by F.H.C. Ckrick
Medical Research Council Laboratory of Molecular Biology

Hills Road, Cambridge, England
(Received 21 August 1968)

from NationalInstituteForMedicalResearch Website

Introduction

The substance of this paper was originally presented at a meeting of the British Biophysical Society in London on 20 December 1966.

A very brief account appeared shortly after in a letter to Nature (Crick, 1967ª). When this manuscript was in its first draft, Dr Leslie Orgel told me that he had already prepared a draft of a paper on a related theme.

We therefore decided to publish our two papers together and have collated them to some extent to avoid overlap. We have not done this for all passages in the two papers which touch on the same topic, preferring on occasions to let our slightly different points of view be expressed as differences in treatment and emphasis. However, broadly speaking, each of us agrees with the opinion expressed by the other.

Since this paper was originally drafted a very full discussion has appeared in Carl Woese’s book The Genetic Code, which should be consulted for a fuller discussion of many of the points touched on here.

The Structure of the Present Genetic Code

The structure of the genetic code is now fairly well known.

The code is a non-overlapping triplet code. Most, but not all, of the 64 triplets stand for one or another of the 20 amino acids and, in most cases, each amino acid is represented by more than one codon. The best present version of the code is shown in Table 1. This is taken from the 1966 Cold Spring Harbor Symposium on The Genetic Code, to which the reader is referred as a source of references for many of the topics discussed here.

Before starting on a detailed examination of this Table a few words of caution are necessary Although the code shown there has been mainly derived from studies on Escherichia coli, it must be very similar in such widely different organisms as tobacco plants and man. In what follows I shall assume, for convenience of exposition, that it is identical in all organisms, which is very far from being proved.

In fact, it is probably untrue for the starting codons.

This Table shows the “best allocations” of the 64 codons at the time of the Symposium. Some of these allocations are less certain than others. The two codons marked ochre and amber are believed to signal the termination of the polypeptide chain. The codons suspected of being concerned with chain initiation are not indicated here

 

Again the function of the three presumed “nonsense” triplets is not known for certain. It is presumed that UAA (ochre) and UAG (amber) are signals for chain termination and probably UGA as well, at least in bacteria.

In E. coli there appears to be a special mechanism for initiating the polypeptide chain, involving formylmethionine and the codons AUG and GUG. The mechanism in higher organisms (if indeed a special one exists) is unknown.

Finally, it is uncertain whether there are ambiguous codons; that is, codons which represent more than one amino acid. Of course, it is known that mutations can produce errors in the translation mechanism and so make certain codons ambiguous, but it is not known whether ambiguity occurs “normally”. Again in what follows I shall assume that this is not usually the case for present-day organisms.

The basic reason why one can ignore these complications and uncertainties for the moment is that the broad features of the genetic code are not likely to be greatly affected by them. What, then, are the properties of the code which require explanation?

There are some features which are of such a general type that they do not depend at all upon the details of the code. They are:

(1) there are 4 distinct bases in the mRNA
(2) each codon is a triplet of bases
(3) only 20 of the numerous possible amino acids are used. In examining Table 1, however, one is apt to take all these characteristics for granted. What, then, is special about the actual details of the genetic code ?

(4) The 20 amino acids are not distributed at random among the 64 triplets
In fact, several rules can easily be deduced from the Table. For example,

(a) XYU and XYC always code the same amino acid.

(b) XYA and XYG often code the same amino acid. The rare amino acids, methionine and tryptophan, which have only one codon each, appear to be exceptions to this rule.

(c) In half the cases (8 out of 16) XY represents a single amino acid, where the XY implies that all four bases are possible.

(d) In most cases the codons representing a single amino acid start with the same pair of bases. Thus the two codons for histidine both start with CA. There are three exceptions to this:

• Leucine has CU- and UU^A_G

• Serine has UO and AG^U_C

• Arginine has CG- and AG^A_G

(e) If the first two bases consist only of G’s and C’s, then the four codons sharing the same initial doublet all code the same amino acid. That is, the meaning of these codons is independent of the third base. This is in fact true for all codons having C in the second position. More complicated rules along these lines can be produced for the remaining codons but they seem to me to be rather forced.

(5) Even allowing for the grouping of codons into sets, the amino acids do not seem to be allocated in a totally random way. For example, all codons with U in the second place code for hydrophobic amino acids. The basic and acidic amino acids are all grouped near together towards the bottom right-hand side of Table 1.

 

 Phenylalanine, tyrosine and tryptophan all have codons starting with U, and so on. It is very difficult not to imagine regularities in even a random grouping but nevertheless the general impression is that “related” amino acids have to some extent related codons (Epstein, 1966).

(6) The code is universal (the same in all organisms) or nearly so.

Why is the Code Universal?

Two extreme theories may be described to account for this, though, as we shall see, many intermediate theories are also possible.

The Stereochemical Theory

This theory states that the code is universal because it is necessarily the way it is for stereochemical reasons. Woese has been the main proponent of this point of view (see Woese, 1967).

 

That is, it states that phenylalanine has to be represented by UUJÍ, and by no other triplets, because in some way phenylalanine is stereochemically “related” to these two codons.

 

There are several versions of this theory. We shall examine these shortly when we come to consider the experimental evidence for them.

 

 

The Frozen Accident Theory

This theory states that the code is universal because at the present time any change wovld be leihal, or at least very strongly selected against.

 

This is because in all organisms (with the possible exception of certain virases) the code determines (by reading the mRNA) the amino acid sequences of so many highly evolved protein molecules that any change to these would be highly disadvantageous unless accompanied by many simultaneous mutations to correct the “mistakes” produced by altering the code.

This accounts for the fact that the code does not change.

 

To account for it being the same in all organisms one must assume that all life evolved from a single organism (more strictly, from a single closely interbreeding population).

 

In its extreme form, the theory implies that the allocation of codons to amino acids at this point was entirely a matter of “chance”.

The Stereochemical Theory - Experimental Evidence

In its extreme form, the stereochemical theory states that the postulated stereochemical interactions are still taking place today. It should therefore be a simple matter to prove or disprove such theories.

Pele and Welton (Pele & Welton, 1966; Welton & Pele, 1966) nave suggested from a study of models that there is in many cases a specific stereochemical fit between the amino acid and the base sequence of its codon on the appropriate tRNA.

Unfortunately, their models were all built backwards (Crick, 19676) so their claims are without support. Such a theory implies that the expected codon sequence occurs somewhere on each tRNA. For example, no such sequence occurs in the tRNA for tyrosine either from yeast (Madison, Everett & King, 1966) or from E. coli (Goodman, Abelson, Landy, Brenner & Smith, 1968). In our opinion this idea has little chance of being correct.

A more reasonable idea is that the amino acid fits the anticodon on the tRNA. At least this has the advantage that it is always present. A model along these lines for proline has been briefly described by Dunnill (1966), but so far no detailed description has been published, nor has he extended his model-building to other amino acids.

The experimental evidence has already established that when the activating enzyme transfers the amino acid to the tRNA, the interaction is not solely with the anticodon and the common... CCA terminal sequence.

This is shown by the fact that an activating enzyme from one species will not always recognize the appropriate tRNA from a different species although the anticodons must be very similar if not identical in different species (for a summary of the data, see Woese, 1967, p. 125). However, this does not preclude the idea that the interaction is partly with the anticodon and partly with some other part of the tRNA.

The best way to disprove the theory (if indeed it is false) would be to change the anticodon of some tRNA molecule and show that nevertheless it accepted the same amino acid from the activating enzyme.

This has already been done for the minor tyrosine tRNA of E. coli whose anticodon has been changed (in an Su + strain) from GUA to CUA (Goodman et al., 1968) although the experiments need to be done quantitatively. Further examples of such changes are likely to be reported in the near future.

Until this is done we must reserve final judgment on the amino acid-anticodon interaction theory; but we consider it unlikely to be correct, except per-haps in a few special cases.

Even if it were established that the activating enzyme recognizes the anticodon, this would not by itself prove that the recognition is done by inserting the amino acid in a cage formed by the anticodon. Notice that the activating enzyme would have to release amino acid from its own recognition cavity and then insert it into the recognition site on the tRNA.

Moreover, when the amino acid has been transferred to the tRNA and the activating enzyme has diffused elsewhere, the amino acid could not stay in the anticodon cage without blocking the interaction with the codon on the mRNA. None of this is impossible but it is certainly elaborate.

It is not easy to see at this stage what evidence would be needed to prove that the anticodon does indeed form a cage for the amino acid, though if the tRNA (or perhaps a fragment of it) could be crystallized it might be possible to see the amino acid sitting in such a position.

The present experimental evidence, then, makes it unlikely that every amino acid interacts stereochemically with either its codon or its anticodon. It by no means precludes the possibility that some amino acids interact in either of these ways, or that such interactions, even though now not used, may have been important in the past, at least for a few amino acids.

We must now leave the system as it is today and turn to the examination of primitive systems.

The Primitive System

It is almost impossible to discuss the origin of the code without discussing the origin of the actual biochemical mechanisms of protein synthesis.

This is very difficult to do, for two reasons: it is complex and many of its details are not yet understood. Nevertheless, we shall have to present a tentative scheme, otherwise no discussion is possible.

In looking at the present-day components of the mechanism of protein synthesis, one is struck by the considerable involvement of non-informational nucleic acid. The ribosomes are mainly made from RNA and the adaptor molecules (tRNA) are exclusively RNA, although modified to contain many unusual bases. Why is this?

One plausible explanation, especially for rRNA, is that RNA is “cheaper” to make than protein. If a ribosome were made exclusively of protein the cell would need more ribosomes (to make the extra proteins, which would not be a negligible fraction of all the proteins in the cell) and thus could only replicate more slowly.

Even though this may be true, we cannot help feeling that the more significant reason for rRNA and tRNA is that they were part of the primitive machinery for protein synthesis.

Granted this, one could explain why their job was not taken over by protein, since

(i) for rRNA, it would be too expensive
(ii) for tRNA, protein may not be able to do such a neat job in such a small space

In fact, as has been remarked elsewhere, tRNA looks like Nature’s attempt to make RNA do the job of a protein (Crick, 1966).

If indeed rRNA and tRNA were essential parts of the primitive machinery, one naturally asks how much protein, if any, was then needed. It is tempting to wonder if the primitive ribosome could have been made entirely of RNA.

Some parts of the structure, for example the presumed polymerase, may now be protein, having been replaced because a protein could do the job with greater precision. Other parts may not have been necessary then, since primitive protein synthesis may have been rather inefficient and inaccurate. Without a more detailed knowledge of the structure of present-day ribosomes it is difficult to make an informed guess.

It is not too difficult to imagine that the early tRNA molecules had no modified bases (so that no modifying enzymes were needed), but it is much more difficult to decide whether activating enzymes were then essential.

An attractive idea (suggested to us by Dr Oliver Smithies) is that the primitive tRNA was its own activating enzyme. That is, that its structure had a cavity in it which specifically held the side-chain of the appropriate amino acid in such a position that the carboxyl group could be easily joined on to the terminal ribose of the tRNA.

It is thus not impossible to imagine that the primitive machinery had no protein at all and consisted entirely of RNA. This is discussed at much greater length in the companion paper by Dr L. E. Orgel, where the importance of the ease of replication of nucleic acid is emphasized.

We are faced with the question of the origin of all this RNA. Could the appropriate sequences have arisen by chance?

We do not feel this is totally impossible, for three reasons.

(a) Some natural catalyst (such as a mineral) for random nucleotide polymerization may exist. If this were so, RNA may have been made at very many places on the earth’s surface over a very considerable period of time, so that altogether an enormous number of different sequences may have been synthesized. It is difficult to assess the value of this idea, since such a natural catalyst has not yet been discovered. Another possibility is that a crude template mechanism developed at an early stage. This is fully discussed in the companion paper.

(b) The mechanism of “random” synthesis may preferentially produce structures with multiple loops (this is also discussed in the companion paper) so that sequences of this sort (which are indeed found in tRNA and rRNA) may have been synthesized preferentially. Moreover, the actual base-pairs used in the base-paired regions may not be critical for their structures. In short, the synthesis of an acceptable rRNA and tRNA may not have been so unlikely as it seems at first sight.

(c) The base-sequences needed may have been repetitive. For example, the early tRNA molecules may have been very alike, only differing in the anticodon and in the region of the presumed cavity. For all we know, the structure of the large rRNA molecules may have been partly repetitive. These repetitions might have been produced rather easily if there were an RNA replicase available. Possibly the first “enzyme” was an RNA molecule with replicase properties. Thus a system based mainly on RNA is not impossible. Such a system could then start to synthesize protein and thus could evolve very rapidly by natural selection. We shall not discuss here the difficult problem of how the various components were kept together, that is, the origin of a cell.

The point of this sketch is to impress the reader with the great difficulty of the problem.

It would certainly be easier if specific stereo chemical interactions could occur between amino acids and triplets of bases, but even if these are possible the origin of the present ribosomal translation mechanism presents grave difficulties.

The Primitive Code

We must now tackle the nature of the primitive code and the manner in which it evolved into the present code.

It might be argued that the primitive code was not a triplet code but that originally the bases were read one at a time (giving 4 codons), then two at a time (giving 16 codons) and only later evolved to the present triplet code. This seems highly unlikely, since it violates the Principle of Continuity.

A change in codon size necessarily makes nonsense of all previous messages and would almost certainly be lethal. This is quite different from the idea that the primitive code was a triplet code (in the sense that the reading mechanism moved along three bases at each step) but that only, say, the first two bases were read. This is not at all implausible.

The next general point about the primitive code is that it seems likely that only a few amino acids were involved. There are several reasons for this. It certainly seems unlikely that all the present amino acids were easily available at the time the code started. Certainly tryptophan and methionine look like later additions.

Exactly which amino acids were then common is not yet clear, though most lists would include glycine, alanine, serine and aspartic acid.

However, if sterepchemical interaction played a part in the primitive code, this might select amino acids which were available but not particularly common. Again, it seems unlikely that the primitive code could code specifically for more than a few amino acids, since this would make the origin of the system terribly complicated.

However, as Woese (1965) has pointed out, the primitive system might have used classes of amino acids. For example, only the middle base of the triplet may have been recognized, a U in that position standing for any of a number of hydrophobic amino acids, an A for an acidic one, etc.

Even though few amino acids (or groups of amino acids) were recognized, it seems likely that not too many nonsense codons existed, otherwise any message would have had too many gaps. There are various ways out of this dilemma. For example, as mentioned above, only one base of the triplet might have been recognized.

Another possibility, however, is that the early message consisted not of the present four bases, but perhaps only two of them.

The Number of Bases in the Primitive Nucleic Acid

The only strong requirements for the primitive nucleic acid is that it should have been easy to replicate, and that it should have consisted of more than one base, otherwise it could not carry any information in its base sequence.

One cannot even rule out the possibility that the base sequence of the two chains was complementary (as in the present DNA). Perhaps a structure is possible with only two bases in which the two chains run parallel (rather than anti-parallel) and pairing is like-with-like. It would certainly be of great interest if such a structure could be demonstrated experimentally.

Leaving this possibility on one side and restricting ourselves to complementary structures, we see that the number of bases must be even. If there were only two in the primitive DNA, the question arises as to which two. The obvious choices are either A with U (or T) or G with C. A less obvious possibility (suggested some time ago by Dr Leslie Orgel, personal communication) is A with I (where I stands for inosine, having the base hypoxanthine).

It is not certain that a double helix can be formed having a random sequence of A’s and I’s on one chain and the complementary sequence (dictated by A-I or I-A pairs) on the other chain, but it is not improbable, especially as the RNA polymers poly A and poly I can form a double helix.

Several advantages could be claimed for this scheme. Adenine is likely to be the commonest base available in the primitive soup, and inosine could arise from it by deamination.

Thus the supply of precursors might be easier than in the case of the other two alternatives, though how true this is remains to be established. Then again in a random (A, I) sequence I would presumably code in the same way as G does now, at any rate for the first two positions of the triplet.

If we can use the present code as a guide (though we shall argue later that this may be misleading), it is noticeable that the triplets containing only A’s or G’s in their first two bases (the bottom right-hand corner of the Table) do indeed code for some of the more obviously primitive amino acids.

It is important to notice that a scheme of this sort (or even one with like-with-like pairing) does not violate the principle of continuity. To change over from an (A, I) double helix to one like the present one but having A, I, U and C, the only steps required are a change in the replicase to select smaller base-pairs, and a supply of the two new precursors. The message carried (by the “oíd” chain) is unaltered by this step.

Gradually mutations would produce U’s and C’s on this chain and the new codons thus produced could be brought into use as the mechanism for protein synthesis evolved.

Eventually G would be substituted for I. At no stage would the message become complete nonsense. The idea that the initial nucleic acid contained only two bases is thus a very plausible one.

It remains to be seen whether primitive ribosomal RNA and primitive tRNA could be constructed using only two bases.

The Stereochemical Alternative

As stated earlier, it seems very unlikely that there is any stereochemical relationship between all the present amino acids and specific triplets of bases; but it is by no means ruled out that a few amino acids can interact in this way.

If this were possible, it would certainly help in the initial stages of the evolution of the code.

However, sooner or later a transition would have had to be made to the present type of system, involving tRNA’s, ribosomes, etc. It seems to us that this could only happen easily if the code at that stage was fairly simple and only coded a rather small number of amino acids.
 

The Evolution of the Primitive Code

Whatever the early steps in the evolution of the code, it seems highly likely that it went through a stage when only a few amino acids were coded.

At this stage either the mechanism was rather imprecise and thus could recognize most of the triplets, or only a few triplets were used, perhaps because the message contained only two types of base. We must now consider what would happen next.

A complication should be introduced into this simple picture. It could well be that at this stage the recognition mechanisms were not very precise and that any given codon corresponded to a group of amino acids (see Woese, 1965, who has stressed this point).

Thus codons for alanine might also incorporate glycine, those for threonine might also code serine, etc. However, it is by no means certain that this happened. It seems highly likely that a “cavity” to accept threonine would also accept serine to some extent, but the converse mistake is less likely and could depend on the exact nature of the structure involved. Thus, though the early coding machinery probably produced errors, we can only guess at their extent.

We shall argue that by far the most likely step was that these primitive amino acids spread all over the code until almost all the triplets represented one or other of them.

Our reasons for believing this are that too many nonsense triplets would certainly be selected against, so that most codons would quickly be brought into use (Sonneborn, 1965). In addition, it would be easier to produce a new tRNA, altered only in its anticodon, while still recognizing the amino acid, than to produce both a new anticodon and a new recognition system for attaching a new amino acid.

Thus, we can reasonably expect that the intermediate code had two properties:

(i)  few amino acids were coded, and

(ii) almost all the triplets could be read.

Moreover, because of the way this primitive code originated, the triplets standing for any one amino acid are likely to be related.

At this stage the organism could only produce rather crudely made protein, since the number of amino acids it could use was small and the proteins had probably not evolved very extensively.

The final steps in the evolution of the code would involve an increase in the precision of recognition and the introduction of new amino acids. The cell would have to produce a new tRNA and a new activating enzyme to handle any new amino acid, or any minor amino acid already incorporated because of errors of recognition.

This new tRNA would recognize certain triplets which were probably already being used for an existing amino acid. If so, these triplets would be ambiguous.

To succeed, two conditions would have to be fulfilled.

(1) The new amino acid should not upset too much the proteins into which it was incorporated. This upset is least likely to happen if the old and the new amino acids are related.

(2) The new amino acid should be a positive advantage to the cell in at least one protein. This advantage should be greater than the disadvantages of introducing it elsewhere.

In short, the introduction of the new amino acid should, on balance, give the cell a reproductive advantage.

For the change to be Consolidated we would expect many further mutations, replacing the ambiguous codons by other codons for the earlier amino acid when this was somewhat better for a protein than the later one. Thus, eventually the codons involved would cease to be ambiguous and would code only for the new amino acid.

There are several reasons why one might expect such a substitution of one amino acid for another to take place between structurally similar amino acids.

• First, as mentioned above, such a resemblance would diminish the bad effects of the initial substitution.

• Second, the new tRNA would probably start as a gene duplication of the existing tRNA for those codons.

Moreover, the new activating enzyme might well be a modification of the existing activating enzyme.

This again might be easier if the amino acids were related. Thus, the net effect of a whole series of such changes would be that similar amino acids would tend to have similar codons, which is just what we observe in the present code.

It is clear that such a mechanism for the introduction of new amino acids could only succeed if the genetic message of the cell coded for only a small number of proteins and especially proteins which were somewhat crudely constructed. As the process proceeded and the organism developed, more and more proteins would be coded and their design would become more sophisticated until eventually one would reach a point where no new amino acid could be introduced without disrupting too many proteins.

At this stage the code would be frozen. Notice that it does not necessarily follow that the original codons, of the original primitive code (as opposed to the intermediate code) will necessarily keep their assignments to the primitive amino acids. In other words, the evolution of the code may well have wiped out all trace of the primitive code.

For this reason arguments about which base-pair carne into use first on the nucleic acid should not depend too heavily on the assignments of the present code.
 

The idea described above is crucial to the evolution of the code. It seems to me not to be the same as the idea, suggested by several authors (Sonneborn, 1965; Goldberg & Wittes, 1966), that the code is designed to minimize the effects of mutations. The implication is that the mutations are those occurring in the many proteins of the organism, and in fact are still occurring today.

This is not quite the same as the idea that it is the situation produced by the introduction of a new amino acid to the developing code that we have to consider. Moreover, the disturbances had to be minimized not to the present day proteins but to the small number of more primitive proteins then existing.

The minimizing of the effects of mutations is in any case likely to have only a small selective advantage even at the present time, and I think it unlikely that it could have had any appreciable effect in molding the genetic code. Woese (1967) has made the same point.

An idea rather close to the one presented above has been developed by Woese (1965). He emphasizes in his discussion the fact that the early translation mechanism would probably be prone to errors. This is indeed an important idea and may well be what actually occurred but it is not identical to the idea suggested above, as can be easily seen by making the rather unlikely assumption that the early mechanism was rather accurate.

In this case Woese’s ideas are irrelevant and one is driven to the scheme outlined above.

Nevertheless, Woese’s discussion (Woese, 1967) follows much the same line as that presented here. However, he argues that by this mechanism it is unlikely that the code could reach the truly optimum code.

There is no reason to believe, however, that the present code is the best possible, and it could have easily reached its present form by a sequence of happy accidents. In other words, it may not be the result of trying all possible codes and selecting the best. Instead, it may be frozen at a local minimum which it has reached by a rather random path.

On the other hand, the basic idea has been very clearly stated by Thomas H. Jukes (1966) in his book Molecules and Evolution (p. 70) though he does not give it any particular emphasis.

There is one feature of the process by which new amino acids were added to a primitive code which is far from clear. This is why several versions of the genetic code did not emerge. It is, of course, easy to say that in fact several did emerge and only the best one survived, but the argument is rather glib. A detailed discussion of what was likely to have happened at this period would involve the consideration of genetic recombination.

Did it occur at a very early stage, perhaps even before the evolution of the cell, and, if so, what form did it take? Surprisingly enough, no writer on the evolution of the code seems to have raised this point. Naturally only rather simple processes would be expected, but the selective advantages of such a process would be very great. Perhaps a simple fusion process would suffice for the origin of the code (a suggestion made by Dr Sydney Brenner, personal communication).

This would provide spare genes for further evolution and in as far as the code for the fusing organisms differed it would produce fruitful ambiguities.

One might even argue that the population which defeated all its rivals and survived was the one which first evolved sex, a curious twist to the myth of the Garden of Eden.

General Features of the Code

We must now go back and ask whether we can explain the general features of the code in terms of the ideas sketched above.

The Four Distinct Bases
We have argued that originally there may have been only two bases in the nucleic acid. Why should there be four today ? The likely answer seems to be that four were stereochemically possible (i.e. could fit into a double-helical structure) and that two was too restrictive a number.

 

If only the first two bases of the triplet were originally distinguished, the mechanism could only code for four things (three amino acids and a space?), and even if the present “wobble” mechanism applied only a maximum of eight things could be coded. This could well be too few to construct really efficient proteins.

Whether six distinct base-pairs are stereochemically possible has been discussed elsewhere (Rich, 1962; Crick, 1964). It should be possible to settle this point experimentally.

 

Why a Triplet?
We have argued that the code must have been basically a triplet code from a very early stage, so that one is not entitled to use sophisticated arguments which would apply only to a later stage, although one could argue that early organisms with doublet or quadruplet codes actually existed but became extinct, only the triplet code surviving.

However, we are inclined to suspect that the reason in this case may be a structural ‘ one. If indeed there is no direct stereochemical relationship between an amino acid and a triplet, the problem of constructing an adaptor to recognize the codon may be a difficult one to solve.

 

In effect, one wants to perform a rather complicated act of recognition within a rather limited space, since two adaptors need to lie side by side, and attached to adjacent codons on the mRNA, during the act of synthesis. This is probably very difficult to perform if protein is used for the adaptor. On the other hand, nucleic acid, by employing the base-pairing mechanism, can do a very neat job in a small space.

For various reasons the adaptor cannot be too simple a molecule. For example, the amino acids on adjacent adaptors need to be brought together - this is probably done at the present using the flexible ... CCA tail. It must have, to some extent, a definite structure and this is likely to be based on stretches of double-helix. Thus .the diameter of a double-helix (since two may have to lie side by side) may have dictated the size of the codon, in that a doublet-code (moving along two bases at a time) would present an impossible recognition problem.

 

The 20 Amino Acids
According to the theory sketched above, both the number 20 and the actual amino acids in the code are at least in part due to historical accident.

First note that if the wobble theory of the interaction between codon and anticodon is correct, then the maximum number of things which can be coded in a positive way is 32 (say 31 amino acids and a chain terminator) not 64. Thus, the multiple representation of eight of the amino acids is not excessive.

 

On this view, only eight of the 21 things coded appear more than once. If the code evolved as I have suggested, it would in fact be surprising if each amino acid did occur only once. However, the theory of wobble must not be trusted too far, if only because it does not easily explain the fact that UGA codes differently from both UG^U_C and UGG.

Discussion of the actual amino acids used in the code may not be very profitable. Some less common amino acids, such as cysteine and histidine, would clearly seem to have an advantage because of their chemical reactivity; but whether, say, methionine could be justified in this way seems less obvious. It might be more useful to consider which amino acids are not used in the code.

 

However, the answer, if this general scheme is correct, really depends upon very complicated considerations, partly accidental, during the early evolution of the code. In particular, it would depend on the exact nature of the primitive proteins. It seems unlikely that one could come to any firm conclusions by following this line of argument.

As already mentioned, the theory does explain in a general way why similar amino acids often use similar codons. This does not answer the question whether the allocation of particular amino acids is entirely due to chance.

 

However, if it is assumed that the primitive code used tRNA molecules and that the recognition site for the amino acid was distinct from the anticodon, then even if activating enzyme did not exist at this stage and instead the amino acid fitted into a specific cage in the tRNA, the association between amino acid and anticodon could be due to pure chance.

 

Thus, a code with this property is not outrageous. Always remember that the present tRNA molecules must necessarily have evolved at some time or another.

The Two Theories Contrasted

The evolution of the code sketched here has the property that it could produce a code in which the actual allocation of amino acid to codons is mainly accidental and yet related amino acids would be expected to have related codons.

The theory seems plausible but as a theory it suffers from a major defect: it is too accommodating.

In a loose sort of way it can explain anything. A second disadvantage is that the early steps needed to get the system going seem to require rather a lot of chance effect. A theory of this sort is not necessarily useless if one can get at the facts experimentally. Unfortunately, in this problem this is just what is so difficult to do.

A theory involving stereochemical relationships between amino acids and triplets, on the other hand, not only makes it easier to see how the system could start but there is at least a reasonable chance that well-designed experiments could prove that such specific interactions are possible. It is therefore essential to pursue the stereochemical theory.

However, vague models of such interactions are of little use. What is wanted is direct experimental proof that these interactions take place (expressed as binding constants) and some idea of their specificity.

REFERENCES

• Crick, F. H. C. (1964). In Proc. Plenary Sessions 6th Int. Gong. Biochem. p. 109. Int. Union BiocKem. vol. 33. Federation of American Societies for Experimental Biology.

• Crick, F. H. C. (1966). Gold Spr. Harb. Symp. Quant. Biol. 31, 3.

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