Whole mount electron microscopy of meiotic chromosomes and the synaptonemal complex
- 76 Downloads
The mechanism by which homologous chromosomes pair and crossover has been a major unsolved problem in genetics. Thin section electron microscopy of the synaptonemal complex has not provided enough details to allow any significant insight into this problem. Whole mount preparations of the testis of mice, quail, crayfish, and frogs provided a striking improvement in visualization of the morphological features of meiotic chromosomes. These studies, when combined with the use of deoxyribonuclease and trypsin allowed the following conclusions. 1. The synaptonemal complex (lateral and central elements with connecting L-C fibers) is composed of protein. 2. Contrary to common speculation the central element is not the pairing surface of homologous chromosomes. 3. The L-C fibers, averaging 75–100 Å in width, extend from the lateral elements and meet to form the central element which is usually composed of four fibers. 4. During leptotene, homologous axial elements, although unpaired for most of their length, attach next to each other at the nuclear membrane. 5. Short segments of the chromatin fibers attach to the lateral elements. These points of attachment are clustered, producing the chromomeres seen by light microscopy. 6. The chromatin fibers extend out from the lateral element as loops. Lampbrush chromosomes are thus not restricted to oogenesis but are common to all meiotic chromosomes.
Since the morphological features of the central element of the synaptonemal complex persist despite extensive deoxyribonuclease digestion, pairing is perhaps best visualized as a two-step process consisting of a) chromosomal pairing during which the proteinaceous synaptonemal complex pulls homologous chromosomes into approximate association with each other, and b) molecular pairing, which probably takes place in the area around the synaptonemal complex.
KeywordsChromosomal Pairing Central Element Homologous Chromosome Synaptonemal Complex Meiotic Chromosome
Unable to display preview. Download preview PDF.
- Anderson, T. F.: Techniques for the preservation of three-dimensional structure in preparing specimens for the electron microscope. Trans. N.Y. Acad. Sci. 13, (Part II) 130–134 (1951).Google Scholar
- Brinkley, B. R., Bryan, J. H. D.: The ultrastructure of meiotic prophase chromosomes as revealed by silver-aldehyde staining. J. Cell Biol. 23, 63–78 (1964).Google Scholar
- Holliday, R.: Genetic recombination in fungi. In: Replication and recombination in genetic material (W. J. Peacock and R. D. Brock, eds.), p. 157–174. Canberra: Australian Academy of Science 1968.Google Scholar
- Hoyer, B. H., McCarthy, B. J., Bolton, E. T.: A molecular approach in the systematics of higher organisms. Science 144, 959–967 (1964).Google Scholar
- Makino, S.: A review of the chromosome numbers in animals. Japan: Hokuvyukan 1956.Google Scholar
- Menzel, M. Y., Price, J. M.: Fine structure of synapsed chromosomes in F1 Lycopersicon esculentum-Solanum lycopersicoides and its parents. Amer. J. Bot. 53, 1079–1086 (1966).Google Scholar
- Mirsky, A. E., Ris, H.: The deoxyribonucleic acid content of animal cells and its evolutionary significance. J. gen. Physiol. 34, 451–462 (1951).Google Scholar
- Moses, M. J.: Chromosomal structures in crayfish spermatocytes. J. biophys. biochem. Cytol. 2, 215–218 (1956).Google Scholar
- —: Synaptinemal complex. Ann. Rev. Genetics 2, 363–412 (1968).Google Scholar
- —: Structure and function of the synaptonemal complex. Genetics 61, Suppl. 41–51 (1969).Google Scholar
- —, Coleman, J. R.: Structural patterns and the functional organization of chromosomes. In: The role of chromosomes in development (M. Locke, ed.), p. 11–49. New York: Academic Press Inc. 1964.Google Scholar
- Nebel, B. R., Coulon, E. M.: Enzyme effects on pachytene chromosomes of the male pigeon evaluated with the electron microscope. Chromosoma (Berl.) 13, 292–299 (1962).Google Scholar
- Ris, H., Chandler, B. L.: The ultrastructure of genetic systems in prokaryotes and eukaryotes. Cold Spr. Harb. Symp. quant. Biol. 28, 1–8 (1963).Google Scholar
- Thomas, C. A.: Recombination of DNA molecules. In: Progress in nucleic acid research and molecular biology (J. N. Davidson and W. E. Cohn, eds.), vol. 5, p. 315–337 (1966).Google Scholar
- Whitehouse, H. L. K., Hastings, P. J.: An analysis of genetic recombination on the polaron hybrid DNA model. Genetic Res. 6, 27–92 (1965).Google Scholar
- Wolfe, S. L., Hewitt, G. M.: The strandedness of meiotic chromosomes from Oncopeltus. J. Cell Biol. 31, 31–42 (1966).Google Scholar
- —, John, B.: The organization and ultrastrueture of male meiotic chromosomes in Oncopeltus jasciatus. Chromosoma (Berl.) 17, 85–103 (1965).Google Scholar
- Wolstenholme, D. R., Meyer, G. F.: Some facts concerning the nature and formation of axial core structures in spermatids of Gryllus domesticus. Chromosoma (Berl.) 18, 272–286 (1966).Google Scholar