[Villi (OP)]

Assuming that viruses are the smallest form of living matter, is there evidence for something biological and smaller than them?

[Kryptid]

You might be interested in something called "viroids": https://en.wikipedia.org/wiki/Viroid

[chris]

Good call, @Kryptid

There's also the case of the "virophage" - viruses that prey on other viruses by creaming off the substrates that they manufacture. "Sputnik" is one example: https://en.wikipedia.org/wiki/Sputnik_virophage

[Villi (OP)]

DNA, RNA, protein aside, is there anything smaller? I was thinking of the boundaries between biology and chemistry/physics.

[evan_au]

In genetic terms, there are some pretty big viruses out there - some in seawater were originally mistaken for a bacterium. Some have over 1 million base pairs, which makes them bigger than the genome of some bacteria.
See: https://en.wikipedia.org/wiki/Mimivirus#History

At the other end of the spectrum, prions are a much simpler molecule than DNA or RNA; they replicate by forming a distorted template which causes naturally-occurring proteins in the human body to adopt the same distorted form. This kills nerve cells, causing a variant of Jakob-Creutzfeld disease..
See: https://en.wikipedia.org/wiki/Prion

[chris]

Yes, I was about to say that if you really want to go small then there are prions, but these are literally infectious proteins, so they are stretching the definition of life and I wouldn't define them as "alive". Indeed, some people don't define viruses as life forms because they regard them as too simple and, given their reliance on a cell to replicate, not strictly alive. As such "life", some say, is a self-replicating entity.

[Villi (OP)]

Imagining a virus with 1 million bps is absurd. It's capsid proteins must be unusual. For a virus to be larger than protozoans and bacteria, I don't know. Unless of course the virus has found a way to efficiently pack the DNA and RNA more so than cellular lifeforms.

[Bored chemist]

Imagining a virus with 1 million bps is absurd.

No need to imagine them, they are real.
https://en.wikipedia.org/wiki/Pandoravirus

[Villi (OP)]

Those things must be gold mines for genes. I only recall learning that a dozen transcription/translation targets were packed into a virus.

[chris]

Those things must be gold mines for genes. I only recall learning that a dozen transcription/translation targets were packed into a virus.

It's a few more than that, generally, but these giant viruses (giruses) are a whole new ballgame...

[alancalverd]

Indeed, some people don't define viruses as life forms because they regard them as too simple and, given their reliance on a cell to replicate, not strictly alive. As such "life", some say, is a self-replicating entity.

That's a bit selfdefeating. Every cell relies on something external to replicate, otherwise the daughters would just get smaller and simpler. Now whilst bacteria and plant cells only require simple compounds to replicate, both viruses and hugely complicated parasites require very complex organisms. If a virus isn't alive, then neither is the Large Blue butterfly!

[Bored chemist]

given their reliance on a cell to replicate

Every cell relies on something external to replicate,

Those are not the same thing.
A virus requires something alive to replicate, a yeast cell requires (dead) stuff like sugar and ammonium phosphate to reproduce.

[Danne]

Virus isn't a living thing. It doesn't live on its own... it just like some form of matter which needs a host to begin it's cycle.

I bet the smallest thing couldn't be smaller than a size of 1 DNR or RNR molecule. I can't find a logical explanation how that could be.