This old idea got a fresh breath thanks to a recent discovery giant virusessuch as pandoraviruses or mimiviruses. They are not only very large (the particle diameter of the mimivirus reaches 750 nm – for comparison, the size of the influenza virus is 80 nm), but they also carry an exceptionally long genome (1.2 million nucleotide links in mimivirus versus several hundred in common viruses), encoding many hundreds of proteins. Among them there are also proteins necessary for copying and “repair” (repair) of DNA, for the production of messenger RNA and proteins.
These parasites are much less dependent on their hosts, and their origin from free-living ancestors looks much more convincing. However, many experts believe that this does not solve the main problem – all the “additional” genes could appear from giant viruses later, borrowed from their hosts. After all, it is difficult to imagine a parasitic degradation that could go so far and affect even the form of the carrier of the genetic code and lead to the emergence of RNA viruses. Unsurprisingly, another hypothesis about the origin of viruses is equally respected – the exact opposite.
Progressive. From simple to complex
Let’s take a look at the retroviruses whose genome is a single-stranded RNA molecule (for example, HIV). Once in the host’s cell, such viruses use a special enzyme, reverse transcriptase, converting it into ordinary double DNA, which then penetrates into the cell’s “holy of holies” – the nucleus. This is where another viral protein comes into play, integrase, which inserts the viral genes into the host’s DNA. Then the cell’s own enzymes begin to work with them: they produce new RNA, synthesize proteins on their basis etc.
Visual science
Human Immunodeficiency Virus (HIV)
This mechanism resembles the reproduction of mobile genetic elements – DNA fragments that do not carry the information we need, but are stored and accumulated in our genome. Some of them, retrotransposons, are even capable of multiplying in it, spreading with all new copies (human DNA consists of such “garbage” elements by more than 40 percent). For this, they may contain fragments encoding both key enzymes – reverse transcriptase and integrase. In fact, these are almost ready-made retroviruses, devoid of only a protein coat. But its acquisition is a matter of time.
Incorporating into the genome here and there, mobile genetic elements are quite capable of capturing new host genes. Some of them might be suitable for capsid formation. Many proteins tend to “self-assemble” into more complex structures. For example, the ARC protein, which plays an important role in the functioning of neurons, in free form spontaneously is taking shape into virus-like particles that can even carry RNA inside. It is assumed that the inclusion of such proteins could take place about 20 times, giving rise to large modern groups of viruses that differ in the structure of their envelope.
Parallel. Shadow of life
However, the youngest and most promising hypothesis turns everything upside down again, assuming that viruses appeared no later than the first cells. A long time ago, when life had not yet gone so far, the proto-evolution of self-replicating molecules, capable of copying themselves, proceeded in the “primordial soup”. Gradually, such systems became more complex, transforming into ever larger molecular complexes. And as soon as some of them acquired the ability to synthesize a membrane and became proto-cells, others – the ancestors of viruses – have become their parasites.
– .
