RNA molecules have one property that contrasts with those of polypeptides: they can directly guide the formation of copies of their own sequence. This capacity depends on complementary base-pairing of their nucleotide subunits, which enables one RNA to act as a template for the formation of another. As we have seen in this and the preceding chapter, these complementary templating mechanisms lie at the heart of DNA replication and transcription in modern-day cells.
large pool of double-stranded DNA molecules, each with a different, randomly generated nucleotide sequence large pool of single-stranded RNA molecules, each with a different, randomly generated nucleotide sequence only the rare RNA molecules able to phosphorylate themselves incorporate sulfur molecules that fail to bind to the rare RNA molecules that can catalyze their own phosphorylation using ATP as a substrate
But the efficient synthesis of RNA by such complementary templating mechanisms requires catalysts to promote the polymerization reaction: without catalysts, polymer formation is slow, error-prone, and inefficient.
Because RNA has all the properties required of a molecule that could catalyze a variety of chemical reactions, including those that lead to its own synthesis (Figure 6–92), it has been proposed that RNAs served long ago as the catalysts for template-dependent RNA synthesis. Although self-replicating systems of RNA molecules have not been found in nature, scientists have made significant progress toward constructing them in the laboratory. While such demonstrations would not prove that self-replicating RNA molecules were central to the origin of life on Earth, they would establish that such a scenario is plausible.
How Did Protein Synthesis Evolve?
The molecular processes underlying protein synthesis in present-day cells seem inextricably complex. Although we understand most of them, they do not make conceptual sense in the way that DNA transcription, DNA repair, and DNA replication do. It is especially difficult to imagine how protein synthesis evolved because it is now performed by a complex interlocking system of protein and RNA molecules; obviously the proteins could not have existed until an early version of the translation apparatus was already in place. As attractive as the RNA world idea is for envisioning early life, it does not explain how the modern-day system of protein synthesis arose. Although we can only speculate on the origins of the genetic code, several experimental observations have provided plausible scenarios.
In modern cells, some short peptides (such as antibiotics) are synthesized without the ribosome; peptide synthetase enzymes assemble these peptides, with their proper sequence of amino acids, without mRNAs to guide their synthesis. It is plausible that this noncoded, primitive version of protein synthesis first developed in the RNA world, where it would have been catalyzed by RNA molecules. This idea presents no conceptual difficulties because, as we have seen, rRNA catalyzes peptide bond formation in present-day cells. However, it leaves unexplained how the genetic code—which lies at the core of protein synthesis in today’s cells— might have arisen. We know that ribozymes created in the laboratory can perform specific aminoacylation reactions; that is, they can match specific amino acids to specific tRNAs. It is therefore possible that tRNA-like adaptors, each matched to a specific amino acid, could have arisen in the RNA world, marking the beginnings of a genetic code.
Once coded protein synthesis evolved, the transition to a protein-dominated world could proceed, with proteins eventually taking over the majority of catalytic and structural tasks because of their greater versatility, with 20 rather than 4 different subunits. Although these ideas are highly speculative, they are consistent with the known properties of RNA and protein molecules.
All Present-Day Cells Use DNA as Their Hereditary Material
If the evolutionary speculations embodied in the RNA world hypothesis are correct, early cells would have differed fundamentally from the cells we know today in having their hereditary information stored in RNA rather than in DNA (Figure 6–93). Evidence that RNA arose before DNA in evolution can be found