Cytosine Ratio Research Articles

Composition of various ribonucleic acid fractions from micro-organisms of different deoxyribonucleic acid composition.

THE ratio of guanine plus cytosine to adenine plus thymine (GC/AT) in the deoxyribonucleic acid (DNA) of a given species of a micro-organism is constant, but the GC/AT ratio of different kinds of microorganisms can vary by a factor of as much as five times1. The ratio of GC content to AU (uracil) content of cell ribonucleic acid (RNA) varies only slightly from one organism to another; but this ratio tends to be slightly higher in organisms the DNA ratio of which is high2. These facts raise a question as to whether all the RNA of a cell has the same composition or whether perhaps a small RNA sub-fraction has the same base ratios as its corresponding DNA (uracil being equivalent to thymine). Recent experiments have demonstrated an RNA sub-fraction in yeast which does have a composition almost identical to the corresponding DNA (ref. 3). The present work is concerned with analysing the nucleotide ratios in sub-fractions of the RNA from organisms with varying GC/AT ratios in the DNA.

Similarity of the structure of DNA from a variety of sources.

DNA's from diverse cells of different species and from diverse tissues give the same x-ray diffraction pattern. The presently observable structure of DNA appears, then, to be the same in all cells. Thus, DNA in the resting state-the stored genetic material, from sperm of Paracentrotus lividus, Arbacia lixula, and salmon and from T(2) and T(7) bacteriophage-gives a pattern indistinguishable from DNA from very rapidly dividing cells, e.g., human acute leukemic leukocytes, human leukemic myeloid cells, mouse sarcoma 180, and bacteria-E. coli and pneumococci-during their logarithmic growth. The same x-ray patterns are given by DNA's from more slowly dividing tissues, e.g. calf liver, calf thymus, and human normal and leukemic lymphatic tissue. DNA from chicken erythrocytes-a DNA presumably metabolically inert-gives a similar picture. DNA's from several sources with a wide range in nitrogen base ratios, prepared independently by different workers using various methods, have given final products in varying yield; these all gave the same x-ray pattern, suggesting that all DNA is in the double-helical configuration. Finally, separation of the DNA molecule into a number of fractions with a varying adenine + thymine:guanine + cytosine ratio, but a constant adenine:thymine and guanine:cytosine ratio, each giving the same x-ray pattern as the original whole molecule, suggests that DNA cannot exist in significant amounts in forms other than the double-helix. X-ray diffraction photographs of sperm heads, extracted nucleoprotamine, calf thymus nuclei and extracted nucleohistone, and of chicken erythrocyte nuclei, are not all as well defined as those given by extracted DNA, but it is clear from the general characteristics of the pattern that much of the DNA bound to protein in these nuclei has the usual helical configuration, and that the double-helical structure of DNA exists in the cell and is not an artifact.