(b) Surface structure of an anaphase chromosome of
barley.Published in Probe Volume 5(2): April-June 1995
Regina Martin, Gerhard Wanner, and Reinhold G. Herrmann
Botanisches Institut der Ludwig-Maximilians-Universität,
Monzinger Stabe 67
D-80638 Munchen, Germany
The past two decades have seen enormous progress towards understanding eukaryotic genes and genomes. Recombinant DNA technology has provided this power and hardly any contemporary experiment of gene structure and function is done today without recourse to methods of molecular biology. Considerable detail is known of the fine structure of genes and regulatory proteins that control not only single metabolic steps but even complicated developmental processes.
It became obvious, however, that a gene can no longer be portrayed only one-dimensionally and that an understanding of the dimensions between the macromolecule DNA and the cytological entity chromosome, with its periodically recurring conformational changes, will be required. Yet the relationships between topographical, physical, and genetic distances of genes and their impact on genetic events, gene expression, and evolution have largely escaped analysis.
It is therefore desirable to unravel chromosome structure and gene arrangement at the ultrastructural level. Modern high-resolution field emission scanning electron microscopy (FESEM) provides a powerful tool for studying such ultrastructural detail. The resolution ofthe instrument is in the order of 1-2 nm, and the detection of fibrillar structures in chromosomes of less than 10 nm corresponding to the elementary fiber has recently become possible. An optimized preparation technique allows the production of a high number of spreads of surface-exposed chromosomes that are appropriate for examination of chromosomal fine structure at the different stages of the cell cycle (Martin et al., Chromosome Res. 2, 411-415, 1994).
Metaphase and anaphase chromosomes are characterized by a highly compact structure of chromatin. High resolution shows a smooth surface and a preponderance of fibers of about 30 nm (fig. 1). Because of the high compaction of chromatin fibers, it is impossible to detect the basic chromatin organization at this stage. More detail can be seen in condensing or decondensing chromosomes (fig. 2), for instance, in prophase chromosomes that exhibit a less compact fiber arrangement. Furthermore, in situ hybridization by signal detection of gold-labeled probes in chromosomes with a relatively low degree of condensation in the FESEM may aid in unraveling the unknown detail of sequence (gene) arrangement as well as the higher three-dimensional order of chromatin fibres.
(b) Surface structure of an anaphase chromosome of
barley.
(a) Electron micrograph of prophase chromosomes of barley, cv.
Marinka.
(b)
Telomeric region of a prophase chromosome from (a).