Identification of human and mouse chromosomes in human-mouse hybrids by centromere fluorescence

Maintenance of replication patterns in human-mouse hybrids retaining only one human chromosome

The time of termination of DNA replication of human chromosomes in human-mouse hybrids retaining only one human chromosome was analyzed. Hybrids between SV40-transformed human skin fibroblasts and mouse peritoneal macrophages were used for these studies. Data obtained from hybrids containing only human chromosome 7 or 17 were compared with data from related hybrids containing additional human chromosomes. When either human chromosome 7 or 17 was present alone, it terminated replication at the same stage of the S phase as in hybrids in which other human chromosomes were present (relative to the time of termination of replication of the mouse chromosomes). In comparing the hybrids containing single human chromosomes, it was found that chromosome 17 terminated replication much earlier than chromosome 7. Therefore, the relationship between the replication times of these chromosomes normally observed in human cells was maintained in the hybrids in the absence of all other human chromosomes. The results also indicate that the presence of SV40 gene sequences in chromosomes 7 and 17 did not alter the relative times of termination of replication of those chromosomes.

DNA repair defects and chromosome instability disorders

Xeroderma pigmentosum (XP), Fanconi anaemia (FA), ataxia telangiectasia (AT) and Bloom disease (BS) are four rare autosomal recessive disorders in which there is defective DNA repair and/or chromosome instability and proneness to malignancy. Between 80 and 90% of patients with XP have a defect, demonstrable at cell level, of excision of DNA lesions induced by ultraviolet rays, while the remainder have a cellular error of post-replication repair. XP cells are also deficient in repairing DNA damage caused by a variety of chemical mutagens. There are at least five different complementation groups of the first, or classical, type of XP (A to D, etc.) Apparently group C patients, as well as those with defective post-replication repair, do not show the progressive neurological illness found in a proportion of the other patients. AT is heterogeneous clinically and genetically. Clinically it presents with a progressive neurological illness, progressive telangiectases and a developmental disorder of the thymus. AT is characterized by sensitivity to X-rays and AT cells are unable to repair gamma-ray-induced damage to bases in the DNA. It appears that in many cases of the disorder a chromosomally marked cellular clone is found. In BS the main defect, which results in growth retardation, sun-induced lesions of the face and susceptibility to infection, appears to be a slow DNA chain maturation during DNA synthesis. An increase of sister chromatid exchanges is characteristically seen in the chromosomes of cultured BS cells. In FA, in which there is progressive pancytopenia with eventual bone marrow exhaustion and a tendency to haemorrhage and infection, the cellular defect seems to consist of faulty removal of repair of cross-links in the DNA. In this condition, as in BS and AT, various structural chromosome changes are detected in cultured cells. Patients with XP develop skin cancers in early life and often maligant melanomas. In the other three disorders, in which an immune deficiency is often present, leukaemia and related proliferative disorders are a frequent cause of death while other malignancies also occur. There is some evidence that points to an increased risk of malignancy in heterozygotes who carry the FA and AT genes.

The use of base pair specific DNA binding agents as affinity labels for the study of mammalian chromosomes

The fluorochromes Hoechst 33258 and olivomycin are base pair specific DNA binding agents. The fluorescence enhancement of Hoechst 33258 and olivomycin in the presence of DNA can be directly related to the A--T and G--C content of the interacting DNA respectively. Cytological observations of metaphase chromosomes treated with these two compounds suggest that the fluorescent banding patterns produced are the reverse of one another.--Non-fluorescent base pair specific DNA binding agents have been used as counterstains in chromosome preparations to enhance the contrast of the banding patterns produced by the base specific fluorochromes. The non-fluorescent G--C specific antibiotic actinomycin-D enhanced the resolution of fluorescent bands produced by the A--T specific fluorochrome Hoechst 33258. Similarly the non-fluorescent A--T specific antibiotic netropsin was found to enhance resolution of the bands produced by the G--C -specific fluorochrome olivomycin. Netropsin was also found to increase the differential fluorescent enhancement of complexes of olivomycin with DNAs of various base composition in solution. These findings suggest that counterstaining agents act through a base sequence dependent inhibition of subsequent binding by base pair specific fluorochromes.--The base specific DNA binding agents have been used to differentiate different types of constitutive heterochromatin in mammalian species, and to facilitate chromosome identification in somatic cell hybrids.

Mechanisms of chromosome banding. VIII. Hoechst 33258-DNA interaction

The interaction of Hoechst 33258 with DNA has been examined to help clarify the mechanisms of banding. 1. In agreement with previous studies Hoechst fluorescence is enhanced to a greater degree in AT-rich compared to GC-rich DNA. 2. Hoechst causes an increase in the DNA Tm which is greater at the higher AT content of the DNA. 3. There is a decrease in extinction coefficient and shift in the adsorption spectra to a higher wavelength when Hoechst binds to DNA. 4. DNA is completely precipitated at a ratio of one dye molecular per base pair, and this precipitation is not affected by salt. 5. There is no increase in viscosity or change in the circular dichroism of DNA when bound to Hoechst. These findings suggest Hoechst does not bind to DNA by intercalation or by ionic interaction with the phosphate groups, but rather binds by an attachment to the outside of the double DNA helix by interacting with the base pairs. This type of binding allows greater sensitivity to the base composition than occurs with intercalating agents. In this respect its binding is similar to that of dibutyl proflavine (Muller et al., 1973).