Multicentric chromosomes are often found in tumor cells and certain cell lines. How they are generated is not fully understood, though their stability suggests that they are non-functional during chromosome segregation. Growing evidence has implicated microtubule motor proteins in attachment of chromosomes to the mitotic spindle and in chromosome movement. To better understand the molecular basis for the inactivity of centromeres associated with secondary constrictions, we have tested these structures by immunofluorescence microscopy for the presence of motor complexes and associated proteins. We find strong immunoreactivity at the active, but not inactive, centromeres of prometaphase multicentric chromosomes using antibodies to the cytoplasmic dynein intermediate chains, three components of the dynactin complex (dynamitin, Arp1 and p150 Glued ), the kinesin-related proteins CENP-E and MCAK and the proposed structural and checkpoint proteins HZW10, CENP-F and Mad2p. These results offer new insight into the assembly and composition of both primary and secondary constrictions and provide a molecular basis for the apparent inactivity of the latter during chromosome segregation.

No abstract

No abstract

Mitotic chromosomes from human peripheral lymphocytes studied at the junction of metaphase and anaphase show that the centromeres of various chromosomes separate in a nonrandom, apparently genetically controlled sequence. It does not depend upon the position of the centromere in the chromosome, the length of the chromosome or total amount of detectable C-chromatin. In man, several chromosomes e.g. 18, 17, 2, separate very early. Such "early" cells do not include nos. 1, 13, 14, 15, and Y and very rarely nos. 21 and 22. The last separating chromosomes are those from group D, G, no. 1, 16, and Y. The possible implication of these findings in evolution, non-disjunction and the control of centromere separation sequences is discussed.

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 cells 100-250 foci are usually detected. On average, individual foci range between 0.5 and 2 microns in diameter and can be seen as clusters of more than ten average-sized (60-100 kb) synchronously activated replicons. In this study, employing minor modifications of the previous methods, we report the visualization of small replication foci of about 0.3 micron diameter (mini-foci). Some foci are clustered into folded chains consisting of 2-40 subunits. DNA content of one mini-focus is estimated to be 50-120 kb and there are 500-1500 mini-foci per cell in the early S-phase. Experimentally induced decrease in replicon size does not affect the size of mini-foci, suggesting that these represent elementary units of DNA replication in mammalian nuclei and are probably identical to the basic structural DNA loop domains.