Genetica 78: 57-62, 1989 © Kluwer Academic Publishers, Dordrecht - Printed in the Netherlands 57 Evolution of digestibility by Hind III: an analysis by light and electron microscopy J. M6ndez l, A. M. Gonz~tlez 1, A. M. Insua ~ & V. J. Goyanes 2 1Departamento de Gendtica, Colegio Universitario de La Corufla, Universidad de Santiago, 15071 La Corufla, Spain 2Secci6n de Gen~tica, Hospital "T. Herrera" (INSALUD), 15006 La Corufia, Spain Reprint requests to be addressed to J. M~ndez Received 8.2.1988 Accepted in revised form 16.9.1988 Abstract Digestion of Chinese hamster metaphase chromosomes from the Don ceil line by Hind III restriction en- donuclease followed by Giemsa staining were analysed by light and electron microscopy. The evolution of di- gestibility was studied and four digestion stages were characterized by different levels of chromosome structure. Three different condensation stages were established according to morphological criteria of length, width and separation among chromatids. It was observed that there are statistically significant differences in the diges- tion progress at the three condensation stages previously defined. Introduction Restriction endonucleases have been used to test the differential organization of chromatin at different chromosomal regions on mammal metaphase preparations. These enzymes have been described as able to induce different banding patterns on fixed chromosomes (Sahasrabuddhe et al., 1978; Mez- zanotte et al., 1983; Miller et al., 1983). Such patterns include C-, g- and NOR banding. Chromosomes treated with restriction en- donucleases show the same conventional chro- mosomal pattern obtained by other G-bands and NOR methods, but it has been established that mechanisms are different in each case (Comings, 1978; Sumner, 1982; Lica & Hamkalo, 1983). C- banding patterns obtained by endonuclease diges- tion are not coincident with those previously described (Bianchi et al., 1984). The effect of digestion with restriction en- donucleases followed by Giemsa staining suggests that structural organization of chromatin plays a primary role in determining enzymatic activity on different chromosomal DNA regions (Sahas- rabuddhe et al., 1978; Lica & Hamkalo, 1983; Mez- zanotte et al., 1983). In this work, fixed Chinese hamster metaphase chromosomes have been treated by Hind III en- donuclease in an attempt to determine a relationship between chromosomal digestion and chromatin fiber packing. Metaphase preparations have been analysed by light and electron microscopy. We have established three different condensation stages: low condensa- tion (LC), medium condensation (MC) and high condensation (HC), and evaluated the different digestion degree at each stage. Material and methods Chromosome preparation and digestion by restriction endonuclease Metaphase chromosomes were obtained from Chi-