Another difficulty is that the best scintillating solvents are not the best chemical solvents for most biological materials. The solubility problem is also aggravated by the low temperatures at which liquid scintillation counters are usually operated for more effective instrument performance. With the method we have described, we can obtain a fairly accurate idea of the rate of RNA synthesis in a given tissue. There are other things we would like to know about RNA. The first of these is the kind of RNA being synthesized. During alkaline digestion all kinds of RNA are broken down into their component nucleotides; we must therefore use other methods if we wish to know the kind of RNA in which the radioactivity of the precursor has been incorporated. Isolation of RNA Native RNA, that is, RNA not broken down into its smaller constituents, can be obtained in a variety of ways, but the most popular one makes use of phenol extraction, which removes DNA and proteins and leaves RNA in solution. If this phenol-purified RNA is dissolved in a concentrated sugar solution and spun in a centrifuge at a very high velocity, it will separate into three major components. These components separate because they have different molecular weights, and the larger the molecule, the faster it forms a sediment in the centrifugal field. Two of these components are s-RNA, the lightest of all, and r-RNA, which is divided into two subfractions. We can also identify a third component, m-RNA, with the centrifuge system but only with some difficulty and only after labeling it with a radioactive precursor, because the amount of m-RNA in a cell is very small. [Illustration: Figure 20 _Diagram of ascending paper chromatography._] Quantitative Analysis Another important feature of RNA (or DNA, for that matter) is its base composition, that is, the percentage of each of the nucleotides that make it up. The four bases that, with ribose and phosphoric acid, comprise the RNA molecule are guanine, adenine, cytosine, and uracil. It will be noted that three of the four—guanine, adenine, and cytosine—are the same as those in DNA, but thymidine in DNA has been replaced by another base, uracil. To determine the percentage of each base in a given RNA molecule, we must digest RNA with alkali to produce mononucleotides, which are smaller molecules, each consisting of a base, ribose, and phosphoric acid. We can now separate the four nucleotides by using paper chromatography (see Figure 20). [Illustration: Figure 21 _A paper chromatography showing separation of amino acids in two directions. Radioactivity in samples then produced this record by radioautography._]