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Building a Protein Molecule

Let us return now to the synthesis of proteins in living cells. The mRNA molecule transcribed from a DNA gene contains the instructions for arranging the amino acids in the chain of a specific kind of protein. But the actual construction of the protein molecule is carried out by an exceedingly complex cellular constituent called a ribosome, of which there may be thousands in one cell.

Ribosomes in eukaryotic cells are constructed of about 78 protein molecules and a roughly equal weight of RNA in four different chains. Prokaryotic ribosomes are slightly smaller, containing only 54 protein molecules plus RNA. It is the complex ribosome which attaches to an mRNA molecule, moves along its chain of codons, and constructs a protein molecule in accordance with the information contained in the codons. Each codon, you will recall, orders the attachment of a particular amino acid molecule at a particular point in the protein chain.

But how is the codon command translated into the addition of the proper amino acid to the chain? There are no known specific attractions between amino acid molecules and the nucleotide codons of mRNA. The solution of this problem is provided by special RNA molecules called "transfer RNA" (tRNA). Transfer RNA molecules are relatively small, containing only about 77 nucleotides. There are one or more special tRNA molecules for each of the twenty amino acids found in proteins. At one end of the tRNA molecule is an anti-codon for the amino acid it is designed to transfer. The amino acid molecule is activated(given energy) by a high-energy ATP(adenosine triphosphate) molecule and then attached to the other end of the tRNA by a strong bond formed under the direction of a special enzyme (called an aminoacyl-tRNA synthetase) which recognizes and operates only for one amino acid and its tRNA molecule. There are about 40 to 60 different tRNA synthetase enzymes in any cell.

It is not yet entirely clear how an aminoacyl-tRNA synthetase molecule recognizes its specific amino acid. Accurate recognition of the correct amino acid is crucial in protein synthesis, because once the synthetase bonds an amino acid to a tRNA molecule which carries the anti-codon for a particular amino acid, the attached amino acid will be placed in a position in a protein molecule corresponding to the anti-codon of the tRNA, regardless of whether the amino acid attached to it is the correct one or not. This is just another example of the exquisite precision and reliability of the many transfers of information which occur in living cells. Without this precision and reliability life would be an impossibility.

Once the tRNA is attached to its amino acid, its anti-codon for that amino acid is attracted to a corresponding codon in the mRNA molecule. The ribosome moves along the mRNA strand, building a protein chain in accordance with the instructions contained in the codons of the mRNA molecule. When the ribosome comes to a particular codon, it binds the growing protein chain to the amino acid. This action is catalyzed by a special enzyme, as is the removal of the tRNA molecule. Thus the protein chain grows by this stepwise process at the rate of 20 to 40 amino acid residues per second.

It should be emphasized that the foregoing, description gives a highly simplified and incomplete picture of a most complex network of cell activities which still is not completely understood, although much knowledge has been gained in recent years. The general structure and functioning of the ribosome is now known, but the complexity is so great that important details may well defy explanation. But if this piece of machinery--ribosomes and their array of cooperating enzymes and RNA molecules--is so sophisticated that its operation cannot be fully explained by scientists, how presumptious it is for them to insist dogmatically that anyone who rejects the evolutionary faith that the machine came into being with a Designer is being "unscientific."

So we see that there is substantial mystery still remaining about protein synthesis in living cells. Enough is known about this most complex process to make clear the fact that cycles of operations are involved, all of which are in some way dependent upon others. So which came first, DNA or protein? Furthermore, each protein synthesis is controlled by a promoter-repressor system or some other means which certainly would be unnecessary without the synthesis process. Yet it is difficult to imagine how a process to synthesize an enzyme would be useful without a control mechanism to turn it on and off at the proper times. Another problem which is still only partially solved is understanding how and why the long protein chains consistently fold up in just the right way to form the final three-dimensional structure which the functional protein must have.