Half-chromatids have often been observed by light microscopy. Similar structures were looked for by electron microscopy of whole-mount preparations of mitotic cells from seven different species of mammals and birds. Of over 3000 plates examined showing all degrees of dispersion of the chromatin fibers, there was no evidence that the chromatid was made up of any subunits larger than the chromatin fiber. In an occasional specimen treated with sodium dodecyl sulfate the chromatid showed dense peripheral regions similar to the half-chromatids seen by light microscopy following trypsin treatment. However, the resolution provided by the electron microscope demonstrated that this was not a true subdivision of the chromatid. It is suggested that the observation of half-chromatids is valid but the conclusion that they represent independent portions of the chromatid, or that they provide any evidence as to the strandedness of chromosomes, is not.
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