[Evidence of single-strandedness in eukaryotic chromosomes]

Polytene salivary gland chromosomes of Chironomus thummi were stretched in pronase solution by 176 +/- 26 times up to their break (fig. 1). The DNA packed arrangement coefficient, determined as a ratio of DNA length, equal to 85 +/- 5 mm in a haploid set, to the length of 520 +/- 40 microns of a set of polytene chromosomes was found to be 164 +/- 22. The coincidence of these two values is a very strong evidence in favour of the uninemity of chromatids of the Chironomus chromosomes. The effect of ethidium bromide on elastic properties of chromosomes, preliminary stretched in pronase (fig. 2), and the lengthening of these chromosomes after ethidium staining prove that DNA molecules are double-stranded and supercoiled until chromosomes are broken. This enables us to conclude that each chromatid of Chironomus consists of a single DNA molecule or, more probably, of a single chain of linked DNA molecules whose both ends are located in telomeres of chromosomes.