| 摘要 |
Accurate quantitative determination of signals produced on film by biological molecules of interest, such as DNA and RNA, identified by hybridization with radioactively-labeled probes comprising homologous sequences (DNA or RNA) is assured by means of preimaging of a density gradient reference series on the film to calibrate the linear range of the film's response. The reference may include a gradient of incremented density levels or it may include multiple steps produced by differing amounts of a standard radiant material. The film may be preimaged with a step-wedge density gradient by means of a film holder with arrangements to optically isolate a substantial portion of the film. Densitometric tracings of the signals generated by the radioactively-identified molecules can be adjusted to lie within the linear range of response of the film defined by the tracing of the step-wedge image by adjustment of the film exposure or sample concentrations. Accurate quantitation can be assured if the heights of the peaks on the densitometric tracings of bands on the film produced by hybridization of probes containing sequences from test and reference chromosomes lie within the linear range of response of the film. Computation of the ratios of areas of test and reference sequences on each sample can then be compared to those of samples of known genetic constitution under similar conditions. The method described can be used to detect alterations (changes from normal) in DNA content of cells within individual samples by single tracings. It can thus be used to diagnose, prenatally or postnatally, any genetic conditions, (including those such as Down Syndrome and Alzheimer's Disease), whose basis involves an abnormal quantity of DNA in cells.
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