VISUALIZATION OF ELEMENTARY DNA-REPLICATION UNITS IN HUMAN NUCLEI CORRESPONDING IN SIZE TO DNA LOOP DOMAINS

Newly synthesized DNA in mammalian nuclei is concentrated in discrete nuclear granules called replication foci. These foci may be visualized using antibodies against 5-bromodeoxyuridine. In the early S-phase ce lls 100-250 foci are usually detected. On average, individual foci ran ge between 0.5 and 2 mu m in diameter and can be seen as clusters of m ore than ten average-sized (60-100 kb) synchronously activated replico ns. In this study, employing minor modifications of the previous metho ds, we report the visualization of small replication foci of about 0.3 mu m diameter (mini-foci). Some foci are clustered into folded chains consisting of 2-40 subunits. DNA content of one mini-focus is estimat ed to be 50-120 kb and there are 500-1500 mini-foci per cell in the ea rly S-phase. Experimentally induced decrease in replicon size does not affect the size of mini-foci, suggesting that these represent element ary units of DNA replication in mammalian nuclei and are probably iden tical to the basic structural DNA loop domains.