The occurrence of circular structures in highly reiterated DNA of the rat

Interspersion and transcription of repeated sequences of rat DNA

Repetitive rat DNA reassociated to Cot=0.1 and deprived of "foldback" sequences showed close interspersion with unique sequences. As measured by electron microscopy, the average length of repetitive segments was about 600 +/- 400, and of unique segments 1800-3600 base pairs. Pyrimidine tracts over 80 nucleotides in length were found mainly in foldback and repetitive fractions. Oligo(dT) tracts, 20-30 bases in length prevailed in the DNA fraction reassociated to Cot=0.1. Repetitive and unique DNA fractions were annealed to Millipore filters and hybridized with hnRNA. Up to 1.6% of repetitive DNA reassociated to Cot=0.05 showed base complementarity with hnRNA, whereas the comparative figures for DNA reassociated to Cot=10 and for the unique fraction were 0.8% and 0.3% respectively. When hybridization of hnRNA was carried out in solution in vast DNA excess, no hybrid formation with repetitive sequences reassociated to Cot=0.1 was observed, although hybridization with DNA reassociated to Cot=10 was noticeable.

An electron microscope study of mitochondrial DNA in spontaneous human tumours and chemically induced animal tumours

MtDNA was extracted by a phenol method from transplanted and primary DAB induced hepatomata in male Wistar rats, normal rat liver, spontaneous human tumours (2 Wilm's tumours, one neuroblastoma and one adrenal carcinoma), as well as 2 specimens of normal human kidney, BNU induced "leukaemias" in mice and CHO fibroblasts in monolayer culture. The proportion of monomers, catenated dimers and oligomers, open dimers and small circles was determined by electron microscopy of the fractions comprising lower and middle DNA bands in a CsCl-EthBr gradient. Tumours were compared where possible with their normal tissue of origin. Open dimers were found in 2 Wilm's tumours and their attached "normal-looking" kidney tissue but not in normal, non-malignant kidney or any other tissue studied. In Wilm's tumours, the occurrence of open dimers is far from being an all-or-none phenomenon. Malignancy produced little change in the relative proportions of catenated dimers and oligomers in the tissues studied. Small circles were found associated with mtDNA from every tissue. Tumour mtDNA was not more heterogeneous in length than monomers from the corresponding normal tissue, neither was the mean length of tumour mtDNA significantly different from its corresponding normal mtDNA.