What's the difference between a dead virus and an alive virus?

In AP Biology, I was taught that a virus acted like an inanimate object unless it came under certain conditions… e.g., unless it was reproducing, it was indistinguishable from an inanimate object.

That being said, how do we know if a virus is dead versus inanimate… e.g., what is the real difference between killed-virus vaccines and live-virus vaccines? Can an otherwise intact virus ever truly be considered to be “dead”?

I am not an organic chemist or biologist, but from what I’ve read and gathered, the question of whether a virus is alive or not (I don’t think “dead” is a good word to use here) is still the subject of much debate.

But, honestly, I would have to say no. To me, it just seems like a strand of genetic code that happens to have a chemical reaction when it bumps into the right organism. As far as I know, anything that is truly alive can happily reproduce on its own and doesn’t need to hijack another organisms reproductive mechanisms to do so.

Alive virus - its little genetic code can reproduce.

Dead virus - tie-dyed protien shell pining for gerry.

(A dead virus is a snippet of a virus that can’t reproduce but contains the information that antibodies can attach to. Used in making some vaccines.)

I’m not a doctor or a biologist, but here’s my understanding.

A virus is a replicating strand of RNA or DNA in a protien case. It infects cells by binding to them. Then the DNA or RNA leaves the protien case and enters the cell itself, “reprogramming” the cell to produce copies of the virus, instead of other copies of the cell. One of the ways the body responds to this is by producing antibodies, which are chemicals designed to destroy the outer protein shell. Each kind of antibody reacts to a specific kind of protein. So, if you get the flu, your body produces the antibodies that act against the protein shell around that particular flu virus. If you then get infected by the measles virus, the flu antibodies your body made won’t react to the protein surrounding the measles virus, and your body will have to make a new set of antibodies to respond to that.

A “live vaccine” is a vaccine made up of weakened copies of the virus you’re vaccinating against. So, to give you a vaccination against measles, I actually inject you with the measles virus. But, it’s a weakened copy of the measles virus, and it can’t reproduce very quickly, which makes it easy for your body to defeat. You won’t get sick because your body will be able to produce antibodies before it can effectively take over cells.

A dead vaccine is just made up of viral protein casings. There’s no DNA or RNA in them (or the DNA or RNA has been tampered with so that it doesn’t work) Your body still sees that there are foreign proteins in your bloodstream, though, so it produces antibodies against them. So, in the future, if you’re exposed to the actual virus, you’ll already have the antibodies against it, and it won’t infect you.

Thank you for the excellent exception. I had not considered that a “killed” virus would simply be a protein shell without RNA.

Not all ‘dead’ viruses are just shells, however any genetic material has been deactivated by some means. They are quite inert.

It’s more common in technical situations to refer to inactivated or inert viruses rather than dead viruses. However to keep the public’s mind at ease the term dead virus is used when referring to vaccines because it’s easier to understand. Most people don’t understand what a vaccine is, trying to explain that something that isn’t alive has been stopped from reproducing is just way to complicated to be worth it.

Good point… thank you.

How can a virus be rendered inert? (My WAG is that it’s coated in something such that it can’t escape the protein shell…)

One of the easiest ways to make a virus inert is to zap it with large amounts of UV light or gamma rays. Instant inert virus. Various chemicals also work. What you’re trying to do is fragment the DNA/RNA so much that it ceases to function.

AWESOME, you really can get an answer to everything on the SDMB! Thank you! :cool:

But you want to keep the protein casing intact, correct, so the proper antibodies will pick up on it. I seem to recall from AP Bio, though, that most proteins denature at about the same temperatures as do nucleic acids, and I would also assume that they would take about the same dosages of hard radiation. So how do you destroy the nucleic acids while leaving the intact protein shell?

Nucleic acids are actually much more vulnerable to radiation than proteins are. Think of health problems that result from radiation exposure - most of the time, people with low to medium grade exposure don’t have any obvious immediate symptoms like radiation burns, nausea, hair loss, etc., which would result from the radiation interfering with their proteins. The major problem is cancer years down the road, which is the result of radiation-induced mutations in their DNA.

Just like he said - UV light or gamma radiation.

OK. Was I at least right in saying that they’re about equally vulnerable to heat?

Speaking pretty broadly, yeah. Obviously, different proteins have pretty different ranges in which they’re functional - think of the heat-loving bacteria that live in hot springs that are about boiling, much hotter than many of your enzymes can function. But for most proteins, I think it’s pretty close. (IANABiochemist, though.)

So it’s not the virus itself that is alive, but rather the system of virus plus host cell where it replicates plus the creature that the host cell is in and supported by…?

BTW, did someone actually build a functioning biological virus from spare parts (chemicals and whatnot)?

Depends on your definition of life.

Sort of. No one’s started from off the shelf chemicals and created a viable virus, but scientists have made viable virus particles by taking the individual parts from different sources and putting them together.