Chromosomal compositional mapping at a 850-band resolution

Compositional mapping can also be done at the chromosomal level by in situ hybridization of compositional DNA fractions corresponding to different isochore families. In the first investigation of this kind (Saccone et al., 1992), it was shown that the hybridization of a DNA fraction derived from H3 isochores produced the highest concentration of signals on two largely coincident subsets of R bands (Figs. 7.4, 7.5) (i) the T bands (Dutrillaux, 1973), which are the most heat-denaturation-resistant R bands and (ii) the chromomycin 

A3-posilivc, DAPI-iicgalivc bands (Ambros and Sumner, 1987), which are the GC-richcsl bands of human chromosomes (DA PI is 4,6 diamino-2-phenyl indole). n situ hybridization of H3 isochore DNA established that T bands comprise GC-rich, gene-rich, single-copy DNA. Indeed Ihc contribution of rcpctilivc DNAs was suppressed by competition with excess unlabeled total human DNA and Alu sequences. The latter are the most abundant type of short interspersed elements, which have their highest density in the GC-richest isochorcs H2 and H3 (see Part 6). This also ruled out the possibility that T bands corresponded to GC-rich satellite DNAs, which was a distinct possibility until then. 

In subsequent investigations (Saccone et al., 1993), the chromosomal locations of the other isochorc families Ll-fL2, H1 and H2 were studied by using an improved competition protocol with excess Q11 DNA, which eliminated the need for a statistical approach. This established (i) that T bands contain not only H3 isochores, but also H2 and some HI isochorcs (ii) that R bands i.e., R bands exclusive of T or H3 bands) arc formed, on the average and to almost equal extents, by HI and I, isochores, H2 and H3 isochores being only rarely present and (iii) that G bands essentially consist of T isochorcs, HI isochores being present a I low levels. 

In further work (Saccone ci al., 1996), il was shown that (i) the number of bands 

On the basis of the above results and of the estimated band sizes (i rancke, 1994), it can be calculated that about 17% of all bands at a 850-band resolution contain H3 isochores 

---