The DNA of a particular active gene manufactures a molecule of m-RNA by
the same kind of replication that it uses for making more DNA. In m-RNA
the sequence of bases is the same as in the parent DNA segment; for this
reason, m-RNA is also called DNA-like RNA. As shown in Figure 12, a
cytosine molecule in m-RNA corresponds to a cytosine molecule in DNA, a
guanine to a guanine, and so on, except that the m-RNA has uracil in all
the places where thymine occurs in DNA. The order of the nucleotides in
the m-RNA is the same as that in the DNA, so the m-RNA carries the
genetic code of the gene that made it. This process, all of which occurs
in the cell nucleus, is one of copying, or transcription, rather than
translation, since the same “codewords” (the nucleic-acid bases) are
reproduced.
The new m-RNA molecule then travels from the nucleus to the cytoplasm
and attaches itself to an unoccupied ribosome (see Figure 27). Here it
fits to a molecule of r-RNA and blends its shape geometrically, or
spatially, with the shape of the r-RNA in lock-and-key, or
jigsaw-puzzle, fashion. The combined new RNA molecule is now capable of
manufacturing a specific protein.
[Illustration: Figure 27 _Protein synthesis in a ribosome (microsome),
and its control by DNA in the nucleus, using RNA as an intermediary._
Adapted from _Principles of Biology_, Neal D. Buffaloe, Prentice-Hall,
Inc., 1962, with permission.]
At this point an s-RNA molecule arrives, bringing with it one amino-acid
molecule, which then combines with other amino acids in the specific
order dictated by the RNA to form a specific protein. After the amino
acids have been formed into the protein molecule, they detach themselves
from the s-RNA molecule. The s-RNA molecule has two recognition sites by
which it matches up to its neighbors: One recognizes, or “fits”, the
amino acid, and the other recognizes a corresponding triplet of bases on
m-RNA. There is thus a particular s-RNA molecule for each amino acid and
a particular triplet of bases on the m-RNA molecule for each triplet of
bases that is specific to the s-RNA molecule.
In this process the machinery has translated the nucleic-acid code into
the protein code; that is, it has translated a sequence of the bases
into a sequence of amino acids. This process is therefore called
translation of the genetic message. Once the protein has been
synthesized, it will become active in performing some of the cell’s
metabolic activities.
The gene-action system actually is somewhat more elaborate than this.
There are feedback mechanisms, genes that control the activity of other
genes, either directly or through the production of specific proteins,
and so on. However, the scheme just outlined gives a fair, if
simplified, idea of how the genetic message is carried to the entire
cell and how it is translated into actual life processes.