The DNA World

Double-stranded DNA molecules are more stable than ssRNA. It would thus be advantageous for living systems to store heritable information in DNA molecules rather than RNA molecules. The 2' OH of RNA can attack an adjacent phosphodiester bond, rendering RNAs much more labile than DNA s. This autocatalytic process may have been accelerated by the harsh conditions on the primitive earth. As cells became more complex, their genome sizes had to increase. If early eugenotes had segmented RNA genomes, at least one of each segment would have to be present in each daughter cell for its survival. To enhance the probability that progeny cells are provided with a full genome, natural selection would favor production of polycistronic genomes. But the larger the RNAgenomic segments are, the less stable they would become because of autocatalysis. Thus, it would be advantageous for more stable polycistronic DNA molecules to take over genomic functions of RNA, leaving the RNAs to carry out functions that need not require long-lived molecules. The earliest anucleate cells containing DNA genomes (and all subsequent such cells) are known as prokaryotes.

At least four major processes were required to complete this transition:
(1) synthesis of DNA monomers by ribonucleoside diphosphate reductase;
(2) reverse transcription of DNA polymers from RNAgenomes;
(3) replication of DNA genomes by a DNA polymerase; and
(4) transcription of DNA genomes in functional (nongenomic) RNA molecules such as tRNA, mRNA, and rRNA.

The split genes of modern eukaryotic cells consist of coding regions (exons) and noncoding regions (introns). The interruption of the gene provided by introns offers an evolutionary advantage. Apparently, exons from different genes can sometimes be recombined by natural mechanisms to code for proteins of different functions but containing related amino acid domains. Each of these domains may have a specific function (e.g., binding to a receptor, forming an α-helix, etc.). This process, termed exon shuffling, is inferred to have been used extensively in the DNA world of early eukaryotes.