An electron microscope study of DNA molecules from Chironomus tentans and Chironomus thummi.

I N recent years substantial information has been gained concerning the size and topology of DNA (deoxyribonucleic acid) molecules of viruses (for reviews see VINOGRAD and LEBOWITZ 1966; COHEN 1967; THOMAS and MACHATTIE 1967) , bacterial plasmic and sex factor (ROTH and HELINSKI 1967; HICKSON, ROTH and HELINSKI 1967), Mycciplasma hominis (BODE and MOROWITZ 1967), bacteria (CAIRNS 1963) and mitochondria (see for example, VAN BRUGGEN, BORST, RUTTENBERG, GRUBER and KROON 1966; SINCLAIR and STEVENS 1966; DAWID and WOLSTENHOLME 1967, '(968; HUDSON and VINOGRAD 1967; CLAYTON and VINOGRAD 1967). Few studies, however, have been made on the size and form of DNA molecules of chromosomes of higher organisms, even though such information is an essential prerequisite to an understanding of the way is which chromosomes are constructed. HOTTA and BASSEL (1965) examined rotary shadowed DNA from isolated wheat nuclei and boar sperm in the electron microscope and concluded that the native mtolecules of both higher plants and animals vary markedly in length, up to more than 100 p. They also found a number of circles of DNA from boar sperm of cointour lengths varying from 0.5-16.8 p. SOLARI (1965, 1967) reported DNA molecules at least 108.5 p long in electron microscope preparations from sea urchin sperm and isolated nuclei. By applying the autoradiographic technique of CAIRNS (1 963) to Chinese hamster cells grown in vitro, HUBERMAN and RIGGS (1966) obtained rows of grains of various length up to about 1.8 mm, which they attributed to single DNA molecules. In similar preparations of HeLa cells (CAIRNS 1966) and human lymphocytes ( SASAKI and NORMAN 1966) rows of grains up to 500 ,p and 2.2 cm, respectively, were found. The present report relates the results of experiments on the form and size distribution of DNA molecules from polytene salivary gland chromosomes of larvae of Chironomus teiztans and Chironomus thummi.