Viruses are defined by their need to reproduce in another organism, so the answer is no.

It takes a lot of cellular machinery to reproduce. By measuring the number of genes, we can get a sense of how much machinery is required for a cell to reproduce and survive. Scientists think that there are around 250 genes that are required for a living cell. However, in nature, the smallest genome of a non-parasitic bacteria is 1,389 genes.

Therefore, just to have an organism reproduce by itself requires over 1000 genes. In contrast, most viruses have far fewer genes, with the smallest viruses only have a few genes. There are actually a few viruses with genomes of over 1000 genes, but they still can’t reproduce by themselves. If these large viruses could reproduce by themselves, then they would no longer be considered viruses.

The answer is no! This is what makes viruses one of the most interesting and curious things in our world. If they are not in a host cell, they are lying dormant on whatever surface they may happen upon. That means each specific virus has an specific life span outside of a host. A virus, like the retrovirus human immunodeficiency virus (HIV) cannot survive very long outside of a host (hours) , but the small pox virus can survive for up to 2 years! Sadly, this was exploited by the British in their war against the indigenous Americans as a biological warfare agent. They let people infected with smallpox infect blankets and other items, which they then gave to the tribes as "gifts". Take a read on this Wikipedia page if you want to learn more about that incident!

No, and this is part of the definition of a virus: viruses do not contain the information necessary to build all of the enzymes and other stuff necessary to replicate DNA. Living cells do have all of this information, and in order to reproduce, the virus must co-opt the enzymes of an existing living cell. This is why viruses are only sort-of alive.

A virus cannot reproduce without a living cell It must have a living cell to be able to replicate itself. The virus contains the bare minimum - DNA that encodes for its proteins and a small amount of its own proteins. This is then injected into a living cell (bacterial or eukaryotic) where the viral DNA (or RNA) is then reproduced, transcribed, translated and new virus particles are made. The virus is interested in replicating and producing more of itself but it cannot do so without the presence of another living cell.

Viruses cannot reproduce without a host, because viruses do not have all the parts needed to reproduce.

Viruses all have the blueprint of reproduction, either as DNA or as RNA. They all have a protein coat that protects the DNA or RNA, but they do not have other proteins or RNAs like living cells do. After viruses infect their hosts, they can hijack the host replication system to multiply. Some viruses, though, can survive a long time without hosts.

No. A virus can't reproduce outside a living cell.

I'm now flying home from a conference about the origins of life and astrobiology; and one of the speakers said they think viruses formed and infected cells soon after there were cells to infect.

Viruses also carry DNA from one cell to another, which is called Horizontal Gene Transfer - HGT.

This is a really interesting question because it points out some important differences between viruses and cells.

We know that cells can reproduce through the process of mitosis but can viruses do that too? It turns out that they cannot. They are missing a few important structures that would allow them to reproduce. One of which are ribosomes. Ribosomes are the structures that translate the mRNA message into an amino acid sequence.

Since viruses do not have ribosomes they are unable to create the proteins that are necessary to build new viruses. They need to infect cells and force them to create new viruses for them. Without the help of a host cell, viruses could not reproduce at all! This brings up a very interesting question: are viruses alive? What do you think? Are there any other organisms that cannot reproduce on their own? Thank you for your question!

I cannot think of any example of a virus that could reproduce without another host organism. If it could do that, then it would be self-replicating, and that would make it alive and not really a virus anymore.

I think of viruses more as a container with some DNA or RNA inside. This is very similar to a computer code without a computer or any of the software or operating system to interpret and run the code. The virus must borrow the machinery it needs to read and copy its genetic code from the host organism in order to reproduce itself.