Karyotyping chromosomes by electron microscopy. II. A method for the sequential examination of spread and banded metaphases by light and electron microscopy

New application of the comet assay: chromosome--comet assay

The comet assay is a well-established, simple, versatile, visual, rapid, and sensitive tool used extensively to assess DNA damage and DNA repair quantitatively and qualitatively in single cells. The comet assay is most frequently used to analyze white blood cells or lymphocytes in human biomonitoring studies, although other cell types have been examined, including buccal, nasal, epithelial, and placental cells and even spermatozoa. This study was conducted to design a protocol that can be used to generate comets in subnuclear units, such as chromosomes. The new technique is based on the chromosome isolation protocols currently used for whole chromosome mounting in electron microscopy, coupled to the alkaline variant of the comet assay, to detect DNA damage. The results show that migrant DNA fragments can be visualized in whole nuclei and isolated chromosomes and that they exhibit patterns of DNA migration that depend on the level of DNA damage produced. This protocol has great potential for the highly reproducible study of DNA damage and repair in specific chromosomal domains.

Evolution of digestibility by Hind III: an analysis by light and electron microscopy

Digestion of Chinese hamster metaphase chromosomes from the Don cell line by Hind III restriction endonuclease followed by Giemsa staining were analysed by light and electron microscopy. The evolution of digestibility was studied and four digestion stages were characterized by different levels of chromosome structure. Three different condensation stages were established according to morphological criteria of length, width and separation among chromatids. It was observed that there are statistically significant differences in the digestion progress at the three condensation stages previously defined.

Electron microscopy of the parameres formed by the centromeric heterochromatin of human chromosome 9 at pachytene

The structure and arrangement of the parameres, which are small bodies representing part of the heterochromatin of human chromosome 9 at pachytene, were studied using transmission and scanning electron microscopy. Parameres appear to be denser than other parts of the chromosomes but have a similar fibrous substructure. The most common arrangement is clusters on the axis of the bivalent, consisting of varying numbers of parameres of variable size. The parameres are joined to each other and to the rest of the chromosome by interconnecting fibres. No evidence was obtained for the organisation of parameres into paired lateral loops, as proposed by previous workers using light microscopy. The combination of osmium impregnation of pachytene chromosomes with a backscattered electron detector in the scanning electron microscope produced very clear images of the pattern of chromomeres. This procedure may prove valuable for pachytene mapping of chromosomes because of the greatly improved resolution compared with light microscopy.