Science in Christian Perspective
Implications of Molecular Biology for
Creation and Evolution
ROBERT L. HERRMANN
Boston University School of Medicine
Boston, Massachusetts 02118
From: JASA 27 (December 1975): 156-159.
Survey of Molecular Biology
In 1953 Watson and Crick1 proposed the doublehelical structure of DNA, the polynucleotide molecule carrying the cell's genetic informaton. Four types of heterocyclic nitrogenous substances (bases) were bound into its structure by means of the sugar 2-deoxyribose, and phosphoric acid. The combination of a given base, a sugar and phosphoric acid is called a nucleotide (See Figure 1).
The crucial feature of the
(Figure 2) was that the two chains of nucleotide building blocks were
Every time an adenine nucleotide (A) is present in one chain, the
bears a thymine nucleotide (T). Likewise, every time a guanine nucleotide (C)
appears in one chain, the other chain bears a cytosine nucleotide
(C). The unique
pairing is the basis of precise duplication of the genes which is so necessary
for the hereditary mechanism. Gene duplication occurs by separation
of these two
chains and the synthesis of a new matching strand for each, so that there are
then two double-stranded structures where before there had been only one. Each
"daughter" molecule now carries the exact arrangement of nucleotide
units as the "parent" molecule, because the unique pairing
of the nucleotide
units prescribes that this be so. This is of utmost importance
because the linear
sequences of nucleotide units are eventually translated into linear sequences
of amino acid units for all of the protein molecules which make up the living
By 1960, experiments in many laboratories indicated that the cell's protein molecules were synthesized by a process involving transcription of the DNA sequence into a second polynucleotide, messenger RNA, which, in conjunction with various elements of cell sap including complex structures called ribosomes, could cause incorporation of radio-active amino acids into protein-like polypeptide material (See Figure 3).
The great breakthrough in understanding this process came about when Nirenberg and Matthaei found that synthetic RNA molecules could catalyze the protein synthetic process in these simple cell-free systems derived from bacteria.2 The synthetic polynucleotides, produced with an enzyme called polynucleotide phosphorylase, could be made with various combinations of the component building blocks of natural RNA and then the protein synthesized subsequently from these compounds in the cell-free system could be analyzed. In this way it was discovered that the code signal for the insertion of a given amino acid into a protein structure was a sequence of three nucleotide units of the polynucleotide. For example, three uridine nucleotides (a trinucleotide) in a sequence of the RNA specifies the positioning of one molecule of the amino acid phenylalanine in the sequence of the protein.
Later a more precise method of determining the coding sequence (the "codon") corresponding to a given amino acid was discovered, based upon the
Figure 1. The combination of a heterocyclic nitrogenous base with the sugar 2-deoxyrihose and phosphoric acid forms one of the nucleotide building blocks of deoxyribonucleic acid (DNA)
Figure 2. A representation of the double-helical model of DNA, illustrating the complementary base-pairing of adenine (A) with thymine (T) and quanine (C) with cytosine (C).
known involvement of a second type of RNA, transfer RNA (t-RNA) in protein synthesis (See Figure 3). This molecule was shown to occur in many formsat least one for each amino acid found in proteinsand to function by adapting its amino acid to the codon through a complementary sequence of nucleotides in its own structure. It was found that even in the absence of protein synthesis, the specific t-RNA molecules bind to complexes of ribosomes and messenger RNA. Furthermore the messenger RNA could be replaced not only by the synthetic polynucleotsdes used in the earlier experiments, but also by simple trinucleotides of precise structure. In this method a given trinucleotide representing a single codon could be examined for its ability to cause binding of various t-RNA molecules with their attached amino acids to the ribosome structure. Those t-RNA molecules which bound must have have been able to recognize that codon as the position for insertion of their particular amino acid. In this way it was possible to assign each codon to a specific amino acid.
The Genetic Code
Figure 4 represents the genetic code as worked out for the bacterium E. coli. Several interesting features are apparent with respect to evolution. The first is the phenomenon called degeneracy. Note that for most of the amino acids there is more than one codon, e.g., phenylalanine is coded for by both UUU and UUC. The third position can vary and specificity still be retained. Because of this variation, it has been suggested that the original code was a doublet instead of a triplet code. Variation in the 3rd position would also allow for the cell to undergo mutational change without that change being necessarily lethal. CT stands for codons which cause termination of a peptide chain (chain termination) and CI stands for chain initiation. Here the amino acid methionine serves as the initiating amino acid and in this case the methionine is first formylated before initiating peptide synthesis. There are also some interesting relationships between amino acids and their codons. Similar amino acids (similar side chains) have similarities in their code words, e.g., all non-polar amino acids (phenylalanine, leucine, isolcucine, valine) have U as the second code letter. Also, aspartie acid and glutamic acid, closely related structurally, both have GA as their first two letters. This suggests another evolutionary possibility; the specific code words for the various amino acids
The implications of a universal genetic code are interesting, fascinating or threatening, depending on your viewpoint.
arose because of some physicochemical relationship between the
and the amino acid which it specifies. This possibility has been
explored by several workers.3,4
A Universal Code
Extension of these experiments to other bacteria, to intermediate forms and to mammals has led to the general conclusion that the genetic code is universalthat the same code words are used in both lower and higher organisms. For example, with rabbit reticulocytes, 22 codons have thus far been shown to be translated into amino acids identical to those in the F. co/i bacterial system. The data, though incomplete, point to a universal code.5
Likewise, the protein-synthetic mechanisms in prokaryotic and eukaryotic systems appear to be quite similar. For example, the chain initiating codon which in the bacterium E. coli involves a special form of transfer RNA, which places the amino acid methionine in the chain at that point, is also utilized by yeast, by wheat germ, by mouse liver and rabbit reticulocytes. Other features of the mechanism also appear similar.
The implications of such a mechanism are interesting, fascinating, or threatening, depending on your viewpoint. The existence of a universal code would imply that there was indeed a single precursor of all living things, a primitive system capable of replication and information transfer from which all the present living forms developed.
A Specific Model
In fact, mechanisms have been proposed for the origin of such a system given the necessary building blocks which appear to have been present on the primitive earth. Quastler, in his Emergence of Biological Organization6 suggests one such mechanism. As we have indicated, the genetic material, DNA, is made up of two polynucleotide chains whose most unique feature is the complementary pairing of the nucleotide building blocks, A to T and G to C.
Figure 3. The scheme for protein synthesis. DNA is "read out" in the form of messenger RNA, which travels to the cyptnplasm and binds to structures called ribosomes. Here, a series of transfer RNA molecules, at least one type for each protein amino acid, carry their appropriate amino acid to the ribosome and align with a specific coding sequence on the messenger to form the proper sequence of the protein chain.
Even the informational content of a living system may have arisen from the apparently random way in which the nucleotide building blocks of the first successful system were incorporated into a polynucleotide polymer.
In Quastler's proposal for the origin of the nucleic acid system
(Figure 5), nucleotide
building blocks react with each other to form single polynucleotide
process would he very slow in the absence of enzymes, but Quastler
there would still be 400 periods during geological time available for
The single chains thus formed may then react further with additional nucleotide
units, by the nucleotide pairing principle, to form intermediate
are partly single-chained and partly double-chained. This reaction is much more
favorable than is the original reaction to form the single
Completion of this reaction leads to fully double-chained structures which may
then reversibly separate to form single chains.
The unique feature of such a system is that it gives rise to a kind of "information," in the sense that the first polynucleotide chain to be formed has a far greater chance for survival than any later arrivals. Thus it is able to compete more favorably for nucleotide units, since the reaction of the polynucleotide chain with nucleotides is favored over the original synthesis of the polynucleotide. The first chain thus becomes the progenitor of a unique polynucleotide system made up of itself and its "sister" chain, in which each nucleotide unit is the opposite pairing partner for the other chain-i.e., A opposite T and G opposite C. The information content of the system, as Quastler sees it, is of the nature of an "accidental thought remembered." The original arrangement of nucleotide units in the polynucleotide chain might have been arrived at by purely random interaction, but once the chain is formed, that particular arrangement and that of its sister strand are the only allowable structures. A good analogy would be the numbers of a combination lock. Prior to their choice for the combination, the numbers are of no consequence. But after being introduced as the numbers of the combination they are now information.
The importance of Quastler's argument lies in its demonstration of the way in which the evolutionary
Figure 4. The genetic code as worked out for the bacterium E. coli.
ORIGIN OF A PRIMITIVE NUCLEIC ACID SYSTEM
Figure 5. Quastler's model for the origin of a nucleic acid system. Nucleotides react to form single-stranded polynucleotides. The latter can undergo a more favorable reaction to form partially double-stranded structures which eventually give rise to a double helical polynocleotide with a complementary base-paired structure.
Figure 6. A proposal for the attachment of primitive counterparts of amino acid transfer RNA molecules to the template of a polynucleotide system, with the eventuality of the synthesis of amino acid polymers.
principles of selection and competition can be applied at the chemical level. For here, from apparently random events, a system may be seen to arise that is capable of reproducing and propagating itself and hence acting as a kind of primitive genetic information.
Explanation of Protein Synthesis
The extrapolation of this scheme to an explanation for present mechanisms of protein synthesis may be made on the same principles of chemical evolution (Figure 6). Polynucleotides could react with amino acids with some degree of specificity3-4 to give adapter molecules similar to the amino acyl, t-RNA's of present protein synthesis. Complementary base pairing of these molecules to the original polynucleotide system would provide the opportunity for the system to couple amino acids in a variety of different arrangements, depending upon the sequence of the original polynucleotide, and, bacterium if one or more amino acid sequences proved to have enzymatic activity, there would be the tremendous advantage, by virtue of the self-duplicating property of polynucleotides, for this system to "remember" it.
Thus even the informational content of a living system may have arisen, in its simplest form, from the apparently random way in which the nucleotide building blocks of the first successful system were incorporated into a polynucleotide polymer. Considering the available data on the universality of the code and a theoretical framework for its origin, the description of life's origins in a purely mechanistic sense would appear to lie within the grasp of modern molecular biology.
However, this should not lead to any feeling on the part of the scientist that his explanation of origins excludes other explanations-e.g., a theological one. Jacques Monod may object in his Chance and Necessity 8to the idea of a "necessity rooted in the very beginning of things," but there is certainly no valid reason to exclude such a possibility. The Scriptural view of origins in fact places its primary emphasis on this very idea of purpose and meaning in the creation; life was made with precision and order, with quite precise ends in view.
Part of the concern of many Christians about evolutionary theory is that they fear that a mechanistic explanation negates God. But this problem has been dealt with in an excellent fashion by Donald MacKay in his booklet Science and Christian Faith Today.9 God's activity includes not only his originating activity (Genesis) but also his sustaining activity. The Apostle Paul writes in Colossians 1, speaking of Jesus Christ, "in Him all things hold together" (Col. 1:17) and the writer to the Hebrews speaks of Christ "upholding all things by His Word and power." (Heb. 1:3) MacKay points out that the phrase "upholding all things" might better be translated "holds in being all things" emphasizing God's immanent activity, without which the universe would not just stop but rather without which it would cease to exist.
The picture of God as a kind of machine tender seems completely inadequate in light of this verse. Rather, God's activity is more like that of a master artist, who paints-in a dynamic fashion-a constantly changing picture. Something like this is suggested by the picture that a television receiver presents. The analogy is especially useful because it emphasizes the dynamic aspect of God's activity-"holding in being" the universe. For by simply ceasing his activity, it would be obliterated much as the television picture may he totally altered by simply flipping a switch. By bringing the focus to God's immanent activity, we see also the inapplicability of such arguments as "evolution leaves no room for the God of action, precluding his function except in areas of fast-disappearing links." The true picture is that God acts in all of Reality, not just where we cannot apply a scientific explanation. It is all His! As MacKay says "the whole multi-patterned drama of the universe is His." Also, the emphasis of Scripture is that God has ordered his Creation not by virtue of producing a perfect mechanism but rather because of His complete faithfulness. It is the ultimate basis for things, the raison d'etre, with which the Bible is dealing in its consideration of origins, and the character of the Creator is therefore its primary concern.
Science gives us the view of how life may have come about. Its view is descriptive, and does not in any ultimate sense account for what it describes. The most we can say based on present data is that God may have used an evolutionary mechanism to achieve the purposes delineated in Scripture.
1J. Watson & F. Crick, "The Structure of DNA." Nature 171 736 (1953).
2M. Nirenherg and J. Mattaei, "The Dependence of Cell-free Protein Synthesis in E. coli upon Naturally Occurring or Synthetic Polyribonucleotides. Prnc. Nat. Acad.Sci., U.S. 47 1588 (1961).
3Woese, C. R., "The emergence of genetic organization", in Exobiology, C. Ponnaniperuma, editor., Frontiers in Biology, Vol. 23, North. Holland, 1972 pp. 318-327.
4Pclc, S. R. and Welton, M. C. E. "Steriochemical relationship between coding triplets and amino acids." Nature
209 868-870 (1966).
5J. Lucas-Lenard and F. Lipmann, "Protein Biosynthesis" Annual Review of Biochemistry 40 409 (1971).
6H. Quastler, "Emergence of Biological Organization" Yale U. Press, New Haven, 1964 pp. 7-16.
7D. Reanney and R. Ralph "A Speculation on the Origin of the Genetic Code", J. Theor. Biol. 15 41 (1967).
8J. Monod, Chance and Necessity, Vintage Books, New York. 1972, pp. 143-6.
9D. MacKay, Science and Christian Faith Today, Church Pastoral-Aid Society (London) 1960.