"I
believe," indicated Mr. Mayfield, "we are almost to the end of our conceptual
journey, Professor Yardley." The prosecuting lawyer went to his table and was handed
some papers by his colleague.
As Mr.
Mayfield slowly returned to the area of the witness stand, he was busy going through the
papers. Apparently, he was either looking for something or briefly reviewing the material
prior to launching into the next phase of direct examination.
When he was
near the witness stand, he stopped and studied the papers for a few more seconds. When he
had finished, he asked: "Professor Yardley, what is the so-called Central Dogma of
molecular biology?"
"Essentially,"
Dr. Yardley replied, "it says that DNA makes RNA which makes proteins. In living
organisms, the available evidence is overwhelmingly in support of this principle."
Pursuing the
issue, Mr. Mayfield inquired: "Does this principle raise any problems in relation to
accounts concerning the origins of life?"
"Yes,
it does," the professor responded. "In everyday terms, it leads to the question:
Which came first, the chicken or the egg?"
"Could
you elaborate a little, Professor Yardley?" requested the prosecuting lawyer.
"Briefly
stated," the professor summarized, "if DNA is necessary to make, first, RNA and,
then, proteins, but the synthesis of DNA polymers depends on the presence of catalytic or
enzymatic proteins, then how can one start with DNA which is dependent on the very
molecule which it is suppose to make? On the other hand, if proteins depend on the
existence of DNA and RNA molecules, then how can proteins come into being prior to that on
which they depend?
"If we
have DNA reprise the role of the egg to protein's stirring rendition of the chicken, we,
once again, are faced with an ancient paradox. In the present case, the problem becomes:
which came first, the protein or the DNA?"
"Is
there any plausible way out of this dilemma, Dr. Yardley?" asked the prosecuting
attorney.
"Until
relatively recently, this paradox constituted a major stumbling block to providing an
overall plausible explanation for how life originated from prebiotic beginnings through
purely naturally processes. The situation vis-a-vis this paradox began to change around
1983.
"In
that year, two researchers, Sidney Altman and Thomas Cech, quite independently of one
another, made a breakthrough. They discovered what has come to be known as a 'ribozyme'.
"A
ribozyme is a polymerized or chained sequence of molecules which is drawn from RNA and
exhibits some of the properties of a protein enzyme or catalyst. In the case of the
Altman-Cech findings, the RNA sequence which had been discovered was able to cut and join
pre-existing strands of RNA.
"This
ability to cut into a given sequence of RNA and then to splice such sequences together is
of considerable importance. Broadly speaking, not only do such capacities allow for the
possibility of building longer sequences of RNA, but cutting and splicing constitute tools
which could play fundamental roles in processes of both replication and the rearranging of
RNA sequences to generate, or experiment with, alternative genetic characteristics.
"Most
importantly, ribozymes do not presuppose anything else to accomplish these functions. In
other words, the chicken/egg paradox evaporates since an RNA sequence which is capable of
acting as an enzymatic molecule in relation to other RNA molecules, depends on neither
proteins nor DNA in order to come into being. RNA molecules are serving as both hereditary
blueprints as well as catalytic agents for the generation and development of such
blueprints.
"RNA
molecules have a further advantage, at least in relation to DNA molecules. The
ribonucleotides in RNA - that is, the bonded triads of ribose sugar, phosphate and nucleic
base which are chained together to create sequences or polymers of RNA such
ribonucleotides are more easily synthesized than are the deoxyribonucleotides of DNA.
"On the
other hand, deoxyribonucleic acids are more stable than are ribonucleic acids.
Consequently, whereas the easier path of synthesis would have conferred an evolutionary
advantage on RNA molecules over DNA, the property of greater stability would have
conferred, later on, an evolutionary advantage of DNA molecules over RNA.
"Many
theorists cite this dimension of greater stability as probably one of the primary factors
which led to a gradual evolutionary transition from RNA-based proto cells or life to
DNA-based proto cells or life. At some point, DNA displaced RNA from the latter's role as
keeper of the genetic memory.
"During
the 1960s, some twenty years before the discovery of the first ribozyme, three scientists,
Francis Crick, Carl Woese and Leslie Orgel, all working independently of one another, had
each suggested that RNA may have had evolutionary priority over both DNA and proteins.
Today, the original proposal of these three scientists has evolved, through the
contributions of a variety of theorists and researchers, to become a theory known as 'the
RNA world'.
"In the
RNA world, as one might anticipate, RNA plays a central role. RNA, as the carrier of
genetic information, as well as the agent responsible for catalyzing reactions, becomes
responsible for generating all of the steps considered necessary to produce the first
precursor of life capable of self-replication and evolutionary change.
"One
can lend support to the idea of the RNA world theory with a number of recent findings. For
example, consider the work of Harry Noller Jr..
"He was
doing research on ribosomes which frequently are called the protein factories of a cell.
Ribosomes consist of , on the one hand, ribosomal RNA, which differs in certain ways from
non-ribosomal RNA, and, on the other hand they contain various kinds of protein. Both of
these components are joined together to form what are known as ribonucleoprotein subunits.
"Each
ribosome is assembled from two such subunits. Each of these subunits is slightly different
in size and kind from the other.
"Furthermore,
different kinds of ribosomal subunits can be found in prokaryotic, or non-nucleated
organisms, and in eukaryotic, or nucleated organisms. However, just to complicate matters,
some of the former, prokaryotic kinds of ribosomal units also can be found within certain
eukaryotic intracellular organelles, or membraned-centers, such as the energy-related
factories known as mitochondria and chloroplast.
"In
general terms, ribosomes travel along the length of various strands of messenger RNA.
Messenger RNA is a single-stranded transcription of the triplet nucleic bases which are
carried by DNA molecules. These triplet, nucleic base sequences, or codons, constitute the
letters, so to speak, designating the specific word from the dictionary of twenty amino
acids which is being called for by means of the messenger RNA.
"As a
ribosome travels along the strand of messenger RNA, the ribosome helps forge a linkage,
known as a peptide bond, between amino acids. The ribosome does this by taking the amino
acid called for by one triplet nucleic base sequence of messenger RNA and connecting the
indicated amino acid with another amino acid which is being called for by the subsequent
RNA triplet nucleic base sequence of the same strand of messenger RNA.
"The
ribosome accomplishes its task of fashioning polymers or chains of amino acids - that is,
proteins - with the assistance of a further kind of RNA, known as transfer RNA. This form
of RNA consists of between 70-80 nucleotides which are specially modified or adapted to be
able to interact with the construction area formed by both the ribosome and the strand of
messenger RNA.
"One
portion of the transfer RNA carries the amino acid being called for. Another section,
known as the anticodon, links up with the appropriate codon section of the messenger RNA,
and the final sequence of the transfer RNA links up with the ribosome.
"Thus,
transfer RNA delivers the required amino acid to the active site of the interaction
between the ribosome and messenger RNA. This tri-partite co-operative effort continues,
using a succession of different transfer RNA molecules, until the fully-formed protein,
which is being specified by the collective set of triplet codons of messenger RNA, has
been completed.
"Harry
Noller, the scientist I mentioned earlier doing research into the nature and functioning
of these ribosomes about which I have been talking, discovered something of considerable
importance to the RNA world theory. He found evidence suggesting that ribosomal RNA
appears to play a major catalytic role leading to the formation of peptide bonds between
amino acids being delivered by transfer RNA to the site of interaction between messenger
RNA and the ribosome.
"The
proteins present in ribosomes also have a catalytic role to play. Yet, this role appears
to be limited to one of enhancing the degree of efficiency of the process already set in
motion by the ribosomal component of the subunit.
"This
molecule, consisting of ribosomal RNA and protein, is known as ribonuclease-P, and it is
considered to be a true enzyme. Not only does it accelerate the rate of the formation of
peptide bonds significantly over what would occur in the absence of such a molecule, but
the molecule survives the chemical reaction and is capable of repeating the process with
other transfer RNA molecules.
"On the
one hand, the self-splicing ribozyme mentioned earlier is not considered a true enzyme.
Although that molecule does have an enzyme-like function which involves capacities for
cutting and splicing, nonetheless, at the end of the chemical reaction, the molecule does
not get restored to its original, pre-reaction form.
"On the
other hand, scientists, like Gerald Joyce, have been able to take this research a few
steps further. Through a variety of procedures, he was able to generate ribozymes - that
is, RNA with catalytic properties which could cleave a number of different kinds of
chemical bonds, including the peptide bonds which link amino acids together in biological
organisms."
