It is not very fruitful to discuss whether proteins or nucleic acids are
more important. That question is something like the one about the
chicken and the egg. We cannot think of one without thinking of the
other. Although our insight into the mutual dependence of these two
materials has greatly increased in recent years and although we know the
relation between them is a fundamental factor in such events as
reproduction, mutation, and differentiation (or specialization) of
cells, our understanding of their interplay is far from complete. Real
understanding of the relation between them would give us insight into
the essence of growth—both normal and abnormal—or, indeed, one could
almost say, into the complexity of life itself.
[Illustration: Figure 5 _Photomicrograph of Paramecia, one-celled
animals, magnified 1100 times. Many of the same structures that appear
in Figure 3 can be seen here. This photo was taken with an
“interference” microscope designed to permit continuous variation of
contrast in the subject under study._]
Practically all the DNA of most cells is concentrated in the nucleus.
RNA, on the other hand, is distributed throughout the cell. Some RNA is
present in the nucleus, but most of it is associated with minute
particles in the cytoplasm known as microsomes, some of which are
especially rich in RNA and are accordingly named ribosomes. These are
much smaller particles than the mitochondria.
[Illustration: Figure 6 _Stages of the mitotic cycle in a hypothetical
cell with four chromosomes._]
(a) Interphase
(b) Early prophase
(c) Prophase
(d) Metaphase
(e) Anaphase
(f) Telophase
(g) Interphase, daughter cells