Nucleic Acids,
extremely complex molecules produced by living cells and viruses. Their name
comes from their initial isolation from the nuclei of living cells. Certain
nucleic acids, however, are found not in the cell nucleus but in cell
cytoplasm. Nucleic acids have at least two functions: to pass on hereditary
characteristics from one generation to the next, and to trigger the
manufacture of specific proteins. How nucleic acids accomplish these functions
is the object of some of the most intense and promising research currently
under way. The nucleic acids are the fundamental substances of living things,
believed by researchers to have first been formed about 3 billion years ago,
when the most elementary forms of life began on earth. The origin of the
so-called genetic code they carry has been accepted by researchers as being
very close in time to the origin of life itself (see Evolution;
Genetics). Biochemists have succeeded in deciphering the code, that is,
determining how the sequence of nucleic acids dictates the structure of
proteins.
The two classes of nucleic
acids are the deoxyribonucleic acids (DNA) and the ribonucleic acids (RNA).
The backbones of both DNA and RNA molecules are shaped like helical strands.
Their molecular weights (see Molecule) are in the millions. To the
backbones are connected a great number of smaller molecules (side groups) of
four different types (see Amino
Acids). The sequence of these molecules
on the strand determines the code of the particular nucleic acid. This code,
in turn, signals the cell how to reproduce either a duplicate of itself or the
proteins it requires for survival.
All living cells contain the
genetic material DNA. The cells of bacteria may have but one strand of DNA,
but such a strand contains all the information needed by the cell in order to
reproduce an identical offspring. The cells of mammals contain scores of DNA
strands grouped together in chromosomes. In short, the structure of a DNA
molecule or combination of DNA molecules determines the shape, form, and
function of the offspring. Some viruses, called retroviruses, contain only RNA
rather than DNA, but viruses in themselves are generally not considered true
living organisms (see Virus).
The pioneering research that
revealed the general structure of DNA was performed by the British
biophysicists Francis Crick and Maurice Wilkins and by the American biochemist
James Dewey Watson. Using an X-ray diffraction picture of the DNA molecule
obtained by Wilkins in 1951, Crick and Watson were able to construct a model
of the DNA molecule that was completed in 1953. For their work, the three
scientists received the 1962 Nobel Prize in physiology or medicine. The
American biochemist Arthur Kornberg synthesized DNA from
"off-the-shelf" substances, for which he was awarded, with the
American biochemist Severo Ochoa (for research on RNA), the 1959 Nobel Prize
in physiology or medicine. The DNA that he synthesized, although structurally
similar to natural DNA, was not biologically active. In 1967, however,
Kornberg and a team of researchers at Stanford University succeeded in
producing biologically active DNA from relatively simple chemicals.
Certain kinds of RNA have a
slightly different function from that of DNA. They take part in the actual
synthesis of the proteins a cell produces. This is of particular interest to
virologists because many viruses reproduce by "forcing" the host
cells to manufacture more viruses. The virus injects its own RNA into the host
cell, and the host cell obeys the code of the invading RNA rather than that of
its own. Thus the cell produces proteins that are, in fact, viruses instead of
the proteins required for cell function. The host cell is destroyed, and the
newly formed viruses are free to inject their RNA into other host cells.
The structure of two types
of RNA and their function in protein production have been determined, one type
by a team of Cornell University and U.S. Department of Agriculture
investigators led by Robert W. Holley of Cornell, and the other type by James
T. Madison and George A. Everett of the Department of Agriculture. Important
research into the interpretation of the genetic code and its role in protein
synthesis was also performed by the Indian-born American chemist Har Gobind
Khorana at the University of Wisconsin Enzyme Institute and the American
biochemist Marshall W. Nirenberg of the National Heart Institute. In 1970
Khorana achieved the first complete synthesis of a gene and repeated his feat
in 1973. Since then one type of RNA has been synthesized. Also, in the early
1980s, American biochemists Thomas Robert Cech and Sidney Altman independently
proved that certain types of RNA, called ribozymes, can function as true
catalysts (see Catalysis). See also Heredity.
