We have discovered that single-stranded DNA containing short guanine-rich motifs will self-associate at physiological salt concentrations to make four-stranded structures in which the strands run in parallel fashion. We believe these complexes are held together by guanines bonded to each other by Hoogsteen pairing. Such guanine-rich sequences occur in immunoglobulin switch regions, in gene promoters, and in chromosomal telomeres. We speculate that this self-recognition of guanine-rich motifs of DNA serves to bring together, and to zipper up in register, the four homologous chromatids during meiosis.

Formation of parallel four-stranded complexes by guanine-rich motifs in DNA and its implications for meiosis

▪ Abstract Meiotic chromosomes have been studied for many years, in part because of the fundamental life processes they represent, but also because meiosis involves the formation of homolog pairs, ...

Meiotic chromosomes: integrating structure and function.

▪ Abstract The separation of sister chromatids at the metaphase to anaphase transition is one of the most dramatic of all cellular events and is a crucial aspect of all sexual and asexual reproduction. The molecular basis for this process has until recently remained obscure. New research has identified proteins that hold sisters together while they are aligned on the metaphase plate. It has also shed insight into the mechanisms that dissolve sister chromatid cohesion during both mitosis and meiosis. These findings promise to provide insights into defects in chromosome segregation that occur in cancer cells and into the pathological pathways by which aneuploidy arises during meiosis.

Disseminating the Genome: Joining, Resolving, and Separating Sister Chromatids During Mitosis and Meiosis

A central role for cohesins in sister chromatid cohesion, formation of axial elements, and recombination during yeast meiosis.

Meiosis is a special type of cell division that produces haploid gametes from diploid parental cells. Chromosome number is reduced during meiosis because a single round of DNA replication is followed by two rounds of chromosome segregation (Fig. 1). Fusion of two gametes during sexual reproduction restores the diploid chromosome complement. The second division of meiosis (the equational division) resembles mitosis: Sister chromatids separate and segregate. The first division, however, is unique. Reductional chromosome segregation at meiosis I differs from mitosis and meiosis II in a number of respects. First, sister chromatids remain associated with each other. Second, the two copies of the same chromosome (called homologous chromosomes or homologs) behave in a coordinate fashion, such that one chromosome moves to one pole of the spindle apparatus and its homolog moves to the opposite pole. This coordination in chromosome behavior depends on complex processes and elaborate structures that bring homologs together during meiotic prophase and hold them together until the transition between metaphase I and anaphase I. In most organisms, the relevant processes include alignment of homologs, assembly of the synaptonemal complex (SC, described below), genetic recombination, and the formation of chiasmata (stable connections between homologs formed at the sites of crossovers). These events occur during a very lengthy prophase that is divided into a series of substages based on changes in chromosome morphology (Table 1). This article reviews our current knowledge of the processes and structures that promote and maintain interactions between homologs and thereby ensure proper reductional chromosome segregation. Emphasis is placed on observations made in recent years, particularly those that have enhanced our understanding of the molecular mechanisms underlying meiotic chromosome behavior.

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Meiotic chromosomes: it takes two to tango

The meiotic cytological behavior of chromosomes in maize microsporocytes homozygous for the recessive mutant desynaptic was studied at various stages. It was found that following apparently normal pachytene synapsis there appears to be sporadic precocious desynapsis. By diakinesis bivalents heterozygous for a distal knob have often separated to pairs of univalents, each with a knob-carrying and a knobless chromatid. From the frequency of such events it is inferred that the crossover process is probably not affected by the mutant and that the genetic defect affects instead a distinct function concerned with chiasma maintenance following crossing over. Since precocious separation of dyads to monads at prophase II was also found in the desynaptic material, it is suggested that normal chiasma maintenance until anaphase I and normal dyad integrity maintenance between anaphase I and anaphase II may depend upon the same mechanism; it is also suggested that this may involve a special tendency for cohesiveness of sister chromatids during meiosis, beyond that which is ordinarily found at mitosis.

Evidence for separate genetic control of crossing over and chiasma maintenance in maize

Relative positions of homologous heterochromatic regions of maize chromosomes were studied at premeiotic interphase, at tapetal mitotic interphase and at root tip mitotic interphase. In all three kinds of cell homologues were found to be situated significantly nearer to each other than to heterologues. It is concluded that some degree of homologous chromosome pairing may occur widely at anaphase or telophase (where it is easily overlooked) and that therefore, as has been previously suggested, homologues may be loosely aligned throughout premeiotic interphase in preparation for their subsequent synapsis.

Evidence for homologous pairing of chromosomes prior to meiotic prophase in maize

The need for a chiasma binder

The mechanism of meiotic homologue pairing.

Commonly accepted precepts are challenged : (1) that homologous chromosome pairing is normally mediated by nuclear envelope attachment sites; (2) that crossover site establishment awaits synaptic completion; and (3) that it is the function of the synaptonemal complex to hold homologues in register so that equal crossing over can occur, and perhaps to provide machinery for the crossover process. Although these views may eventually be shown to be true, it is felt that currently available evidence does not warrant their full acceptance, and that alternatives should be considered. As examples of alternatives the following ideas, with some supporting evidence, are suggested: (1) homologous chromsome pairing (in non-haplont organisms) may be accomplished by chance meeting of homologue segments (followed by establishment of invisible, elastic connectors) at congression for a mitotic metaphase (in many cases perhaps the premeiotic mitosis); (2) crossover sites may be established before, during, or immediately following initiation of synapsis; and (3) the synaptonemal complex may somehow function in the crossover process at the inception of its formation, but its complete deployment throughout each normal bivalent may serve some other role, such as mediation of the binding of sister chromatids apparently required for chiasma maintenance until anaphase I.

Marjorie P. Maguire

Papers

Evidence for separate genetic control of crossing over and chiasma maintenance in maize

Evidence for homologous pairing of chromosomes prior to meiotic prophase in maize

The need for a chiasma binder

The mechanism of meiotic homologue pairing.

Homologous chromosome pairing.