Rna värld
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RNA World - The Genetic Material
The site is secure. NCBI Bookshelf. Molecular Biology of the Cell. New York: Garland Science; To fully understand the processes occurring rna värld present-day living cells, we need to consider how they arose in evolution. The most fundamental of all such problems is the expression of hereditary information, which today requires extraordinarily complex machinery and proceeds from DNA to protein through an RNA intermediate.
How did this machinery arise? According to this hypothesis, RNA stored both genetic information and catalyzed the chemical reactions in primitive rna värld. Only later in evolutionary time did DNA take over as the genetic material and proteins become the major catalyst and structural component of cells. If this idea is correct, then the transition out of the RNA world was never complete; as we have seen in this chapter, RNA still catalyzes several fundamental reactions in modern-day cells, which can be viewed as molecular fossils of an earlier world.
The RNA world
Time line for the universe, suggesting the early existence of an RNA world of rna värld systems. In this section we outline some of the arguments in support of the RNA world hypothesis. We will see that several of the more surprising features of modern-day cells, such as the ribosome and the pre-mRNA splicing rna värld, are most easily explained by viewing them as descendants of a complex network of RNA-mediated interactions that dominated cell metabolism in the RNA world.
We also discuss how DNA may have taken over as the genetic material, how the genetic code may have arisen, and how proteins may have eclipsed RNA to perform the bulk of biochemical catalysis in modern-day cells. In principle, an elaborate system of molecular synthesis and breakdown metabolism could have existed on these surfaces long before the first cells arose. But life requires molecules that possess a crucial property: the ability to catalyze reactions that lead, directly or indirectly, to the production of more molecules like themselves.
Catalysts with this special self-promoting property can use raw materials to reproduce themselves and thereby divert these same materials from the production of other substances. But what molecules could have had such autocatalytic properties in early cells? In present-day cells the most versatile catalysts are polypeptides, composed of many different amino acids with chemically diverse side chains and, consequently, able to adopt diverse three-dimensional forms that bristle with reactive chemical groups.
But, although polypeptides are versatile as catalysts, there is no known way in which one such molecule can reproduce itself by directly specifying the formation of another of precisely the same sequence. Polynucleotides have one property that contrasts with those of polypeptides: they can directly guide the formation of exact copies of their own sequence.
This capacity depends on complementary base pairing of nucleotide subunits, which enables one polynucleotide to act as a template for the formation of another. As we have seen in this and the preceding chapter, such complementary templating mechanisms lie at the heart of DNA replication and transcription in modern-day cells. But the efficient synthesis of polynucleotides by such complementary templating mechanisms requires catalysts to promote the polymerization reaction: without catalysts, polymer formation is slow, error-prone, rna värld inefficient.
Today, template-based nucleotide polymerization is rapidly catalyzed by protein enzymes—such rna värld the DNA and RNA polymerases.
Evidence for RNA origins
How could it be catalyzed before proteins with the appropriate enzymatic specificity existed? The beginnings of an answer to this question were obtained inwhen it was discovered that RNA molecules themselves can act as catalysts. We have seen in this chapter, for example, that a molecule of RNA is the catalyst for the peptidyl transferase reaction that takes place on the ribosome.
The unique potential of RNA molecules to act both as information carrier and as catalyst rna värld the basis of the RNA world hypothesis. RNA therefore has all the properties required of a molecule that could catalyze its own synthesis Figure Although self-replicating systems of RNA molecules have not been found in nature, scientists are hopeful that they can be constructed in the laboratory.
The RNA world ‘hypothesis’
While this demonstration would not prove that self-replicating RNA molecules were essential in the origin of life on Earth, it would certainly suggest that such a scenario is possible. An RNA molecule that can catalyze its own synthesis. This hypothetical process would require catalysis of the production of both a second RNA strand of complementary nucleotide sequence and the use of this second RNA molecule as a template to rna värld many more Although RNA seems well suited to form the basis for a self-replicating set of biochemical catalysts, it is unlikely that RNA was the first kind of molecule to do so.
From a purely chemical standpoint, it is difficult to imagine how long RNA molecules could be formed initially by purely nonenzymatic means. For one thing, the precursors of RNA, the ribonucleotides, are difficult to form nonenzymatically. Given these problems, it has been suggested that the first molecules to possess both catalytic activity and information storage capabilities may have been polymers that resemble RNA but are chemically simpler Figure We do not have any remnants of these compounds in present-day cells, nor do such compounds leave fossil records.
Structures of RNA and two related information-carrying polymers. In each case, B indicates the positions of purine and pyrimidine bases. The transition between the pre-RNA world and the RNA world rna värld have occurred through the synthesis of RNA using one of these simpler compounds as both template and catalyst.