The RNA world hypothesis

In modern cells, proteins perform functions in cells, and nucleic acids generally store information; however, ribonucleic acid (RNA) acts as both a protein and an information system. Therefore, scientists think that RNA was a key component in the early evolution of life. Experiments show that RNA replicates and divides, enabling natural selection and evolution.

The central dogma of molecular biology is that RNA governs the fundamental information transfer from DNA to proteins. All the molecules in magenta in Figure 6‑11 are primarily RNA, not proteins. The RNA information transfer system includes helicase, DNAP (DNA polymerase) RNAP (RNA polymerase), mRNA (messenger RNA), tRNA (transfer RNA), and ribosomes. This leads biologists to conclude that information storage and transfer first arose in RNA in a bulk prebiotic solution or possibly in fatty acid vesicle protocells. The RNA world hypothesis is that life began with RNA; however, the beginning may have been a bit more complex than than the RNA world hypothesis. Cyanosulfidic chemistry potentially forms all organic compounds. Thus, proteins and lipids may have been involved in the earliest part of the origin of life. Nevertheless, the combination of information and activity functions of RNA indicates that it may have been at the root of information storage and transfer in protocells.

The following video shows how RNA could replicate and form molecules in prebiotic chemistry. Scientists are getting close to demonstrating evolution of RNA, in which some strands that are better adapted to environmental conditions could replicate more often than other strands. This is survival of the fittest or natural selection.

The following videos describe the RNA information transfer system: DNAP, RNAP, mRNA, tRNA, and ribosomes. In preparation for cell division, DNA replication takes place at the replication fork. Helicase, an RNA enzyme, rotates at a rate of 10,000 revolutions per minute (170 revolutions per second) as it unravels DNA. One strand of unwound DNA enters DNA polymerase (an RNA enzyme, Figure 6-11), which uses the single strand to form a complete DNA double helix. Enzymes in the replication fork reorient the other DNA strand before it enters another DNA polymerase.

DNA transcription is the transfer of the information from a DNA gene to messenger RNA. Transcription factors first assemble at the beginning of a gene. A protein brings RNA polymerase (RNA enzyme, Figure 6-11) and other molecules to the DNA strand at the location of transcription factors. Activator proteins initiate the copying process by RNA polymerase, which unwinds the DNA strand and copies the information to an RNA strand with subunits that enter the RNA polymerase from another direction. The messenger RNA (mRNA) then leaves the RNA polymerase and travels into the cell where a ribosome will transfer the information to a protein. An electron micrograph image shows thousands of RNAP surrounding DNA strands (Figure 6-12).

Messenger RNA (mRNA) strands leave the RNA polymerase (RNAp) and carry the copy of a gene to one of a million ribosomes (Figure 6-11) in each cell. Transfer RNA (tRNA) brings amino acids to the ribosome. It has a complex tertiary structure (Figure 6‑13) like a protein and brings amino acids to the ribozyme for assembly into proteins (Figure 6‑14). The ribosomes (Figure 6‑15) are primarily RNA (nucleotides) but have short chains of amino acids (proteins). The ribosome incorporates the amino acids brought by tRNA into the specified amino acid sequence in the mRNA.