RNA world hypothesis

The RNA world hypothesis is the idea that cellular life used to be based on RNA, before the current system of DNA and protein became more commonplace. This is due to the ability of RNA to act both as an information storage molecule and as a biological catalyst (catalysis by RNA enzymes, or ribozymes).

It is believed that DNA overtook RNA in information storage because DNA is a more stable molecule and thus able to preserve genetic information with a higher degree of fidelity. This is because, in RNA, the OH group on the 2' carbon of the ribose sugar found in ribonucleotides is able to chemically attack the adjacent phosphodiester bond. This can lead to the RNA molecule effectively undergoing self-destruction. In DNA, the 2' carbon (of deoxyribose) lacks a hydroxyl group and has only a H atom, making it inherently more stable in this respect. Also, in RNA, the nucleobase thymine is replaced by uracil. Although uracil is equally stable in a base-pair with adenine, it also is the cause of a number of GC-AU mutations (because cytosine can be deaminated to uracil). The high mutation rate of RNA is energetically expensive to maintain.

Proteins are likely to have overtaken the catalytic function of RNA because while RNA may contain only 4 types of base, proteins may be formed of 20 commonly found amino acids (and some rarer ones). This gives proteins much greater structural diversity, and thus enables them to catalyse a wider range of biochemical reactions. However, RNA catalysis is still observable in nature: the rRNA component of ribosomes catalyses peptide bond formation in protein biosynthesis, and self-splicing introns display autocatalytic ability. These phenomena are often thought of as 'relics' of the RNA world.