The concept of life from the biological point of view refers to the capacities of metabolism, reaction to external stimuli, growth and reproduction, generally also the capacities of being born and dying. physical beings with these abilities would be considered alive as opposed to inert beings.
The idea of biological life is closely linked to organisms with a cellular structure. However, the idea of acellular life has never been ruled out, especially if we think of life in other parts of the Universe, a thought in which we don’t have to take for granted that life would be structurally the same as on Earth.
The existence of acellular life forms on Earth is also not out of the question. The line between being alive and being inert is very blurred in some cases, highlighting the case of viruses. For many the virus is not a living being, for many it is.
The definition of life that requires the simultaneous existence of metabolism, reaction to stimuli, growth and reproduction, is for many scientists to assume that life is as the scientist himself experiences it, and they argue that this assumption may be wrong. For example, some bacterial spores can remain non-growing for extremely long periods, even over a thousand years.1.
Mimivirus and the definition of life
In 2003, a new virus was identified, the Mimivirus, and it was discovered that it could synthesize its own proteins, a fact that led many scientists to consider this virus as a living being.
The mimivirus is as large as some bacteria, similar to Rickettsia conorii or Tropheryma whippiei. Among the 911 genes in the Mimivirus DNA, there are some that have not been found in any other virus and that until their discovery were considered exclusive to cellular life.
For example, they contain the gene for cytochrome P450 and other proteins involved in carbohydrate, lipid and protein metabolism. Mimiviruses do not depend on the host cell to encode these metabolic pathways as if it happens in some obligate intracellular prokaryotes that depend on these pathways encoded in the host cell and which are clearly considered living beings.
Mimiviruses do not depend on the host cell genome to encode some metabolic pathways, but continue to depend on their ribosomes for gene translation. They also lack some characteristics included in many definitions of life: homeostasis, response to stimuli, and growth (they replicate by self-assembly, but do not grow in the ordinary sense of the word).
Mimiviruses are classified within the Mimiviridae family, which are believed to have appeared on Earth a billion years ago, before cellular life emerged. They are believed to have played an important role in the development of life on the planet, as there are still viruses of this family capable of infecting bacteria, archaea and eukaryotes.
Viruses are generally encompassed in the term acytoto-any aphanobiont which includes other acellular biological entities such as plasmids, prions, transposons and viroids, also often referred to as probionts.
In the same way that the definition of life and its relationship to the cell is very fuzzy, assuming that life is based on DNA can be an equally erroneous assumption. Within the concept of “acellular life”, many scientists also include possible extraterrestrial life forms that may or may not use DNA, and even a possible future artificial life where machines are able to reproduce by manufacturing their own structural elements and evolve by improving and adapting. up to the environment.