Cell Biology lnternational Reports, Vol. 16, No. 8, 1992

749

SYNAPTOSOMAL COMPLEXES AND THE ORGAN/ZATION OF CHROMATIN DURING MEIOTIC PROPHASE

C. Heyting ~ and A.J.J. Dietrich 2

tDept. Genetics, Agricultural University, Dreijenlaan 2, 6703 HA Wageningen, NL; and 2Dept. Human Genetics, Univ. Amsterdam, Academic Medical Centre, Meibergdreef 15, 1105 AZ Amsterdam, NL

INTRODUCTION

In the meiotic prophase nucleus, the relation between the structural organization of chromatin and function appears more evident than in any other cell type: The chromatin undergoes a series of well-defined structural alterations. Some of these can be considered as preparations to the requirements which the products of meiosis will have to fulfil. These changes vary widely, depending on the species and sex of the individual in which meiosis takes place. Examples of such alterations are the formation of lampbrush chromosomes with decondensed actively transcribed loops in oocytes of various animals (Bostock and Sumner, 1978), the association of certain testis-specific histones with chromatin of rat spermatocytes (reviewed by Meistrich and Brock, 1987), and the incorporation of specific lamins in the nuclear envelope of primary oocytes of the rat (reviewed by Krohne and Benavente, 1986). Other changes in chromatin organization however, are directly related to the process of meiosis itself, namely pairing of homologous chromosomes, reciprocal exchanges between non-sister chromatids, and segregation of homologous chromosomes. The result of this series of events is that at meiosis I diploid cells divide to produce haploid cells with novel assortments of genes. This review will deal with these meiosis-specific changes. How are they accomplished? And for what purpose? ORDER OF CHANGES IN CHROMATIN ORGANIZATION

The sequence of meiosis-specific rearrangements of chromatin is remarkably similar among species. In (almost) all eukaryotes analysed thus far, the condensation, pairing, recombination and segregation of chromosomes is accompagnied by the assembly and disassembly of nuclear organelles which are specific for meiotic prophase nuclei, the synaptonemal complexes or SCs (Moses, 1968). These arc flat, zippedike structures which appear between paired homologous chromosomes. They consist of two compact proteinaceous axes, one along each homologue, the lateral elements or LEs. The LEs are connected by thin transversal filaments. On the transversal filaments between the lateral elements there is another longitudinal structure, the central element or CE. Both LEs together with the CE make up the tripartite structure of the SC; homologues are called synapsed if they are connected by this tripartite structure. Initially, only exactly corresponding pans of homologous chromosomes appear to be synapsed. However, in some 0309-1651/92/080749-12/$03.00/0

© 1992 Academic Press Ltd

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Synaptosomal complexes and the organization of chromatin during meiotic prophase.

Cell Biology lnternational Reports, Vol. 16, No. 8, 1992 749 SYNAPTOSOMAL COMPLEXES AND THE ORGAN/ZATION OF CHROMATIN DURING MEIOTIC PROPHASE C. He...
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