Multi-dimensional Visualization of Genetic Information: From DNA to Genome
As Mendel exactly speculated more than a century ago, genetic material exists and now we can visualize various statuses of the genetic materials of DNAs, chromosomes, and genomes. Genes with the size of 1 kbp can be visualized by fluorescence in situ hybridization (FISH) even on rice chromosomes with less than 2 mm at metaphase. Several useful genes in rice breeding, such as bacterial blight resistant, leaf blast resistant, and gall midge resistant genes have now directly been mapped on the rice chromosomes by FISH (Ohmido et al. PMB 1998).
DNA regions occupied by the specific nucleotide sequences are also visualized on DNA fibers by direct hybridization of the probe DNAs to the extended DNA fibers (EDFs) extracted from isolated nuclei and fixed on a glass slide. As a result, the spatial resolution of FISH goes down from 2 - 5 Mbp in the case of chromosomal FISH to some hundred base pairs in the EDF-FISH. The exact positioning of rice subtelomeric and telomeric repeats were resolved by using EDF-FISH (Ohmido submitted).
FISH using the total genomic DNA (GISH) is a powerful tool to identify the D genome rice chromosomes of a "lost" rice species with the D genome, which is only maintained in an allotetraploid species with CCDD genome. GISH clearly distinguishes the C genome chromosomes from D genome ones (Fukui et al. TAG 1997).
Comparison of recombination values and physical length of barley chromosomes revealed the specific concentration of the genes at the limited regions of both ends of the chromosomes. The enigma that the genes are separately located at the most distant places on a chromosome, is solved by the three-dimensional FISH. Using centromeric and telomeric probes in intact barley nuclei, the clustered locations of the centromeres and telomeres at the opposite poles were clearly visualized (Wako submitted).
As demonstrated above, we have now visualized structures of genetic materials from DNAs to genomes in the 20 century and are expecting to visualize functional significance of the genetic materials in the next century.
Department of Biotechnology, Graduate School of Engineering,