RNA World: Definition, Hypothesis, and Significance

The RNA World Hypothesis proposes that earlier life forms may have used RNA alone for the storage of genetic material.  This concept was put forth in the 1960s by Carl Woese, Francis Crick and Leslie Orgel. The main reasoning behind the hypothesis is that RNA is capable of self-replication and could therefore have carried genetic information across generations independently. This concept has been highly debated in the scientific world over the last 50 years. In this article, we will learn about RNA World Hypothesis in detail.

RNA- Ribonucleic Acid: Overview

• 1. RNA is ribonucleic acid.
• 2. It is a single-stranded structure that can replicate itself when behaving as genetic material in viruses.
• 3. It is a non-hereditary molecule except for some viruses and viroids.
• 4. In certain viruses, like the retrovirus, it can convert RNA into DNA inside the host cell by a mechanism called reverse transcription.
• 5. It is a polymer of ribonucleotides, which are made of pentose ribose sugar, phosphoric acid and nitrogenous bases.
• 6. Ribose sugar and phosphate groups form the backbone of the RNA chain.
• 7. Bases are joined to ribose sugar by glycosidic bonds.
• 8. RNA is made of adenine, guanine, uracil and cytosine bases. These bases are selectively attracted to other bases and form base pairs, like G pairs with C, A pairs with U.

On the basis of molecular size and function, RNA is of three types:-

• 1. mRNA– helps in protein synthesis.
• 2. tRNA– acts as an amino acid carrier.
• 3. rRNA– forms ribosomes.

RNA World Hypothesis

• 1. The concept of RNA World was first proposed by Alexander Rich in 1962, and the term was coined by Walter Gilbert in 1986.
• 2. In vitro synthesis of RNA molecules of 77 ribonucleotides by H.G Khorana in 1970 gave rise to speculation that probably RNA and not DNA was the primordial genetic material.
• 3. This view was further supported by the discovery of certain ribozymes (RNA molecules having enzymatic properties).
• 4. DNA can store information but cannot act as an enzyme. It needs RNA primer and different enzymes during replication. DNA cannot work independently, so scientists thought that this DNA did not evolve first as genetic material.
• 5. Protein can be considered as the first genetic material as it is a great molecular machine to keep cells alive and healthy, but it cannot store information and make a copy of its own. So, the thought that proteins were the first genetic material also gets discarded. So the question is, which came first as genetic material and made life possible? DNA or protein?
• 6. Then scientists thought of the third type of molecule called RNA, which is thought to be the earliest genetic material of a cell.
• 7. This idea that at the very beginning of life, RNA may be the genetic material is called the RNA World hypothesis. According to this hypothesis:-
  • i. RNA was the first genetic material. Life processes like metabolism, splicing, translation, etc., evolved around RNA.
  • ii. DNA has evolved from RNA by modification in chemical structure which may be as a result of mutation.
  • iii. RNA can act as genetic material as well as a catalyst. Some biochemical reactions need RNA catalysts and not protein enzymes.
  • iv. RNA can replicate on its own without the help of any other molecule.
  • v. DNA is more stable than RNA. DNA is double-stranded and due to the presence of complementary strands is able to resist further modification due to the evolution of the repair process.
  • vi. RNA being single-stranded, during replication, sometimes mutations slip in. This suggests that there is a possibility of evolution with modifications.
• 8. So, the latest view is that first, cells used RNA as their hereditary material and later, DNA evolved from an RNA template only when RNA-based life became enclosed in membranes.
• 9. Finally, DNA replaced RNA as the genetic material for most organisms. This RNA-only World became extinct more than 3 billion years ago. But even today, we can find RNA being genetic material in viruses and viroids and even the presence of catalytic activities of RNA as ribozymes in various living cells.

It is not sure that RNA must be the genetic material of first life. But different evidence is enough that supports that RNA may be the genetic material of the first life that existed in which RNA stored genetic information and also acted as catalysts to catalyse the reactions required in those times. Evidence to support this hypothesis is as follows:-

• i. Some ribosomal RNAs of the ciliate Tetrahymena are involved in splicing operations that cut out an intron within themselves.
• ii. RNA acts as a primer which is required for the initiation of DNA synthesis.
• iii. mRNA, tRNA, and rRNA are required for protein synthesis.
• iv. Ribosomes that help in protein formation are themselves made of rRNA.
• v. ATP is made up of nucleotide that comprises ribose sugar.
• vi. RNase P, Peptidyl transferase 23S rRNA, GIR1 branching ribozyme, Leadzyme, Group I and Group II introns, Hairpin ribozyme, Hammerhead ribozyme, HDV ribozyme are the naturally occurring ribozymes in the living cell.

But now the question is why DNA has evolved as a storage of information in today’s world? The answer to this question is that RNA is comparatively more unstable. The presence of the 2-OH group in ribose sugar, uracil nitrogenous base and RNA being single-stranded generally makes them more prone to mutation and chemical reactions, thus making it unstable.  Therefore DNA has evolved from RNA after mutations, and DNA is more stable than RNA.

• i. DNA has a double helix structure.
• ii. DNA has a 2-H group in deoxyribose instead of 2-OH in ribose, which makes DNA less reactive.
• iii. Thymine is chemically more stable than uracil because of the presence of the methyl group.
• iv. It possesses a process of repair and replication.
• v. DNA is a highly advanced genetic material.

Summary

From the above discussion, we can conclude that RNA may be the first genetic material in the organism and not DNA. Also, certain RNA like ribozymes can act as an enzyme and thus, they can be catalytic in nature. As RNA can perform all the functions necessary for sustaining life, it is considered to be the first genetic material to be evolved. This is called the RNA world hypothesis. RNA can act as genetic material and as a catalyst too. There is much evidence that proves that RNA could be the first genetic material. DNA could not be the first genetic material as it needs proteins for its synthesis. Even proteins cannot be the first genetic material as they cannot replicate themselves. So, RNA has been considered as the first genetic material as it has the capability to store information and act as a catalyst. DNA has evolved from RNA, and DNA is more stable than RNA to store information.

FAQs on RNA World

Q.1. What is the basis for the RNA world theory?
Ans: The basis for the RNA world theory is that RNA acts as genetic material and also as a catalyst. So, maybe RNA was the first genetic material in a cell.

Q.2. What is the main function of RNA?
Ans: The main functions of RNA are as follows:
i. mRNA helps in the formation of proteins.
ii. rRNA helps in the formation of ribosomes.
iii. tRNA acts as amino acid carriers.
iv. In certain viruses and viroids, like HIV, TMV, PSTVd, etc., it acts as genetic material.
v. Certain RNA molecules, like ribozymes, act as catalysts.

Q.3. Does RNA replicate itself?
Ans: Yes, RNA can replicate itself without the help of any proteins or other components.

Q.4. When did RNA first appear on Earth?
Ans: The first RNA appeared on the Earth around 4 billion years ago.

Q.5. Why did the discovery of ribozymes add support to the RNA world hypothesis?
Ans: Ribozymes are enzymes that are actually RNA and can catalyse reactions necessary for life. So, the discovery of ribozymes supported the RNA world hypothesis.

Study About DNA as the Genetic Material

Now that you have a detailed article on the RNA world, we hope you study well. If you get stuck somewhere do let us know in the comments sections. We will get back to you at the earliest.