What, according to biology, makes something come alive

Are Viruses Alive?

Coronavirus, flu pathogens and the like are living beings, many people intuitively assume. But actually they are not - or are they? How viruses can be assigned is a tricky question: we are dealing with tiny “cross-border commuters”.

They attack us and make us sick - viruses seem to be similar to bacteria, one might think. But they differ fundamentally from microbes: In contrast to these single cells, virus particles are actually only packaged genetic material: DNA or RNA, which is surrounded by a protein-covered shell. In contrast, viruses do not have organelles such as mitochondria, ribosomes and cytoplasm. They therefore lack two basic characteristics of living beings: Viruses do not have a metabolism and cannot reproduce on their own.

Lifeless genetic engineers

Therefore others have to take over these functions for them. To make this possible, viruses infiltrate their blueprint into the genome of host cells and thereby turn them into their factories: The victim produces further virus particles, which then travel to hijack other cells. In this way, viruses cause various diseases in humans. However, some also infect bacteria - this makes the difference between the two types of pathogen particularly clear.

Against this background, viruses can be viewed as lifeless bioparticles. That is why some biologists exclude them from the community of living things. But there are also arguments in favor of an assignment to life. Viruses have one characteristic of living things: They have the ability to develop genetically - for example through mutations that enable them to adapt. One can also be of the opinion that viruses are part of life, at least in their integrated state: as soon as their blueprint is built into the DNA of a host cell, they become part of a living system.

Living ancestors?

However, some researchers go even further: They advocate classifying viruses in the family tree of life. The reasoning: There are indications that viruses once developed from living things. This is shown by comparing the protein molecules of bacteria with those produced by the viral DNA. They suggest that distant ancestors of the viruses were still real cells. It was only when they became cell parasites that they saved the cell machinery and used that of their hosts, so the assumption was made. Seen in this way, viruses would be living beings that have produced a bizarre method of reproduction by reducing their characteristics.

The existence of so-called giant viruses also seems to support the thesis. In contrast to tiny creatures such as coronaviruses or flu pathogens, they can grow as large as some types of bacteria and carry a very extensive genetic make-up. Protein folding patterns of these giant viruses also show striking parallels to those of microbes. The assumption is that they could still be comparatively similar to the cellular precursors of all viruses.

Ultimately, however, it still remains a matter of opinion whether one assigns the bizarre giant viruses or pathogens such as Sars-CoV-2, HIV or Ebola to living beings or not. What is clear, however, is that viruses have had an enormous impact on the development of life - and also on human history.

March 20, 2020

© Wissenschaft.de - Martin Vieweg