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The following HTML text is provided to enhance online readability. Many aspects of typography translate only awkwardly to HTML. Please use the page image as the authoritative form to ensure accuracy. The above descriptions are more than a hypothesis. The quick acquisition of a new function by homologous recombination between genes with slightly different sequences was demonstrated in a series of in vitro experiments and observed in vivo during B-cell development in avian species and rabbits. In the in vitro experimental system called DNA shuffling, a pool of DNA molecules carrying a gene with different mutations was randomly fragmented and reassembled by a self-priming polymerase reaction (95). This system is not exactly the same as the homologous recombination involving double-stranded DNA, but incorporates the formation of a heteroduplex by the annealing of complementary strands bearing different mutations. Thus, the consequence of the reactions is equivalent to homologous recombination plus random mutagenesis. The DNA shuffling was shown to be much more efficient than simple random mutagenesis for the directed evolution of a gene product with enhanced activity, altered substrate specificity, and so on, and when a mixture of DNA bearing a common gene(s) from related organisms was used as starting DNA, the efficiency of the directed evolution of the gene(s) was extensively enhanced (96). During B-cell development in chicken, for an example, a unique rearranged V gene is diversified through repeated homologous recombination (gene-conversion type) with a group of homologous pseudogenes serving as donors with various mutations (see refs. 97 and 98 for review). The shuffling of protein parts by homologous recombination does not require introns, which might play an important role in exon shuffling for later stages of protein evolution (99), and would have played a significant role, especially at an early stage of evolution when genetic variability was much more limited than now. The fact that most complex organisms have a DNA genome instead of the RNA genome that very primitive organisms have indicates that DNA has a critical evolutionary advantage over RNA as a molecular carrier of genomic information. Although homologous recombination through heteroduplex joint formation is a general and essential feature of organisms with DNA genomes, homologous recombination of RNA viruses (that replicate without DNA intermediates, thus excluding retroviruses) is generally very rare (see refs. 100 and 101 for review). Significant levels of homologous recombination have been detected only in retroviruses and in a limited group of RNA viruses. We assume that the critical advantage of DNA is its double-stranded structure and capacity for homologous recombination. The double-stranded structure provides a template for the correction of erroneously incorporated bases during duplication and for the repair of base or strand damage. This is essential for maintaining the integrity of a genome whose size is sufficiently large to encode for all of the genomic information necessary for independent cellular life. On the other hand, homologous recombination is essential for the well-organized dynamic property of double-stranded DNA that is necessary for the evolution of genomic information as discussed above. Unlike DNA genomes, homologous recombination of RNA viruses is carried out by a copy-choice (replicative template switch) mechanism. In the copy-choice mechanism, RNA replication is initiated on a template RNA by an RNA replicase, followed by a template switch (see refs. 101–103 for review), and thus, both parental RNA molecules have to be single-stranded. In addition, it is claimed that nonhomologous recombination and homologous recombination of RNA viruses occur at comparable frequencies (101, 104). The presence of the massive and hydrophilic hydroxyl group at the 2′ position of the sugar ring prevents RNA from taking on the extended structure that is induced in DNA upon the binding of RecA/Rad51. These facts suggest that the 2′ methylene-base interaction is essential for the efficient and accurate homologous recombination of double-stranded polynucleotides and gives a critical advantage to DNA over RNA for evolution. Concluding RemarksA transferred NOE study on the three-dimensional structure of RecA-bound oligo-DNA revealed a unique extended DNA structure containing a 2′ methylene-base interaction. This interaction plays a pivotal role in heteroduplex joint formation through homologous pairing and strand exchange by base-pair switch. The observed requirements of RecA/Rad51 to induce the extended DNA structure and the general requirement for RecA/Rad51 for homologous recombination in various organisms suggest that homologous recombination through heteroduplex joint formation is an intrinsic property of a DNA structure induced by RecA/Rad51-family proteins. This function confers on double-stranded DNA, which is otherwise chemically and genetically stable, a well-organized dynamic property that enables the rearrangements of gene segments to create new genes without disturbing their coding frame. We suggest that the two faces of DNA, its stability and its propensity for homologous recombination, might give DNA a critical advantage over RNA for evolution. 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