THE DNA METHYLATION PATTERN OF ALLIUM-CEPA METAPHASE CHROMOSOMES

The DNA methylation pattern of Allium cepa metaphase chromosomes has b een investigated by using a monoclonal antibody to 5-methylcytosine (a nti 5-mC). Our results indicate that 5-methylcytosine (5-mC) mainly oc curs in heterochromatic chromosome regions; indeed a strong binding of the antibody is, as a rule, detectable in the same regions showing C- bands. A preferential binding of anti 5-mC to telomeric regions occurs , but it is also evident in the proximal regions of almost all chromos ome arms and in some intercalary bands. The chromosomal localization o f methylated sequences does not overlap that of highly repeated DNA an d/or satellite sequences. When the frequency of labelling over each ch romosome region is taken into account, remarkable differences result. While the telomeric regions are always methylated, lower percentages o f labelling at centromeric and intercalary bands are, as a rule, detec table. In some cells, a differing methylation pattern is detectable in homologous regions of chromosome pairs V and VII.