Life's Origin: The Beginnings of Biological Evolution

5. The Origin of Biological Information

chapter 5 The Origin of Biological Information leslie e. orgel introduction Organic chemists should have invented the computer scientists’ motto, “Garbage in, garbage out.” Proceeding step by step, purifying the product of one step before using it in the next: this is the orthodox approach to organic synthesis. Under carefully controlled conditions, it is just possible to constrain the chemistry of a pure input compound to give a unique product. But garbage in—an impure compound or a complex mixture of compounds—does as it damn well pleases. And it almost always yields an intractable mixture of products—garbage out. Unfortunately, all of the most impressive prebiotic syntheses produce garbage by the standards of synthetic organic chemistry. For example, Miller’s classic experiment (discussed in chapter 3) produces tar along with a percent or two of a complex mixture of racemic amino acids (Miller 1953), while Oró’s landmark synthesis of adenine from hydrogen cyanide (chapters 1 and 3) produces an ill-characterized brown precipitate along with, at most, a percent or two of the desired nucleoside base (Oró and Kimball 1960). How could chemistry on the primitive Earth proceed in such a messy way, producing informationrich living cells, those exquisitely designed chemical factories, from such unpromising starting materials? This is the central and as yet largely unanswered question facing investigators of the origin of life. We know that certain polymers control the functioning of all living organisms on Earth. The proteins, composed of 20 standard amino acids, are the machines that catalyze almost all of the chemical reactions 140 that go on in cells. The nucleic acids are largely concerned with information : DNA (deoxyribonucleic acid) is a blueprint that dictates which proteins are to be synthesized and when, while RNA (ribonucleic acid) implements DNA’s instructions. And we now have overwhelming evidence (discussed in chapter 6) that all contemporary living organisms are descended from a Last Common Ancestor (LCA). The LCA lived on Earth at least as early as 3,500 million years ago, and the members of this plexus of early-evolved microorganisms had a biochemistry similar to that of modern bacteria. In particular, the LCA employed nucleic acids for the storage of genetic information and used proteins as catalysts. Thus, if we are to understand the origins of life, we must understand the origin of the nucleic acid–protein system. The replication of DNA is a process brought about by protein enzymes called DNA polymerases. Provided DNA replication is sufficiently accurate, and the two DNA copies are distributed one to each daughter when a cell divides, each daughter cell starts with the same genetic information as that of the parental cell. This is the universal origin of heredity in biology. The DNA polymerases are proteins, the amino acid sequences of which are specified by the base sequence of appropriate segments of DNA. Here, we cannot go into the process of protein synthesis in detail. Suffice it to say that in the first step, the DNA sequence is transcribed into messenger RNA (mRNA) by RNA polymerases, in a process similar to that involved in DNA replication. In a second step, the RNA is translated into protein. The process of translation is extremely complicated, involving scores of soluble proteins and an elaborate factory known as a ribosome. The ribosome is made up of three RNA molecules and scores more proteins. The result of translation is the production of a protein in which the nature of each amino acid monomer in the protein sequence is determined by the nature of three nucleotide residues in the nucleic acid sequence. The nature of the relationship between amino acid residues and nucleotide triplets defines the genetic code. This account of the genetic system, admittedly a highly oversimpli- fied one, only emphasizes that the processes of DNA replication and protein synthesis are hopelessly intertwined in modern organisms. But even this inadequate view demonstrates a conundrum: The replication of DNA requires the pre-existence of protein enzymes, but the formation of protein enzymes requires the pre-existence of DNA. Since everyone agrees that the whole complex system could not have come into existence by chance in a single step, we have to ask the classic chickenand -egg question: Which came first, nucleic acids or proteins? The Origin of Biological Information 141 A tentative solution emerged more than thirty years ago (Woese 1967; Crick 1968; Orgel 1968). It was suggested...