materials in addition to cellular life itself is a sensible objective of planetary exploration? Even if we never find another biosphere in which to search for viruses, contemplation of viruses in the cosmic context invites several other questions such as: Does a biosphere always produce non-cellular entities that replicate inside cells? In other words, are virus-like entities an inevitable by-product of a planetary biosphere? Discuss this idea in the context of what you know about viruses and their characteristics.
Berliner, A.J., Mochizuki, T., and Stedman, K.M. (2018). Astrovirology: viruses at large in the Universe. Astrobiology 18: 207–223.
Griffin, D.W. (2013). The quest for extraterrestrial Life: what about the viruses? Astrobiology 13: 774–783.
Viruses are not cellular structures and require cells for replication. Although many people associate viruses with animal disease, many (known as phages) infect bacteria and archaea, and they are known to be infectious agents in extreme environments such as volcanic hot springs and polar lakes.
Viruses are microscopic infectious particles that have a diversity of shapes and forms, from helical to bottle shaped. They are constructed of either single- or double-stranded DNA or RNA surrounded by a protein coat called a capsid. Some of them are surrounded by a structure called the envelope, which is made of lipid and often has glycoproteins within it. The envelope plays a role in facilitating access to the host cell. In some sense, although viruses are not cellular, they still exhibit compartmentalization of their structures. In the case of the Tobacco Mosaic Virus (TMV), the capsid forms a long, thin tube that surrounds the RNA (Figure 5.26). Viruses are typically 20–350 nm in size, much smaller than even prokaryotic cells. Their inability to replicate on their own has caused controversy about whether viruses can be considered “life.”
Figure 5.26 The structure of the Tobacco Mosaic Virus (TMV). The diagram shows its nucleic acid and protein coat. Also shown is a micrograph of the virus.
Source: Schematic reproduced with permission of Thomas Splettstoesser.
The differences in the genetic material of viruses are important. RNA viruses carry the enzymes needed to translate the RNA into proteins, meaning that they can often complete their life cycle in the cytoplasm of eukaryotes. Some RNA viruses such as the retroviruses (which include Human Immunodeficiency Virus, HIV) encode for the enzyme reverse transcriptase, which allows the RNA to be converted to DNA, which is then subsequently incorporated into the host genome as a provirus.
Viruses mediate important processes in the biosphere. They play an immensely influential role in the cycling of carbon in the biosphere by breaking apart or causing the lysis of bacterial cells in which they are reproducing, thus recirculating carbon in the oceans. They can mediate the transfer of genetic information from one cell to another as they infect and reproduce inside cells. Although they may not fit within a classical definition of life that includes the ability to reproduce, they are certainly an important part of life on Earth. There are about 10 times as many virus particles on Earth as there are bacteria – often about 10 billion of them per liter of seawater.
The origin of viruses is not certain. There have been diverse hypotheses. They include: (i) viruses are independent entities that co-evolved with other cells, (ii) viruses represent ancient degenerate cells that lost many genes and became dependent on other cells, or (iii) viruses have evolved from the genes of larger organisms. The structure of RNA viruses has encouraged suggestions that they are the remnants of the RNA World (Chapter 12), an RNA-dominated molecular world that is proposed to have existed prior to the evolution of the first cells. None of these hypotheses is adequate to explain the characteristics of all viruses, and all of them have exceptions. It seems likely that viruses are very ancient and that they may have evolved several times in different ways.
The diverse patterns of nucleic acid structure in viruses and their non-cellular nature mean that they cannot readily be fitted into a traditional Tree of Life (Chapter 8), but instead form their own group of entities separate from cellular life. Nevertheless, their ancestry can be traced, and many of them have ancient lineages. The Herpes viruses, enveloped DNA viruses, form a group of at least 150 viruses and are separated into three groups, the alpha, beta, and gamma subfamilies. In humans, they are responsible for diseases as diverse as chickenpox, cold sores, and genital herpes. They are thought to have evolved about 400 million years ago in the Devonian and would have infected the first animals that emerged onto land. Their ancient lineage also accounts for the fact that they infect all human populations.
A prominent group of viruses is the bacteriophages or phages, a term applied to viral particles that infect prokaryotes. The co-evolution of bacteria and viruses has been a complex and prolonged interaction. Bacteria produce restriction endonucleases, enzymes which splice (cut up) virus DNA after its injection into the bacterial cell by the bacteriophage, preventing successful infection. CRISPR (clustered regularly interspaced short palindromic repeats) sequences are codes of DNA within the bacterial genome that correspond to sequences from viruses that have previously infected them. Using these sequences, bacteria are able to synthesize nucleases and RNA sequences that destroy the nucleic acid of similar phages that infect them at a later stage. This is a type of acquired immunity.
One reason why astrobiologists are interested in viruses is the intimate association between viruses and the prokaryotic world. Their role in information transfer between extant prokaryotes (Horizontal Gene Transfer; Chapter 8) complicates our efforts to build evolutionary trees depicting life on Earth, making it more difficult to unravel the origin of particular metabolic and biochemical pathways. An example of these complications is illustrated by the cyanophages that infect cyanobacteria. These viruses are capable of transferring photosynthetic genes, and it has been estimated that 10% of the world's cyanobacterial photosynthesis is carried out by genes that were originally transferred by cyanophages. There are many other examples of this phenomenon that suggest a need to understand the extent to which the activity of gene transfer by viruses influences the accuracy of inferred evolutionary relationships between organisms based on genetic information.
5.13 Prions
It would be incomplete not to mention prions. These entities are made of misfolded proteins that can have disease-causing characteristics. One of the best characterized of these is the agent responsible for scrapie, one of several transmissible spongiform encephalopathies, which affect brain and neural tissue. Prions can induce normal proteins in cells to misfold into a stable configuration, which can then cause other proteins to misfold, thereby generating a chain reaction of misfolded proteins. These misfolded proteins are folded in such a way as to make them resistant to proteases, which are enzymes that normally break down defective proteins in cells. They have been reported in fungi. Like viruses, they can be reproduced in host cells, but they cannot reproduce by themselves, putting them outside most definitions of life. However, like viruses they invite us to advance our discussion of what operational definition we use for “life.”
5.14 Conclusions
At the cellular level, life is complex. Nevertheless, we can identify cell types, and we can broadly recognize certain features about them that they all share. The basic biochemical structure of cell membranes, made of amphiphilic lipids, is common to all cellular life, although there are different types of membranes in different organisms. Similarly, the reading of the genetic code, from DNA to RNA to protein, is common to life on Earth. The genetic code is exquisitely evolved for its role in storing information, yet we also have seen how it contains redundancy in the amino acids for which it codes (the degeneracy of the genetic code). Eukaryotic cells are more complex than prokaryotic