[Preparation and properties of elongated polytene chromosomes. I. The method]

Compactness of eukaryotic, particularly polytene chromosomes pose difficulties for investigation of their functional organization. For example, 0.1 --1.0 micrometer thick bands of polytene chromosomes contain some dozens microns pieces of DNA molecules. Therefore the useful resolving power of autoradigraphical methods is reduced by 10--100 times of its upper limit. To overcome the mentioned difficulty, a new method has been developed which permits to attain the upper limit of resolution. An isolated polytene chromosome from salivary gland nucleus of Chironomus thummi larva is stretched by microneedles to obtain a bundle of oriented DNP-fibers. A previously chosen small region of the chromosome (band or puff) is stretched simultaneously in a transverse direction by a stick frame made of another chromosome. Electron microscopy of the preparation reveals a meshwork of DNP fibers as presented on fig. 6.