What defines a living organism? Is it having a conscience? No, insects and bacteria are considered living, but don’t have a conscience so far as we know. Does having a genetic code, such as DNA, define if an organism is living? Also no, because there are “things” that have genetic codes, but are not traditionally considered “alive”. These “things”, for lack of a better term, are viruses.

Viruses are weird. They aren’t considered living because they cannot replicate by themselves, and yet, they are able to infect and harm animals, plants, and even bacteria. While viruses have nucleic acids to carry genetic information (which can be organized into either DNA or RNA), they are parasites because they need to hijack the cellular machinery of the cells they infect to replicate, express their genes, and ultimately, survive. 

A typical virus may have a lipid shell, called an envelope (yellow), and a protein coat, called a capsid (blue), that holds either RNA or DNA (red).

Scientists from around the world worked together to sequence numerous new bacteriophage (viruses that infect bacteria) genomes with sizes larger than 200,000 base pairs, with their largest completed genome being around 735,000 bases – the largest known bacteriophage genome. For reference, a typical bacteriophage genome size is around 50 thousand bases, while the human genome is around 3 billion bases. Some human genes alone are larger than entire viral genomes. 

By looking at the pattern of the bases (A, T, C, and G) in the viral genomes, the authors were able to understand what was different about these huge phages compared to the more typical smaller genomes. Inside the large viral genomes, the authors found genes that produced proteins important for the production of proteins and replication of their genomes. They also identified novel CRISPR-Cas systems. CRISPR-Cas systems target genes for degradation, and were discovered to act in bacteria as a form of immune system. The phage CRISPR-Cas systems appeared to target bacteria or other phages. Currently, CRISPR-Cas systems have been engineered for precise gene editing in other animal systems, and are currently used in research and gene therapies. These large viral genomes could hold untapped potential to improve gene therapies through genome editing.

We know comparatively little about these huge phages, but their large genomes, which can even include genes derived from bacteria, may lead us to question the boundary between life and non-life.

Peer edited by Rita Meganck

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