The RNA world hypothesis proposes that RNA, not DNA or proteins, was the first self-replicating molecule and the central player in early life. RNA can both store genetic information and catalyze chemical reactions, acting as both the genome and enzymes. Evidence for this includes the discovery that some RNA molecules can self-replicate and that the ribosome, the cell's protein-building machinery, has RNA as its key component. While still debated, the RNA world hypothesis provides a plausible explanation for how life could have originated and evolved prior to the development of DNA and proteins.

1. RNA world
All modern organisms have DNA as a store of genetic information, RNA as a
messenger and proteins as their major cellular catalyst. This suggests that the
organism from which all cellular life began had these properties.
There are many questions surrounding the origin of life, for example, how did it
all start? Where did DNA come from?
The RNA world is the currently accepted hypothesis which answers these
questions. A key step in the origin of life was the evolution of a molecule that
could copy itself. Once it was discovered that RNA could both carry information
and cause chemical reactions (like those that would be required to copy a
molecule), RNA became the prime suspect for the earliest self-replicating
molecule. In fact, biologists hypothesize that early in life's history, RNA occupied
center stage and performed most jobs in the cell, storing genetic information,
copying itself, and performing basic metabolic functions. This is the "RNA world"
hypothesis. Today, these jobs are performed by many different sorts of
molecules (DNA, RNA, and proteins, mostly), but in the RNA world, RNA did it all.
This is the "RNA world" hypothesis.
The main evidence supporting this hypothesis is the discovery that RNA can act
as both a store of genetic information and as a cellular catalyst. The hypothesis is
also supported by the fact that RNA can effectively self-replicate using
ribozymes. Some ribozymes can faithfully add 20 nucleotides to a single-
stranded RNA template molecule. Both polypeptides and polynucleotides have
the ability to store information by means of their amino acid sequences and their
nucleotide sequences respectively but only polynucleotides have the ability to
replicate. This is due to the fact that one polynucleotide strand can act as a
template for another, by the complimentary base pairing of free nucleotides. For
this self-replication to be fast, efficient, and not prone to errors, a catalyst is
required. This catalyst is the RNA molecule itself.
Although there are many pieces of evidence in support of the RNA world
hypothesis, there are some weaknesses in the theory. Earlier one major
weakness of the theory was that scientists found it exceptionally difficult to
synthesise the component nucleotides of RNA under prebiotic conditions.
However, recent developments by John Sutherland and his team have shown
cytidine and uridine being synthesised under model prebiotic conditions,
suggesting that RNA may have been able to spontaneously form under prebiotic
conditions.
History
The book, The Genetic Code, written in 1967 by Carl Woese, was the first
published material that supported RNA World Hypothesis. Francis Crick and
Leslie Orgel proposed the idea that RNA once did the work of DNA and proteins
in 1968. Their theories were not validated until the work of Nobel Prize laureate
Thomas R. Cech. In the 1970s, Cech was studying the splicing of RNA in a single-

2. celled organism, Tetrahymena thermophila, when he discovered that an
unprocessed RNA molecule could splice itself. He announced his discovery in
1982 and became the first to show that RNA has catalytic functions. The phrase
“RNA World Hypothesis” was then coined later in 1986 by Harvard molecular
biologist and Nobel Prize laureate Walter Gilbert as he commented on the recent
observations of the catalytic properties of RNA. Another major milestone
occurred in 2000 when it was published in Science that "The Ribosome is a
Ribozyme" and the proteins in the ribosomes exist primarily on the periphery.
Finally, the RNA world could easily have evolved into our current day central
dogma of biology, with DNA taking over as the main genetic store due to its
increased stability. and proteins becoming the favoured catalyst due to their
larger array of functions. Therefore, RNA remains as an intermediate molecule
between the two systems.
Properties Supporting Hypothesis
Major evidence that has the scientific community believing that RNA predates
DNA and proteins are as follows:
- RNA has the ability to store genetic data, and pass down hereditary
information.
- It is the main component linking up DNA and gene formation to amino acids
and protein synthesis via transcription and translation.
- RNA's ability to duplicate itself as well as the genetic information it carries, very
much like DNA.
- RNA's complexity is less than that of DNA, and involves fewer types of
molecules in order to self-replicate.
- DNA requires an RNA primer in order to replicate while RNA does not need any
such primer. This shows how DNA seems to depend more on RNA for its
continued existence rather than the other way around.
- RNA is able to catalyze reactions as proteins do. The formation of a protein is
also administered by RNA which points heavily to its preexistence over the
proteins.
- RNA's ability to form double helices similar to DNA, and tertiary structures
similar to those of catalytic proteins.
- The structure of RNA, with an hydroxyl group in the 2' position of the sugar
molecule, makes it a less stable molecule which is capable of attacking a
phosphodiester bond near it as long as the RNA molecule is in a flexible position
and not constrained. This made it susceptible to breakdown and allowed an
adaptation of different conformations which perhaps was beneficial to early life
and rapid evolutionary changes.