Rosalind Franklin day

A Question in Xenobiology: Is all Life Similar?

Introduction

If you know simple biology, you know that all life on Earth has genetic information: both DNA and RNA. Following this line of logic, it is plausible to think that life on another planet may also have to have DNA or RNA or both. What if this is not the case? Perhaps other life forms have completely different macromolecules storing genetic information.

What are DNA and RNA?

Deoxyribonucleic acid (DNA) is found in all living organisms, including many viruses [1]. It codes the genetic information for inheritance. In terms of structure, DNA has two strands wound up together – a double helix. Both of these strands consist of a chain of nucleotides, which have a (deoxyribose) sugar molecule attached to a phosphate group and a nitrogenous base (adenine, guanine, cytosine, or thymine). Basically, our DNA is made up of nucleotides that can only come in four forms.

Ribonucleic acid (RNA) can also be found in all living organisms. In some viruses, RNA, rather than DNA, codes genetic information [2]. In contrast to DNA, RNA is a single stranded molecule, but it does have a similar nucleotide structure. The only difference is that its nucleotides can have adenine, guanine, cytosine, and uracil (not thymine) as their nitrogenous base. RNA plays a major role in the protein synthesis (another word for creation) of the cells of living organisms.

While scientists have a lot of knowledge on the characteristics of these two nucleic acids, their origins remain unclear. One idea in the origin of life studies is that RNA originated before DNA and was the main molecule coding for genetic information. This is called the ‘RNA World’ hypothesis [3]. Other models suggest that proteins were “life’s first molecule” [4].

Synthetic Nucleic Acids

To circle back to the question asked in the introduction, would extraterrestrial life have genetic information similar to ours, meaning, would they also have DNA and RNA? The short answer is we don’t know. In fact, this will probably be the answer until some form of contact with extraterrestrial life is made. However, this doesn’t keep scientists from trying to investigate this question using resources and information from Earth.

A 2019 study by Hoshika et al. funded by The National Aeronautics and Space Administration (NASA) created synthetic DNA and RNA built from eight nucleotide types, rather than four [5]. In their research paper, the scientists state that, “These synthetic systems meet the structural requirements needed to support Darwinian evolution…” Essentially, their creation (dubbed hachimoji DNA and RNA) met the requirements to store genetic information for life.

Again in 2019, researchers at the Medical Research Council Laboratory of Molecular Biology in Cambridge created Earth’s first living organism (an altered Escherichia coli bacterium) with a completely synthetic genome [6]. Most living organisms use 64 codons, which are a sequence of three nucleotides that call for a certain protein in protein synthesis or for protein synthesis to stop. While modifying the E coli bacterium, the researchers set to get rid of unnecessary codons – ones that completed the same task as another codon. The newly formed organism, named Syn61, has a 61-codon genome. While Syn61 is slightly longer than expected and grows at a slower rate, its genome can still sustain life. This result makes one wonder: Even if all life (including extraterrestrial) has some form of DNA or RNA encoding their genetic information, do they need to have 64-codons like us?

Nucleic acids that do not have a ribose or deoxyribose sugar backbone are called Xeno nucleic acids (XNA). According to a 2010 essay published in BioEssays, the following molecules have been used in creating xeno nucleotides: hexose, threose, glycol, cyclohexenyl, and ribose with an additional ‘bridge’ [7]. The fact that XNA can be synthesized in the first place showcases that DNA and RNA may not be a requirement for life.

Conclusion

As far as we know, DNA and RNA are found in every non synthetic living organism. This would imply that extraterrestrial life also has DNA and RNA, though little is known about the genetics of life on exoplanets. Since extraterrestrial life still remains undiscovered, no research can be done on its characteristics. In order to gain information on the genetics of other forms of life, it is vital to determine how DNA and RNA originated on Earth. Furthermore, scientists are looking into synthetic molecules that may encode genetic information. The field of xenobiology is relatively new and future research should bring interesting results.

Bibliography

[1] “DNA | Discovery, Function, Facts, & Structure”. 2020. Encyclopedia Britannica. Accessed July 19. https://www.britannica.com/science/DNA.

[2] Chatterjee, Kunal, and Yao Wan. 2020. “RNA | Definition, Structure, Types, & Functions”. Encyclopedia Britannica. Accessed July 19. https://www.britannica.com/science/RNA.

[3] “What Is The RNA World Hypothesis?”. 2020. Science Alert. Accessed July 19. https://www.sciencealert.com/rna-world-hypothesis.

[4] Cepelewicz, Jordana. 2017. “Life’S First Molecule Was Protein, Not RNA, New Model Suggests”. Quanta Magazine. https://www.quantamagazine.org/lifes-first-molecule-was-protein-not-rna-new-model-suggests-20171102/.

[5] Hoshika, Shuichi, Nicole Leal, Myong-Jung Kim, Myong-Sang Kim, Nilesh Karalkar, Hyo-Joong Kim, and Alison Bates et al. 2019. “Hachimoji DNA And RNA: A Genetic System With Eight Building Blocks”. Science. https://science.sciencemag.org/content/363/6429/884.

[6] Fredens, Julius, Kaihang Wang, and Daniel de la Torre et al. 2019. “Total Synthesis Of Escherichia Coli With A Recoded Genome”. Nature. https://www.nature.com/articles/s41586-019-1192-5.

[7] Schmidt, Markus. 2010. “Xenobiology: A New Form Of Life As The Ultimate Biosafety Tool”. National Center For Biotechnology Information. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2909387/.

Figure References

Fig. 1: “File:Difference DNA RNA-EN.Svg”. 2010. Wikimedia Commons. https://commons.wikimedia.org/w/index.php?curid=9810855.

Fig. 2: Sample, Ian. 2019. “World’S First Living Organism With Fully Redesigned DNA Created”. The Guardian. https://www.theguardian.com/science/2019/may/15/cambridge-scientists-create-worlds-first-living-organism-with-fully-redesigned-dna.

Leave a Reply

Your email address will not be published. Required fields are marked *