The RNA world hypothesis proposes that self-replicating ribonucleic acid (RNA) molecules were precursors to current life, which is based on deoxyribonucleic acid (DNA), RNA and proteins. RNA stores genetic information like DNA, and catalyzes chemical reactions like an enzyme protein. It may, therefore, have played a major step in the evolution of cellular life.
The RNA world would have eventually been replaced by the DNA, RNA and protein world of today, likely through an intermediate stage of ribonucleoprotein enzymes such as the ribosome and ribozymes, since proteins large enough to self-fold and have useful activities would only have come about after RNA was available to catalyze peptide ligation or amino acid polymerization. DNA is thought to have taken over the role of data storage due to its increased stability, while proteins, through a greater variety of monomers (amino acids), replaced RNA's role in specialized biocatalysis.
The RNA world hypothesis is supported by the observation that many of the most critical components of cells (those that evolve the slowest) are composed mostly or entirely of RNA. This would mean that the RNA in modern cells is an evolutionary remnant of the RNA world that preceded ours. Also, many critical cofactors (ATP, Acetyl-CoA, NADH, etc.) are either nucleotides or substances clearly related to them.
Read more about RNA World Hypothesis: History, Properties of RNA, Support and Difficulties, Prebiotic RNA Synthesis, Further Developments, Alternative Hypotheses, Implications of The RNA World
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