Electron microscopy of differentially BrdU-substituted sister chromatids treated with sodium phosphate

A historical overview of bromo-substituted DNA and sister chromatid differentiation

The thymidine analogue 5-bromo-2'-deoxyuridine (BrdU) has been widely used to make sister chromatid differentiation (SCD) evident in metaphase chromosomes of cells grown for two cycles in BrdU and, thus, containing varying amounts of the thymidine analogue. A direct consequence was the possibility of making sister chromatid exchange (SCE) evident without using autoradiographic procedures. The latter phenomenon was first discovered in 1953, and its frequency is considered a reliable marker of pathological cell situations, as well as an indicator of mutagenic compounds. Several experimental procedures were found which produced SCD, such as the use of fluorochromes like 33258 Hoechst or acridine orange, whose observation under fluorescence microscopy was directly recorded by photos or stained with Giemsa to make chromosome preparations permanent. Other treatments followed by Giemsa staining required the use of saline hot solutions, acid solutions, nuclease attack and specific monoclonal antibodies. Basically two molecular mechanisms were invoked to explain the different affinity of Giemsa stain for differential BrdU-substituted chromatid DNA. The first implied debromination of chromatid DNA, whose occurrence would be greater in chromatids containing an amount of BrdU greater than that present in sister chromatids. The second mechanism, although not denying the importance of DNA debromination, postulated that chromatin structural organization, in terms of DNA-protein and/or protein-protein DNA interaction, is responsible for SCD production.

Effects of bromodeoxyuridine substitution on metaphase chromosome structures examined by scanning electron microscopy

Chinese hamster chromosomes were differentially substituted with 50 microM 5-bromodeoxyuridine (BrdU) to obtain chromosomes with bifilarly and unifilarly substituted (BB-TB) and unifilarly and non-substituted (TB-TT) chromatid constitutions. To avoid the effect of Giemsa staining on the ultrastructure of chromosomes, unstained preparations were exclusively used. When TB-TT chromosomes were prepared with the conventional air-drying method followed by the osmium tetroxide-thiocarbohydrazide (OsO4-TCH) technique and examined by scanning electron microscopy (SEM), the TB-chromatid appeared somewhat more slender and showed more conspicuous spiral structures, thereby appearing more loosened compared to the TT-chromatid. At higher magnifications, however, 30 nm chromatin fibres which were seen to constitute both chromatids showed no discernible differences in dimension between the TT- and TB-chromatids. On the other hand, TB-TT chromosomes specially prepared for SEM without the process of air-drying appeared in their entirety less extended and no spiral configuration was observed even in the TB-chromatid. The TB-chromatid instead appeared rather less loosened than the TT-chromatid whereas thick fibre-like structures which in turn seemed to consist of 30 nm fibres were more easily discernible in the TT-chromatid compared to the TB. Such seemingly contradictory results obtained from the two different preparatory procedures were tentatively explained on the basis of our multiple coiling model (Taniguchi and Takayama 1986).

The ultrastructure of R-banded chromosomes

Electron microscopy has been used to study the fine structural organization of R-banded chromosomes prepared by treatment of the chromosomes with a hot NaH2PO4 solution. The results indicate that there is a structural basis for R-banding with this technique. In comparison to untreated control chromosomes, the R-banded chromosomes had a greatly reduced electron density, suggesting that the heat treatment has a general adverse effect on chromosome structure. Chromatin fibers formed a coarse, irregular network throughout the chromosome and were often enlarged, probably as a result of the fusion of two or more native fibers. The chromatin fibers were more aggregated and had an increased electron density in the R-band regions of the chromosome than in the interbands. This indicates that the treatment has a differential effect on the structure of bands and interbands. A comparison of the ultrastructure of R- and G-banded chromosomes demonstrated that the distribution of aggregated chromatin was reversed by these two types of banding techniques; however, the treatments producing the R-banding appeared to induce less extreme differences in the degree of chromatin condensation in band and interband regions than those giving rise to G-banding. It is suggested that alterations of DNA-protein interactions may arise from the differential denaturation of proteins and/or DNA in R-band and interband regions during the heat pretreatment. Such differential alterations in DNA-protein interactions may induce localized changes in the organization of chromatin and may account for the subtle morphological differences observed between the band and interband regions.

Topographic examination of sister chromatid differential staining by Nomarski interference microscopy and scanning electron microscopy

BrdU-substituted Chinese hamster chromosomes were treated with a hog Na2HPO4 solution and stained with Giemsa to produce sister chromatid differential staining (SCD). The process of SCD was examined with the Nomarski differential interference microscope and the scanning electron microscope. After the Na2HPO4 treatment alone, unifilarly BrdU-substituted (TB) chromatids appeared somewhat more severely collapsed than the bifilarly substituted (BB) chromatids. Subsequent Giemsa staining, however, brought about pronounced piling up of the Giemsa dye on the TB-chromatids but not on the BB-ones, causing highly distinct differential Giemsa staining as well as a marked differentiation in surface topography between the sister chromatids. Removal of the Giemsa dye from the differentially Giemsa stained chromosomes resulted in a disappearance of such a pronounced topographic differentiation.

Sister chromatid differential staining by direct staining in Na2HPO4-Giemsa solution and the mechanism involved

The direct staining of BrdU-substituted Chinese hamster chromosomes in a Na2HPO4-Giemsa solution without any pretreatments resulted in a B-dark type SCD istituted (TB) chromatids stained light. Detailed examinations of the staining process suggested that the Na2HPO4 solution acts to collapse chromosomes whereas the Giemsa dye works to reconstruct the collapsed chromosomes, and that during the reconstruction process preferential binding of the Giemsa dye to the BB-chromatids occurs to produce the B-dark SCD. It was revealed that not only the time but the temperature at which chromosome preparations are kept prior to use considerably affect the occurrence of SCD.