Among the available cytogenetic assays are the following four types of studies that are particularly applicable when it is important to determine whether there has been exposure to a potentially carcinogenic agent. Test descriptions have been written with the non-biologist in mind. For more information, of a general or more technical nature, please contact us.

1. Micronucleus test.

This test is based on the idea that pieces of broken chromosomes, or in some cases whole chromosomes, will become disassociated from the rest of the nuclear DNA and form a tiny nucleus on their own. While this technique provides an excellent opportunity for rapid screening of a population, it is not possible to determine the amount or specific identity of the DNA contained in the micronucleus or the events that led to its formation. Moreover, the presence of a micronucleus offers no clue to the nature of other possible abnormalities contained within the nucleus of the cell. Thus, the test is most useful when used in conjunction with G-banded studies, or as an indicator of what direction a possible study should take.

2. Sister Chromatid Exchange (SCE).

Just prior to cell division DNA replicates. When this process is complete each chromosome will have made an exact copy of itself. The two copies are held together at a location called the centromere. In this configuration the chromosome is said to be composed of two chromatids. DNA breaks, sometimes spontaneously but more often because of influences from outside the cell. As soon as DNA breaks it begins its own repair process. Sometimes in the repair process equal portions of the two opposing chromatids (sister chromatids) of a chromosome will exchange places. Special staining techniques allow us to stain one chromatid dark and the other light making any exchange that has occurred quite detectable. Thus SCE is a sensitive indicator of a cell's exposure to hazardous substances. However, with this technique it is not possible to detect damage that might have occurred during DNA replication prior to the time the cells were placed in culture.

3. Chromosome breakage (sometimes referred to as aberrations).

This is test whose results whose results are typically reported as breaks, gaps, and deletions. While this a test also makes it possible to screen large populations rapidly, it has a good number of disadvantages. First, the distinction among the terms, "break", "gap", and "deletion" is a subjective one relating to whether or not the reader feels the deletion is larger or smaller than the width of a chromatid and whether the chromatids remain aligned or not. It is not at all unusual for equally experienced readers to report different results on the same metaphases. Second, slide preparation, among other things, can have an influence on the manifestation of the lesion. Third, because all the chromosomes stain alike it is not possible to identify one chromosome from another with any degree of certainty and therefore it is not possible to identify specific chromosomes involved in rearrangements or to even detect rearrangements where the two chromosome fragments involved are the same size. Finally, non-banded chromosomes are often not what they appear to be. For example, the literature abounds with reports of translocations that clearly cannot be positively identified and with control populations with two or three dicentrics (chromosomes with two centromeres) per one hundred cells, a finding that is simply not consistent with reality.

4. G-Banding.

By using special techniques the cytogeneticist is able to stain chromosomes in such a way that they are easily and quickly identifiable one from the other. A particular chromosome will stain in the same pattern not only from one cell to another within an individual but from one individual to another within a species. Moreover, the "banding" pattern achieved by this staining procedure is so intricate that any minor changes in the structure of the chromosome can be detected by a technologist properly trained in the technique. Although this test is the most demanding in terms of time and skill levels, it is the one we prefer to do, particularly in cases where exposure was in the past and not going on currently or where exposure has occurred at low levels over long periods of time.

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