The invention:
A method for synthesizing the biological molecule
RNA established that this process can occur outside the living
cell.
The people behind the invention:
Severo Ochoa (1905-1993), a Spanish biochemist who shared
the 1959 Nobel Prize in Physiology or Medicine
Marianne Grunberg-Manago (1921- ), a French biochemist
Marshall W. Nirenberg (1927- ), an American biochemist
who won the 1968 Nobel Prize in Physiology or Medicine
Peter Lengyel (1929- ), a Hungarian American biochemist
RNA Outside the Cells
In the early decades of the twentieth century, genetics had not
been experimentally united with biochemistry. This merging soon
occurred, however, with work involving the mold Neurospora crassa.
This Nobel award-winning work by biochemist Edward Lawrie
Tatum and geneticist George Wells Beadle showed that genes control
production of proteins, which are major functional molecules in
cells. Yet no one knew the chemical composition of genes and chromosomes,
or, rather, the molecules of heredity.
The American bacteriologist Oswald T. Avery and his colleagues
at New York’s Rockefeller Institute determined experimentally that
the molecular basis of heredity was a large polymer known as deoxyribonucleic
acid (DNA). Avery’s discovery triggered a furious
worldwide search for the particular structural characteristics of
DNA, which allow for the known biological characteristics of genes.
One of the most famous studies in the history of science solved
this problem in 1953. Scientists James D.Watson, Francis Crick, and
Maurice H. F.Wilkins postulated that DNAexists as a double helix.
That is, two long strands twist about each other in a predictable pattern,
with each single strand held to the other by weak, reversible
linkages known as “hydrogen bonds.” About this time, researchers
recognized also that a molecule closely related to DNA, ribonucleic
acid (RNA), plays an important role in transcribing the genetic information
as well as in other biological functions.
Severo Ochoa was born in Spain as the science of genetics was
developing. In 1942, he moved to New York University, where he
studied the bacterium Azobacter vinelandii. Specifically, Ochoa was
focusing on the question of how cells process energy in the form of
organic molecules such as the sugar glucose to provide usable biological
energy in the form of adenosine triphosphate (ATP). With
postdoctoral fellow Marianne Grunberg-Manago, he studied enzymatic
reactions capable of incorporating inorganic phosphate (a
compound consisting of one atom of phosphorus and four atoms of
oxygen) into adenosine diphosphate (ADP) to form ATP.
One particularly interesting reaction was followed by monitoring
the amount of radioactive phosphate reacting with ADP. Following
separation of the reaction products, it was discovered that
the main product was not ATP, but a much larger molecule. Chemical
characterization demonstrated that this product was a polymer
of adenosine monophosphate. When other nucleocide diphosphates,
such as inosine diphosphate, were used in the reaction, the
corresponding polymer of inosine monophosphate was formed.
Thus, in each case, a polymer (a long string of building-block
units) was formed. The polymers formed were synthetic RNAs, and
the enzyme responsible for the conversion became known as “polynucleotide
phosphorylase.” This finding, once the early skepticism
was resolved, was received by biochemists with great enthusiasm
because no technique outside the cell had ever been discovered
previously in which a nucleic acid similar to RNA could be
synthesized.
Learning the Language
Ochoa, Peter Lengyel, and MarshallW. Nirenberg at the National
Institute of Health took advantage of this breakthrough to synthesize
different RNAs useful in cracking the genetic code. Crick had
postulated that the flow of information in biological systems is from
DNA to RNA to protein. In other words, genetic information contained
in the DNA structure is transcribed into complementary
RNAstructures, which, in turn, are translated into the protein. Pro-
tein synthesis, an extremely complex process, involves bringing a
type of RNA, known as messenger RNA, together with amino acids
and huge cellular organelles known as ribosomes.
Yet investigators did not know the nature of the nucleic acid alphabet—
for example, how many single units of the RNA polymer
code were needed for each amino acid, and the order that the units
must be in to stand for a “word” in the nucleic acid language. In
1961, Nirenberg demonstrated that the polymer of synthetic RNA
with multiple units of uracil (poly U) would “code” only for a protein
containing the amino acid phenylalanine. Each three units (U’s)
gave one phenylalanine. Therefore, genetic words each contain
three letters. UUU translates into phenylalanine. Poly A, the first
polymer discovered with polynucleotide phosphorylase, was coded
for a protein containing multiple lysines. That is, AAA translates
into the amino acid lysine.
The words, containing combinations of letters, such as AUG, were
not as easily studied, but Nirenberg, Ochoa, and Gobind Khorana of
the University of Wisconsin eventually uncovered the exact translation
for each amino acid. In RNA, there are four possible letters (A, U,
G, and C) and three letters in each word. Accordingly, there are sixtyfour
possible words. With only twenty amino acids, it became clear
that more than one RNAword can translate into a given amino acid.
Yet, no given word stands for any more than one amino acid. A few
words do not translate into any amino acid; they are stop signals, telling
the ribosome to cease translating RNA.
The question of which direction an RNA is translated is critical.
For example, CAA codes for the amino acid glutamine, but the reverse,
AAC, translates to the amino acid asparagine. Such a difference
is critical because the exact sequence of a protein determines its
activity—that is, what it will do in the body and therefore what genetic
trait it will express.
Consequences
Synthetic RNAs provided the key to understanding the genetic
code. The genetic code is universal; it operates in all organisms, simple
or complex. It is used by viruses, which are nearly life but are not
alive. Spelling out the genetic code was one of the top discoveries of
the twentieth century. Nearly all work in molecular biology depends
on this knowledge.
The availability of synthetic RNAs has provided hybridization
tools for molecular geneticists. Hybridization is a technique in which
an RNA is allowed to bind in a complementary fashion to DNA under
investigation. The greater the similarity between RNAand DNA,
the greater the amount of binding. The differential binding allows for
seeking, finding, and ultimately isolating a target DNAfrom a large,
diverse pool of DNA—in short, finding a needle in a haystack. Hybridization
has become an indispensable aid in experimental molecular
genetics as well as in applied sciences, such as forensics.
See also :
Artificial hormone; Cloning; Genetic“fingerprinting”;
Genetically engineered insulin; In vitro plantculture;
Synthetic amino acid ; Synthetic DNA , Small interfering RNA
Further Reading :
