RNA world An Introduction
The RNA world hypothesis is an attempt to provide an adequate answer to problems facing origin-of-life researchers in relation to the original information storage medium on primitive earth. DNA is responsible for housing the information that the cell requires to fold proteins into the correct shape critical to their respective function. Practically every cellular and extracellular structure is constructed from proteins. Given this importance, the information housed in DNA defines lifes most fundamental operations and structures.
When cells undergo replication, DNA and the information it stores is copied and subsequently passed on to the daughter cells. Biochemical blueprints are conveyed to the next generation through DNA replication. This process generates two daughter molecules which are identical to the parent DNA molecule. Once replication is complete, the two generated DNA molecules are distributed between the daughter cells produced during cell division.
Building proteins requires genetic information in DNA, but information in DNA cannot be processed without many specific proteins and protein complexes. Mutual interdependence of DNA and proteins has stood as a major stumbling block for Darwinian paradigms with regards to lifes origin since the mid-1980s. Origin-of-life researchers even refer to this conundrum as the chicken-and-egg paradox. Because proteins are so fundamental to the means by which DNA replicates, DNA and proteins could not simultaneously arise from a primordial soup.
RNA world A Solution?
The RNA world hypothesis has been proposed as a resolution to this paradox. This model maintains that RNA preceded DNA and proteins as the initial fundamental information storage medium. RNA can simultaneously store information (like DNA) and catalyse chemical reactions (like proteins). Thus it is contended that the RNA world eventually evolved into the DNA-protein world of contemporary biochemistry, with RNA currently functioning as an intermediary between DNA and proteins.
While the RNA-world hypothesis sidesteps the need for an interdependent system of DNA and proteins in the earliest living system on paper, in practical terms it appears largely untenable. Numerous difficulties abound for the RNA world hypothesis. For example, the formation of the first RNA molecule would have necessitated the prior emergence of smaller constituent molecules, including ribose sugar, phosphate molecules, and the four RNA nucleotide bases. It turns out, however, that both synthesizing and maintaining these essential RNA building molecules (particularly ribose) and the nucleotide bases is profoundly problematic, if not impossible to perform under realistic prebiotic conditions.
Another major difficulty confronting proponents of the RNA-world hypothesis is that naturally occurring RNA molecules possess very few of the specific enzymatic properties of proteins. Ribozymes can perform a small handful of the thousands of functions performed by proteins.
The inability of RNA molecules to perform many of the functions of protein enzymes raises a third and related concern with regards to the tenability of the RNA-world paradigm. To date, no plausible explanation has been advanced as to how primitive self-replicating RNA molecules could have made the transition into modern cellular systems which rely heavily on a variety of proteins to process genetic information. Consider the transition from a primitive replicator to a system for building the first proteins. Even if such a system of ribozymes for building proteins had arisen from an RNA replicator, that system of molecules would still require information-rich templates for building specific proteins. There is no foreseeable account of the origin of that information.
RNA world Conclusion
In summary, RNA can perform only a few minor functional roles and even then usually as the consequence of researchers intentionally engineering the RNA catalyst in question. Even in the face of extreme difficulty, most neo-Darwinians remain convinced that the RNA world must have existed, subsequently paving the way for the DNA-protein world. If it did not, the chicken-and-egg paradox -- from a materialistic perspective -- cannot be resolved.