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._]