MEDICAL WORLD

January 5, 1962 Vol. 3, No, 1

NIH RESEARCHERS CRACK
THE GENETIC CODE

In pioneering experiments, two young biochemists have begun
to decipher the RNA molecules that carry cellular blueprints

he enigma of genetic coding, con-

sidered a fundamental secret of
life, may be on the verge of solution.

In just-published and about-to-be-
- published papers, several research
teams are reporting experimental
proof of what has been largely theory:
the intricate process by which struc-
ture and function of living organisms
are shaped. One group has begun to
crack the DNA-RNA code—the key
to the whole mystery. Soon they expect
to decipher the entire set of instruc-
tions by which genetic messengers di-
rect the manufacture of proteins—the
basic stuff of life.

The major achievement in RNA
research is the work of two young
biochemists at the National Institute of
Arthritis and Metabolic Diseases, Drs.
Marshall W. Nirenberg and J. Hein-
rich Matthaei. Behind their work,
however, is a whole series of investi-
gations which has produced the basic
theory and its preliminary experi-
mental support.

Fundamentally, the theory states
that the hereditary “blueprints” of cell
structure and function are coded with-
in the cell nucleus as long-chain
molecules of desoxyribonucleic acid
(DNA). These plans are transmitted,
in a series of steps, to the cytoplasmic
“assembly line” where they direct the
synthesis of each cell’s characteristic
products.

In all species, from bacillus to man,
DNA molecules are organized around
the same general plan, chains of
only four nitrogen-containing units
(bases): adenine, guanine, cytosine
and thymine. The specific order of
these bases, like the order of letters
in words, is meaningful. In effect, the

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coded “message” of heredity is writ-
ten in a four-letter alphabet.

DNA molecules can duplicate
themselves. This property plays a key
role, not only in the passage of heredi-
tary characteristics from one genera-
tion to the next, but also in the devel-
opment of multicellular organisms.
The DNA in a fertilized human ovum
reproduces thousands of billions of
times to form the myriad cells of the
mature individual. And each cell con-
tains a complete copy of the genetic
“specifications” for the human species,
as well as those for the individual’s
unique collection of hereditary char-
acteristics.

This duplicative process was re-
produced in the test tube by Dr.
Arthur Kornberg of the Stanford Med-
ical Center, who won the 1959 Nobel
Prize in medicine and physiology for

the accomplishment. He prepared so-
lutions containing the four DNA bases
and other ingredients, added a dash
of “natural” DNA as a primer—and
found that the primer would assemble
the bases into exact copies of itself.

So far as is known, DNA functions
only in the nucleus, like an original
blueprint kept in the foreman’s office.
From its headquarters, the DNA
transfers its genetic information to
ribonucleic acid (RNA), which con-
stitutes the “working drawings” used
in the cytoplasmic factory.

RNA, like DNA, also consists of
chains built up of four bases. Three
of the bases—adenine, cytosine and
guanine — are the same as those in
DNA, but instead of thymine, RNA
has uracil. (Dr. Severo Ochoa of the
NYU College of Medicine shared the
1959 Nobel Prize with Kornberg for

 

 

NATURAL RNA

 

 

BIOCHEMICAL CODE depends on sequence of four kinds of units (gray) in RNA mole-
cule. Various sequences select different amino acids (colof) for protein synthesis.

MEDICAL WORLD NEWS
developing methods of synthesizing
RNA.)

At NYU, the University of Chi-
cago, and the University of St. Louis,
researchers are now studying trans-
mission of coded information from
DNA to RNA. One NYU team, Drs.
John Furth and Jerard Hurwitz, told
the American Association for the Ad-
vancement of Science that they have
now transferred, in vitro, the letter-
for-letter nuclear code to a material
which is known to pass through the
nuclear membrane into the cytoplasm.

Exactly as Predicted

They prepared a solution contain-
ing quantities of the four RNA bases
—unassembled components of the
long molecules—and a special enzyme
obtained from E. coli. Then they add-
ed various species of DNA. The DNA,
presumably acting as a sort of tem-
plate, assembled the bases to form
RNA molecules. And in each case,
the proportion of the bases in the syn-
thesized RNA reflected the propor-
tions in the DNA, precisely as the
theoretical model predicted. For ex-
ample, for every hundred adenine
units in the DNA, there were a hun-
dred uracil units in the RNA.

Not only the proportion, but also
the order of the bases was transferred.
In one experiment, the two investi-
gators used a synthetic DNA in which
every other base was adenine, In the
RNA, every other base was uracil.

This is only the first step. Once
the coded plans reach the cytoplasm
they are translated into action—syn-

 

DR. NIRENBERG spells out an explanation of his work in biological cryptanalysis.

thesis of enzymes and other proteins.
The properties of these proteins are
determined by the sequence of their
hundreds or thousands of component
units. Each of these units is one of the
20-odd amino acids, and biochemists
have speculated that each amino acid
is specified by a different genetic “code
word”—a group of three or four bases
in the RNA molecule. A sequence of
several thousand such “words” could
specify the complete structure of even
the most complex protein. But the
pivotal question has been: Which
word signifies which amino acid?
The answer to this is the major
contribution just now emerging from
the laboratory of Drs. Nirenberg and
Matthaei. For the first time, they have
brought about controlled RNA man-
ufacture of amino-acid chains in the
test tube. And by utilizing known

 

SYNTHETIC (“POLY-U”’) RNA

 

 

 

 

DECODING EXPERIMENT uses RNA with only one kind of unit, uracil (dark gray). This
simple sequence forms chains of only a single amino acid, phenylalanine (dark color).

January 5, 1962

sequences of RNA bases, they have
succeeded in equating certain sequen-
ces with certain amino acids.

Drs, Nirenberg and Matthaei are
using E. coli solutions containing the
20 amino acids, energy-rich adenosine
triphosphate (ATP), ribosomes (tiny
intracellular particles where proteins
are synthesized) and cytoplasmic flu-
ids (cell sap).

To this mixture. they add ‘“‘poly-
U,” a synthetic RNA containing only
one kind of base: uracil. The RNA
triggers the formation of amino-acid
chains (polypeptides), But most sig-
nificantly, out of all available amino
acids, the poly-U selects just one —
phenylalanine—to incorporate in the
polypeptide. Evidently, the code-word
U-U-U  (uracil-uracil-uracil) trans-
lates as “phenylalanine.”

Within a few months, the NIAMD
researchers expect to report on the
code sequences associated with at least °
15 different amino acids.

Nirenberg and Matthaei’s tech-
niques, which have yielded such ex-
citing results, are now being taken up
by half a dozen other groups. At least
two Nobel laureates—Ochoa at NYU
and Fritz Lipmann at Rockefeller In-

‘stitute—are heading research teams

that are tackling protein synthesis.
Ochoa’s group, including Drs. Carlos
Basilio, Joseph F. Speyer and Peter
Lengyel, has already achieved a -pre-
liminary picture of RNA sequences
associated with 14 amino acids. For
each sequence they have determined
the names and proportions of the
bases, but not their order.

Dr, Nirenberg predicts that “within
another six months or so most of the
genetic code will be cracked,” =

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