Chromosome-sized DNA molecules from Drosophila

Abstract

Measurements of viscoelastic retardation times of detergent-Pronase lysates of Drosophila cells demonstrated the presence of large numbers of DNA molecules of a size commensurate with that of the chromosomes. The values estimated from the retardation times for the molecular weights of the largest molecules ranged from about 20×10⁹ to 80×10⁹ daltons depending on the species of Drosophila. The molecular weights of the DNA molecules were independent of the metaphase shapes (i.e., metacentric or submetacentric), but were proportional to the DNA contents of the chromosomes in the case of translocations or deletions. It was concluded, therefore, that the DNA molecules must run the length of the chromosome and cannot be discontinuous at the centromere. When compared with the values of the DNA contents of Drosophila chromosomes determined by other methods, the results were consistent with the model of one, or possibly two, DNA molecules per chromosome; the simplest conclusion, that there is only one DNA molecule per chromosome (for simple chromosomes), rests on a long extrapolation of an empirical relation between retardation time and molecular weight, but is also favored by indirect evidence. Further possibilities which could not be excluded were that the large DNA molecules contained Pronase-resistant, non-DNA links, or that a fraction of smaller DNA molecules might also have been present in the chromosomes. Chromosome-sized DNA molecules were obtained almost quantitatively from unsynchronized cultured cells, suggesting that the size of the chromosomal DNA is conserved throughout much of the cell cycle. The molecules were stable for periods of up to several days at 50° C in solutions containing detergent, Pronase, and EDTA.