We have mentioned previously that there are two main types of nucleic
acids: DNA, the genetic material itself, and RNA, the molecule that
translates the genetic message from DNA into terms the cell can use as
“instructions” for making protein. Cells differ from each other on the
basis of kinds of proteins they contain, and, since differences among
cells determine differences among organisms, it follows that differences
in the composition of DNA serve to explain the variety in living
organisms populating the world. However, if differences between two
organisms can be explained by differences in the chemical composition of
their respective DNA’s, how can we explain differences between cells of
the same organism? How can we explain that cells of the human pancreas
secrete insulin, whereas other cells in man produce no insulin? Or how
can we explain that certain cells make bone and others make fat? If
indeed all cells in the same organism contain the same amount and kind
of DNA (since all DNA in an organism derives from the duplication of the
DNA of the fertilized egg cell and its descendants), it would seem, at
first glance, that DNA is not the molecule responsible for differences
among the cells. The clarification of this apparent contradiction is
found in the remarkable properties of the other nucleic acid, the
translator molecule, RNA.