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RNA-DNA World Hypothesis?

How did life start as we know it? In the scientific community, the RNA World Hypothesis has many adherents. Many believed that life came about as a result of the existence of the simplest molecule, such as RNA. Perceptibly, RNA shows signs of somewhat being “alive” — at least in the sense that it carries a genetic code and capable of self-replicating. In essence, RNA could be the earliest biomolecule. Subsequently, other organic molecules came about. However, a new hypothesis is gaining a grip. Accordingly, RNA and its close relative – DNA – might have existed side by side during the primordial times, even before life began.

RNA World Hypothesis

RNA world hypothesis
According to RNA World Hyopothesis, RNA dominated the ancient Earth and served as descendants to life. [Photo: public domain, from Pxhere]

In RNA world hypothesis, primitive life is presumably RNA-based. This assumption arose from the notion that RNA could act both as a genetic material and as a catalyst. In due time, primitive RNA-based entities have transitioned into compartmentalized life forms (in the form of cells) for over many millions of years. Possibly, the RNA-based life dominated the primitive Earth and then served as the descendant of the present-day living organisms.1 Carl Woese, an American microbiologist and biophysicist, is hailed as the originator of the RNA World hypothesis. Accordingly, he conjectured in 1967 that the earliest self-replicating life entities could have relied on RNA.2

Theory on the Origin of RNA

How RNA emerged or came about still puzzles scientists. Where did RNA come from? Did it come from the Earth’s more nascent, rudimentary units?or perhaps, the building blocks for RNA came down to Earth from the outer space? According to scientists, RNA seemingly came from, and synthesized in, the asteroids from the outer space. Apparently, they reached the Earth through meteorites. NASA reported that they found RNA and DNA nucleobases (e.g. adenine, guanine) in meteorites. These could have led to the spontaneous creation of RNA and DNA on Earth.3 In March 2015, researchers reported that pyrimidines uracil, cytosine, and thymine formed in their laboratory under outer space conditions and using precursors such as compounds present in meteorites.4

RNA-DNA world?

In essence, DNA is a more complex compound than RNA. While RNA occurs as a single strand, DNA exists as two strands that typically wound in a helix. Similar to RNA, DNA consists of multiple nucleotides covalently bonded by 3′, 5′ phosphodiester linkages. Each nucleotide, in turn, contains phosphoric acid, a deoxyribose sugar (5-carbon), and a nucleobase (particularly, cytosineguanine, adenine, and thymine). Thymine is a distinctive structural feature of DNA. In DNA, thymine takes the place of uracil. Having discovered that these building blocks could form under pre-biotic conditions causes scientists to rethink the origin of life.

A research team reported how chains of nucleic acids could form in a pre-biotic environment and how RNA could easily turn into DNA components even without the assistance of enzymes. Ramanarayanan Krishnamurthy, one of the researchers, said, “These new findings suggest that it may not be reasonable for chemists to be so heavily guided by the RNA World hypothesis in investigating the origins of life on Earth.”5

They surmised that the primitive Earth is no pure RNA. DNA might have existed side by side with RNA and it probably even competed for supremacy, until the DNA system eventually reigned over.

They published their report in Nature.6

— written by Maria Victoria Gonzaga

References:

1  Biology-Online Editors. (2014, May 12). Ribonucleic acid. Retrieved from Biology-online.org website: [Link]

2  Woese, C. (1967). The Genetic Code: the Molecular basis for Genetic Expression. New York: Harper & Row.

3  NASA – NASA Researchers: DNA Building Blocks Can Be Made in Space. (2011, January 1). Retrieved NASA website: [Link

4 Marlaire, R. (3 March 2015). “NASA Ames Reproduces the Building Blocks of Life in Laboratory”. Retrieved from NASA website: [Link]

5 McRae, M. (2019). DNA And RNA May Have Existed Together Before Life Began on Earth. Retrieved April 6, 2019, from ScienceAlert website: [Link]

6 Xu, J., Green, N. J., Gibard, C., Krishnamurthy, R., & Sutherland, J. D. (2019). Prebiotic phosphorylation of 2-thiouridine provides either nucleotides or DNA building blocks via photoreduction. Nature Chemistry. [Link]