Metaphase and chromomere banding are distinct entities of chromosome substructure

Insight into magnesium ions effect on chromosome banding and ultrastructure

Magnesium ion (Mg²⁺ ) plays a fundamental role in chromosome condensation which is important for genetic material segregation. Studies about the effects of Mg²⁺ on the overall chromosome structure have been reported. Nevertheless, its effects on the distribution of heterochromatin and euchromatin region have yet to be investigated. The aim of this study was to evaluate the effects of Mg²⁺ on the banding pattern and ultrastructure of the chromosome. Chromosome analysis was performed using the synchronized HeLa cells. The effect of Mg²⁺ was evaluated by subjecting the chromosomes to three different solutions, namely XBE5 (containing 5 mM Mg²⁺ ) as a control, XBE (0 mM Mg²⁺ ), and 1 mM EDTA as cations-chelator. Chromosome banding was carried out using the GTL-banding technique. The ultrastructure of the chromosomes treated with and without Mg²⁺ was further obtained using SEM. The results showed a condensed chromosome structure with a clear banding pattern when the chromosomes were treated with a buffer containing 5 mM Mg²⁺ . In contrast, chromosomes treated with a buffer containing no Mg²⁺ and those treated with a cations-chelator showed an expanded and fibrous structure with the lower intensity of the banding pattern. Elongation of the chromosome caused by decondensation resulted in the band splitting. The different ultrastructure of the chromosomes treated with and without Mg²⁺ was obvious under SEM. The results of this study further emphasized the role of Mg²⁺ on chromosome structure and gave insights into Mg²⁺ effects on the banding distribution and ultrastructure of the chromosome.

Chromosome preparation and high resolution banding techniques. A review

High resolution banding techniques enable detection of chromosome rearrangements even within major bands. Banded chromosomes prepared for light microscopic studies of intact metaphase plates are, however, highly modified structures compared with native chromosomes, and the high resolution banding techniques only seem possible because the following methods were standardized and combined. The use of colcemid, which prevents formation of the spindle and thereby collects cells at the metaphase-anaphase border, is routinely used for chromosome preparations. For high resolution banding studies, short exposure time and concentrations near the threshold value have been recommended by several authors. Several agents interfere with chromosome contraction processes, but only a few have had a lasting influence on high resolution banding studies. The most used agents are ethidium bromide, actinomycin D, and Hoechst 33258, which all partially inhibit chromosome contraction. Treatment with hypotonic solutions induces swelling of animal cells, and the methanol in the fixative denatures and precipitates protein by dehydration. The acetic acid coagulates nucleoproteins and causes swelling of the cells. The fixative penetrates the cells rapidly and preserves the chromosome structure. To obtain long segmented chromosomes suitable for high resolution banding hypotonic treatment with .075 M KCl, frequent changes of fixative and overnight fixation at 4 degrees C have been recommended. The use of cell synchronization, 5-bromodeoxyuridine incorporation into DNA, and fluorochrome-photolysis Giemsa (FPG)-staining have improved the quality of high resolution banding. Synchronization techniques, which select for lymphocyte populations in early divisions, provide excellent materials for chromosome preparations and induction of high resolution banding. The banding techniques seem to enhance differences already present in the chromosomes, and the differential Giemsa staining has recently been explained by interactions between the hydrophobic dye complex, the supercoiled DNA helix, and the denaturated histone core of the nucleosomes.

A structural basis for R- and T-banding: a scanning electron microscopy study

The structure of reverse (R)-banded and telomeric (T)-banded chromosomes was studied by examination of the same chromosomes first in the light microscope (LM) followed by the scanning electron microscope (SEM). This procedure demonstrated a structural basis to both the R- and T-banding techniques. A direct correlation was shown between the LM staining patterns and the structural patterns observed in the SEM. In the R-banded chromosomes the positively stained R-bands, viewed by LM, corresponded to highly fibrous three-dimensional regions in the SEM. The negatively stained R-interbands corresponded to flatter regions from which material appeared to have been extracted. These structural observations strongly support the suggestion that chromosomal material is preferentially lost from the R-interbands with aggregation of fibres in the R-bands. T-banded chromosomes showed a similar structure to the R-banded chromosomes. The positively stained T-bands located at the telomeres corresponded to regions of highly aggregated fibres. The remainder of the chromosome, corresponding to the negatively stained area, had a flattened and extracted appearance. These similarities in morphology between the T- and R-banded chromosomes support the view that T-bands result from a progressive breakdown of the R-banded chromosome structure.