RNA World
RNA stands for the ribonucleic acid, which is one of the two most important nucleic acids in present an organism, the other one being the DNA or deoxyribonucleic acid. Even though RNA is the genetic material in some of the organisms, DNA is considered to be the predominant genetic material in almost all the organisms. According to the RNA world hypothesis, ribonucleic acid is the first genetic material in which all genetic information was stored and the first life had arisen from it. It is known to be a self-replicating molecule. In simple words, the RNA is known to be the precursor to every life form which exists today on the earth. It is believed that the modern cells arose from them and all the essential processes which occur in the living organisms evolved around the RNA. The RNA world hypothesis was greatly accepted by the scientific community. In this article, we will study about the RNA world hypothesis and the RNA world.
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RNA World Hypothesis
The RNA World Hypothesis is a concept that was put forth in the1960s by Francis Crick, Carl Woese, and Leslie Orgel. It proposes that the earlier life forms might have used RNA alone for the storage of their genetic material.
Walter Gilbert, which was a Harvard molecular biologist, was the first one to use the term "RNA World" in one article that was published in the year 1986. The hypothesis says that the DNA later became the genetic material as a result of the evolution since RNA was a relatively unstable molecule. According to the RNA World Hypothesis, about 4 billion years ago, the RNA was the only primary living substance, largely because of the RNA’s ability to function as both the genes and the enzymes.
The main reason behind the hypothesis is that the RNA is much capable of self-replication and might, therefore, have carried the genetic information across all the generations independently. This concept has been highly debated all across the scientific world for over the last 50 years.
Experts now generally agree that the non-living chemicals could not have given rise to the bacterial cells in one single step and that the intermediate, and pre-cellular life forms must, therefore, have existed. Of all the possible pre-cellular life models that were considered, the most popular one is the RNA World.
In the year 1968, Sir Francis Crick proposed that the RNA would have been the primary genetic material since it is capable of self-replication, owing to its ability for acting as an enzyme. Moreover, the RNA can also be converted to the DNA by the reverse transcription, that further strengthens the idea that the RNA world might have been the initial pathway to the cells.
RNA World and Ribosomes
It was thought previously that the only biomolecules which could catalyze the essential chemical reactions in the cells were the proteins. However, Sidney Altman, Thomas Cech and other colleagues discovered a class of RNAs which is capable of the catalyzing chemical reactions called ribozymes. Altman and Cech were hence awarded the Nobel Prize in Chemistry in the year 1989 for this discovery.
The discovery of the ribozymes supported the RNA World Hypothesis. The strongest argument in proving the hypothesis is probably that the ribosome, which assembles the proteins, is itself a ribozyme. Despite the fact that the ribosome is composed of both the RNA and the protein, the processes involved in the translation are not catalyzed by the protein, but by the RNA, which indicates that the early life forms might have used the RNA to catalyze the chemical reactions before they used the proteins.
RNA Organisms
Research over the past few years has demonstrated the useful properties of the random RNA sequences, like the production of the active RNA ligases from the random sequences of the RNA. But, this is not enough to prove about the RNA world hypothesis. Based on what is referred to about the present-day organisms, it is not possible to conclude that the self-replicating molecule was the primary source of the catalytic mechanisms that are fundamental to the living systems.
Research on the RNA World Hypothesis is still ongoing, although the recent findings threaten to disprove the hypothesis. NASA stated in one 1996 report that the difficulties which are surrounding the RNA World concept are the chemical fragility of the RNA and its poor range of the catalytic activities.
The biochemist Harold S Bernhardt shares the same kind of concerns and in one 2012 biology paper, he called the concept to be the worst theory of the early evolution of life. His paper states that the complexity of the RNA molecule means that it could not have arisen prebiotically and that the RNA is unstable and that it possesses only limited catalytic properties.
FAQs on RNA World Hypothesis
1. What is the RNA World Hypothesis as explained in Biology for Class 12?
The RNA World Hypothesis proposes that earlier forms of life on Earth may have relied solely on ribonucleic acid (RNA) to store genetic information and to catalyse chemical reactions. According to this concept, RNA was the precursor to all current life, which now primarily uses DNA for information storage and proteins for catalysis. RNA's ability to perform both these functions makes it a plausible candidate for the first self-replicating molecule that initiated life.
2. What is the primary evidence that gives credibility to the RNA World Hypothesis?
The main evidence supporting the RNA World Hypothesis is the discovery of ribozymes. These are RNA molecules that can act as biological catalysts, a role previously thought to be exclusive to proteins (enzymes). The fact that the ribosome, the cellular machinery for protein synthesis, is itself a ribozyme strongly suggests that RNA had a central catalytic role before proteins became dominant. This discovery proved that RNA could both store information and perform work.
3. Who were the key scientists to propose the RNA World concept?
The concept behind the RNA World Hypothesis was independently proposed in the 1960s by several prominent scientists, including Carl Woese, Francis Crick, and Leslie Orgel. The term "RNA World" itself was later coined by Nobel laureate Walter Gilbert in 1986 to describe this hypothetical early stage of evolution.
4. Why is RNA considered a better candidate than DNA for the first self-replicating molecule?
RNA is considered a better candidate than DNA as the first life molecule due to its functional versatility. Unlike DNA, which is primarily a passive information storage molecule, RNA can perform dual roles:
- Information Storage: Like DNA, RNA can carry genetic code in its nucleotide sequence.
- Catalytic Activity: RNA can fold into complex three-dimensional structures and act as an enzyme (a ribozyme), enabling it to catalyse its own replication and other reactions.
5. If the RNA World existed, why did life evolve to use DNA for storing genetic information instead?
Life likely evolved to use DNA for genetic storage due to its superior chemical stability. The double-helix structure of DNA makes it more resistant to degradation and mutation compared to the typically single-stranded and more reactive RNA molecule. This stability provides a more reliable and secure way to store the genetic blueprint over long periods, reducing errors in replication. This led to a biological division of labour: DNA for stable storage, RNA as a temporary messenger, and proteins as highly efficient catalysts.
6. What are the main limitations or scientific challenges to the RNA World Hypothesis?
Despite being a popular theory, the RNA World Hypothesis faces several significant challenges:
- Prebiotic Synthesis: It is difficult to explain how complex RNA molecules could have formed spontaneously from the simple non-living chemicals available on early Earth.
- Chemical Instability: RNA is inherently less stable than DNA, making it prone to breaking down, especially in the harsh conditions of a prebiotic world (e.g., in water or under UV radiation).
- Limited Catalytic Ability: While ribozymes are functional, they are generally not as efficient or versatile as protein enzymes, raising questions about their ability to orchestrate all necessary life processes.
7. How does the RNA World Hypothesis explain the origin of life's essential processes before proteins existed?
The hypothesis suggests that ribozymes were the original engines of cellular metabolism. In this early world, RNA molecules would have catalysed all the necessary reactions for survival and replication. For instance, a ribozyme could have functioned as an RNA polymerase, making copies of other RNA molecules. Similarly, the formation of peptide bonds, a fundamental step in making proteins, is today catalysed by ribosomal RNA (rRNA). This indicates that RNA-based catalysis could have established life's core processes, like replication and metabolism, before the evolution of more efficient protein enzymes.
