3 Chromosome Organization, Mitosis, and Meiosis

Abstract

I. INTRODUCTION
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...