The RNA world hypothesis
It proposes that self–replicating ribonucleic acid (RNA) molecules were precursors to current life, which is based on deoxyribonucleic acid (DNA), RNA and proteins.
Once the organic polymers formed and became organized into protobionts, they needed a way to copy themselves. This is a key point in any discussion about life's origin. Most modern organisms use a DNA–based replication system, but this is believed to have been too complex for early life forms. It is generally accepted that before DNA, there was an "RNA world".
Modern RNA polymers provide much insight into the proposed function of RNA as the first hereditary unit. RNA has properties similar to DNA and proteins because it is a genetic molecule with enzymatic action. RNA is the sole genetic material for some viruses, and it serves as a carrier of genetic material in many living organisms.
RNA is able to polymerize by using clay or other substrates as a catalyst. RNA can self–replicate short strands even. The enzymatic properties of RNA were discovered by Cech and his co–workers in 1980s. They found that there are RNA molecules that help catalyze the synthesis of new RNA, remove some sequences from mRNA, and join peptides to form proteins.
In an "RNA world", there would have been single strands of RNA with a genotype and characteristic phenotype. A single strand of RNA folds back onto itself, and portions that are complementary pair.
Thus, the order of nucleotides would have provided the genotype and the 3–D folding and pairing would have provided the phenotype. This is unlike a "DNA world", where double–stranded DNA has a genotype and the proteins produced determined the phenotype. In the laboratory, some single–stranded RNA is more stable and replicates better under certain conditions. Thus, this RNA is more likely to occur in the next generation of molecules.