphase chromosome. Microtubule attachment is distributed along the chromosome, and the chromosomes move broadside on toward the spindle poles. Chromosomes with this structure are referred to as holocentric or holokinetic, in contrast to monocentric chromosomes in which a single centromeric region may be distinguished at the primary constriction. Holocentric chromosomes typically behave differently in meiosis and mitosis. In meiosis, the nonlocalized kinetochore is absent, and in most organisms that have been examined, no structural differentiation of a kinetochore can be seen. Instead, microtubules appear to insert directly into the chromatin. The ends of the chromosomes are also said to adopt “kinetic activity” in meiosis, referring to the fact that in the meiotic divisions, the chromosomes move end on toward the spindle poles.

Studies on mitotic and meiotic segregation of C. elegans chromosomes have established that despite their holokinetic organization, these chromosomes share many features and behaviors with the more commonly studied higher eukaryotic monocentric chromosomes. They have similar telomere sequences and a trilaminar kinetochore structure which resembles that of monocentric chromosomes. C. elegans chromosomes undergo all of the classically described stages of meiotic prophase, culminating in a reductional division at meiosis I and equational division at meiosis II. Further homologous chromosomes in C. elegans rely on the formation of crossovers to ensure their proper disjunction at meiosis I. Thus, the study of mitotic and meiotic chromosome behavior in C. elegans can provide insight about conserved mechanisms governing chromosome behavior as well as about mechanisms specific to the segregation of holokinetic chromosomes.

In this chapter, we first review what is known about the structure and organization of mitotic chromosomes and discuss mitotic segregation of fusion chromosomes, chromosome fragments, and microinjected DNA in the context of holokinetic chromosome organization. We also briefly mention several mutations affecting mitotic segregation and cell division. In the second part of the chapter, we review the meiotic process, beginning with a cytological description of meiotic prophase and the meiotic divisions. This includes a discussion of the role of chiasma formation in determining the orientation of homologous chromosomes in late prophase and metaphase I, evidence that chromosome ends may adopt some centromeric functions during both meiosis I and meiosis II, and the genetic identification of trans-acting factors involved in assembly of the oocyte meiotic spindle. We then go on to discuss how the requirement for, and regulation of, crossover formation during meiosis is reflected in the organization of the genetic and physical maps. We review evidence for, and properties of, cis-acting chromosomal features that have key roles in promoting pairing and crossover formation between homologous chromosomes. Finally, we discuss how the analysis of mutants defective in meiotic segregation has led to the identification of numerous genes encoding trans-acting factors involved in pairing and crossover formation.

Copyright © 1997, Cold Spring Harbor Laboratory Press.
Bookshelf ID: NBK20131


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