http://apologeticspress.org/APContent.aspx?category=12&article=2317
The RNA World Hypothesis Explained and Unexplained
| by |
Kathleen Hamrick
Will Brooks, Ph.D. |
[Editor’s Note: The following article was written by A.P.
auxiliary staff scientist Will Brooks and one of his students. Dr.
Brooks holds a Ph.D. in Cell Biology from the University of Alabama at
Birmingham and serves as Assistant Professor of Biology at
Freed-Hardeman University.]
One of the goals within the discipline of biology is to define life.
This goal, however, is no simple task. While we can have an intuitive
understanding of what it means to be alive, forming this understanding
into a precise definition of life poses a dilemma for scientists. Life
comes in many shapes, sizes, colors, and forms, so placing all these
variations of life into one nice definition is seemingly impossible. To
circumvent this problem, scientists have defined life by stating
characteristics shared by all life forms. To be considered “alive,” a
system of molecules must possess each of these characteristics. Examples
include (1) the ability to sense and respond to stimuli, (2) the
ability to acquire and utilize materials for energy, (3) the ability to
store genetic information in the form of
DNA, and
(4) the ability to self-replicate. All living organisms share these
basic characteristics, and those systems of molecules which lack even
one of these basic characteristics is not considered to be a living
organism.
Deoxyribonucleic acid (
DNA) is the genetic material used by all living organisms to code for life.
DNA
can be thought of as the genetic fingerprint of each organism because
it is unique to each species of organism. During the process of
self-replication, this genetic code is duplicated and identical copies
(discounting rare instances of mutation) are given to each progeny of an
organism, maintaining the fingerprint and thus the identity of that
organism. The function of
DNA as the genetic
material of an organism is to provide a code for the production of
another group of molecules known as proteins. Proteins serve a host of
functions for an organism. They are known, appropriately, as the
workhorses of a cell, because they carry out the vast majority of
organismal tasks, including catalysis.
A catalyst is any substance capable of increasing the speed of a
chemical reaction. Within each living organism on Earth, millions of
chemical reactions take place every minute. The majority of these
reactions are prompted by a very large group of protein catalysts known
as enzymes. These enzyme-mediated chemical reactions range from those
used to synthesize various metabolites to those used to break down
ingested foods. By serving as enzyme catalysts, proteins play a crucial
role in all living organisms. For without enzymes, organisms would be
both unable to break down the food that they ingest and unable to make
the necessary metabolites needed to sustain life.
While the vast majority of functional enzymes within living organisms
are proteins, scientists have discovered that another group of
molecules, known as ribonucleic acids (
RNAs), are also capable of catalyzing some chemical reactions (Kruger, et al., 1982).
RNAs are very similar in structure to
DNA, differing only in the type of sugar used to form the molecules—
DNA utilizes deoxyribose and
RNA utilizes ribose. While
DNA is the vital genetic code that is passed down between parents and offspring,
RNA also plays an important role. Ribonucleic acids are a messenger system that carries the
DNA code from the cell’s nucleus, the home of
DNA, to the cellular cytoplasm where proteins are synthesized. These are known as messenger
RNAs (m
RNA). Furthermore, another group of
RNAs, known as ribosomal
RNAs (r
RNAs),
is used along with proteins to build the cellular structure known as
the ribosome, which is the cellular location at which proteins are made.
So,
RNA plays several related roles in the process of protein production: (1) it carries the genetic code from
DNA to the ribosome, (2) it helps form the structure of the ribosome, and (3) it functions in catalysis.
While there are a few other examples (reviewed in Fedor and Williamson, 2005), the catalytic properties of
RNA
are best seen in the ribosome. When proteins are synthesized by an
organism’s cells, small units known as amino acids are chemically linked
together to form a long, linear chain. This chain of amino acids is
known as a polypeptide or protein. The chemical bond that links together
each amino acid in the chain is called the peptide bond. Because each
of the 20 amino acids are very similar in structure, the same peptide
bond is formed between every unit of the polypeptide chain. The chemical
reaction that forms this peptide bond requires catalysis. The protein-r
RNA
complex that we know as the ribosome has long been known to serve as
the site as well as the catalyst in forming the peptide bond. But,
scientists were surprised to discover that the protein component only
serves as a structural element of the ribosome. It is the
RNA component of the ribosome that serves as the catalyst (Nissen, et al., 2000). This catalytic
RNA has thus been termed a ribozyme.
Later it was discovered that yet another group of
RNAs, the small nuclear
RNAs (sn
RNA), were also capable of catalyzing a chemical reaction (Valadkhan and Manley, 2001). When produced by the cell, m
RNA
must undergo a series of maturation steps before it is fully functional
as a genetic message (Alberts, et al., 2002, pp. 317-327). One of these
steps toward maturity is the process of splicing. Newly synthesized m
RNA
contains large regions, spread throughout its length, that do not
directly code for protein production. These non-coding regions are
called introns. To make the m
RNA mature and functional as a code, each intron must be removed from the m
RNA
and the remaining coding regions, known as exons, must be linked or
spliced back together. These “cut-and-paste” events occur within the
cell’s nucleus within a structure that we call the spliceosome. Like the
ribosome, the spliceosome is a large complex of both protein and
RNA, in this case sn
RNA. Amusingly, these protein-
RNA complexes have been dubbed small nuclear ribonucleoproteins or “snurps.” Interestingly, scientists found that not protein, but
RNAs were responsible for catalyzing the chemical reactions that take place during these splicing events.
RNAs were carrying out chemical reactions on other
RNAs.
Scientists were very excited by these revolutionary findings. Now, they had a single type of molecule,
RNA, that possessed two very important properties. First, it was very similar in structure to
DNA
and thus theoretically could also store genetic information. Second, it
could function as a catalyst like proteins. In 1986, Walter Gilbert
coined the phrase “
RNA World” and initiated what is now known as the
RNA World Hypothesis (Gilbert, 1986). This hypothesis on the origin of life states simply that because
RNA has the dual ability to both store genetic information and catalyze chemical reactions, it must pre-date
DNA and proteins, both of which supposedly evolved after and perhaps from the
RNA.
The
RNA World Hypothesis is widely accepted by
evolutionists, because it provides an alleged solution to a
long-recognized problem in evolutionary theory. Consider how
proteins are made by a cell. First,
DNA which holds the genetic code is converted into
RNA
through a process known as transcription. This process is similar to
how one would copy a letter from one piece of paper onto another sheet.
The contents of the letter remain unchanged, only the medium—the
paper—has changed.
RNA carries this information
to the ribosome, where it is read and used as a code to make a protein
through a process known as translation. This process can be compared to
translating the copy of the letter from one language into another.
Nucleic acid (
DNA and
RNA) is changed into another molecule altogether: protein. This linear progression of
DNA to
RNA
to protein is known in biology as the Central Dogma of Molecular
Biology (Alberts, et al., 2002, p. 301). Of the three components in the
path, only
DNA has the capacity to be replicated. So, while
DNA
stores the genetic code and can be replicated, it cannot perform any
chemical reactions. And, while protein can perform chemical reactions,
it cannot store genetic information. So, in evolutionary thinking, which
came first—
DNA or protein? Making the problem even more difficult,
DNA relies upon proteins during its own replication.
DNA
does not self-replicate of its own accord. It must have protein enzymes
to facilitate this process. So, what came first—the chicken or the egg?
DNA or protein? Each relies upon the other. You should begin to see how
RNA might solve this problem. If
RNA
can both store genetic information and catalyze chemical reactions, and
if it evolved first, we have a single molecule that stores information
and can catalyze its own replication, a self-replicating genetic
material.
In order to prove this theory plausible, a set of conditions must be created to favor the spontaneous formation of
RNA molecules without the aid of a biological catalyst. This would have had to be the starting point for an
RNA world. One necessary component for
RNA formation would be a steady supply of nucleotides, the building blocks of
RNA.
Scientists speculate these nucleotides were created from other small
molecules present, or were generated in space before arriving on earth.
Ribose, the sugar used in
RNA, is assumed to have
arisen from formaldehyde via the formose reaction. The mystery of the
addition of nucleotides onto a ribose backbone remains unsolved by
scientists attempting to create conditions of a primitive Earth (Müller,
2006, 63:1279-1280). Once these
RNA molecules
were formed completely by chance, they would have to have possessed or
evolved the ability to catalyze reactions leading to self-replication.
After sustaining itself through several replications, the
RNA
would then need to gain the ability to create a barrier between the
extraneous materials surrounding it, in order to isolate the beneficial
products from those proving non-functional. Thus, a membrane of sorts
would have had to evolve and be maintained (Müller, 63:1285-1286). These
steps are only the basics, proving the task much too complicated to
occur by mere chance.
In all known organisms living today,
DNA and not
RNA is the genetic material.
DNA has advantages over
RNA which make it a more suitable molecule to store the very important genetic code. First,
DNA is a double-stranded molecule while
RNA 
is single-stranded. The double-stranded nature of
DNA gives it the ability to be replicated in a much simpler series of steps. When
DNA
is replicated, each of the two complimentary strands serves as a
template on which to build another strand. The result is that in one
step, each strand of
DNA is replicated to produce four total
DNA strands or two identical double helices.
RNA,
however, is single-stranded. In order for it to be replicated, two
sequential rounds of replication would be required. First, a
complimentary strand would need to be synthesized from the original
parental strand. Only then could that new complimentary strand be used
to re-make the parental strand. As stated before,
DNA and
RNA differ in the sugar which makes up the molecule’s backbone. Deoxyribose, the sugar used in
DNA, differs from ribose used in
RNA,
by lacking one organic functional group known as alcohol. The absence
of this alcohol group greatly increases the stability of
DNA over
RNA. In ribonucleic acids, this
–OH group is capable of initiating chemical reactions which favor breakdown of the
RNA molecule. For these and other reasons,
DNA is a much more stable and preferable genetic material. This is made obvious by the fact that all living organisms use
DNA, not
RNA, as their permanent storage medium of genetic information. It also indicates that
RNA would be an unsuitable medium by which to initiate life.
Evolutionists would have us to believe that non-living elements and
molecules joined together and developed increasing biological
capabilities. Those who believe in intelligent design reject this
hypothesis, insisting that neither
RNA nor living
cells are able to evolve spontaneously. While some disagreement exists
among those in the evolutionary community on the time frame for such
alleged reactions to occur, the consensus is that, given large amounts
of time, single-celled bacteria were formed.
But all known biological principles militate against this notion. Even billions of years could not provide mechanisms for the reaction products to evolve advantageous characteristics and form
DNA and cell proteins, let alone create strings of
RNA
nucleotides, arriving at just the right sequence in order to code for a
functional protein. The four nucleotide bases that form
RNA
(adenine, guanine, cytosine, and uracil) can be arranged in an
exponential array of combinations and lengths. For an actual, functional
protein to be coded, a precise sequence of nucleotides must be
obtained.
Forming the code for even one protein by evolutionary means is impossible, without even considering the necessity of the number that work together in a single cell.
There is no scientific evidence to suggest that
RNA
is spontaneously being created and capable of forming pre-cellular life
today. While some artificial ribozymes have been created in the
laboratory (reviewed in Chen, et al., 2007), there are still significant
holes in reproducing an
RNA world to support the
hypothesis. The ribozymes created artificially lack the abilities to
sufficiently process themselves, and there is no evidence of them
producing large quantities of advantageous nucleotide sequences.
Moreover, no system has ever created cellular life. There is even
significant debate among scientists over the conditions and constituents
of a “prebiotic Earth” model.
The
RNA World Hypothesis is simply another
attempt by scientists to explain the origin of life to the exclusion of
the divine Creator. Given the absolute impossibility of life originating
from the reactions of non-living matter, it can be justified that
RNA did not predate other biological molecules. All biological molecules were created together to work in concert.
RNA was designed to be the essential intermediate between
DNA
and proteins, making our cells capable of sustaining life as it was
created. The designer of this system must be the intelligent Designer,
the God of the Bible.
REFERENCES
Alberts, Bruce, et al. (2002),
Molecular Biology of the Cell (Oxford: Garland Science).
Chen, Xi, et al. (2007), “Ribozyme Catalysis of Metabolism in the
RNA World,”
Chemistry and Biodiversity, 4:633-656.
Fedor, Martha and James Williamson (2005), “The Catalytic Diversity of
RNAs,”
Nature Reviews Molecular Cell Biology, 6(5):399-412.
Gilbert, Walter (1986), “The
RNA World,”
Nature, 319:618.
Kruger, Kelly, et al. (1982), “Self-splicing
RNA: Autoexcision and Autocyclization of the Ribosomal
RNA Intervening Sequence of Tetrahymena,”
Cell, 31(1):147-57.
Müller, U.F. (2006),
“Re-creating an
RNA World,”
Cellular and Molecular Life Science, 63:1278-1293.
Nissen, Poul, et al. (2000), “The Structural Basis of Ribosome Activity in Peptide Bond Synthesis,”
Science, 289:920-930.
Valadkhan, Saba and James Manley (2001), “Splicing-related Catalysis by Protein-free sn
RNAs,”
Nature, 6857:701-707.
