Viral Infections
I spent the last two days in Jerusalem, attending a nice conference that my advisor unexpectedly found two days earlier.
The title was “From Macromolecules To Cell Biophysics”, and it included some nice talks. One of the talks reminded me of the way viruses enter cells, and then I thought this is neat enough to share with the world.
Viruses are composed of two main parts: the caspid, which is a small capsule; and the genetic information (which can be either a DNA or an RNA strand, depending on the type of the virus), which is stored inside the caspid.
It is not easy: a DNA is a very long and electrically charged molecule: you can imagine it like a long electrified spaghetti. The caspid looks like a tiny pea; so packing the DNA into the caspid is something like folding a spaghetti into a pea - not a very simple task, and certainly one that requires lots of energy (left alone, the spaghetti will tend to occupy more space than available inside the caspid, so the virus actually has to spend energy in order to put there the DNA. There’s a special “motor” near the entrance, that takes charge of this. The energy comes, of course, from the infected cell).
Actually, one can calculate the pressure that is exerted on the caspid from the DNA inside it. You arrive at considerably large numbers - something in the order of 50 atmospheres, which is much higher than I had initially imagined. This implies, that if you make a hole in the caspid, the DNA will start to leave it in a passive process, just injected out by the immense pressure inside the caspid. This accounts for the entrance of the first two-thirds of the DNA strand into the infected cell.
As the DNA leaves the caspid, the pressure drops; eventually there is not enough pressure to push the end of the strand out of the caspid (there are special rules for these kind of movements; physicists usually call them “low Reinolds Number movement” and the main thing there is that there is practically no acceleration - the moment you stop excerting force, the damn thing stands still). So how does the “tail” of the strand enter the cell? This is the neatest part.
The DNA is very, very, long. By the time two-thirds of it have entered the cell, the cell had already noticed that something happens. In fact, it “sees” free DNA, and immediately starts to replicate it. During the replication, a cellular motor attaches to the molecule - and pulls it inside. So actually, the cell pulls the viral genome into it, while replicating new viruses.
I think it is very clever of the viruses. Almost as clever as those viruses that went the few next steps, and cut their genome into separate strands, further - stored each strand in a separate caspid - so you actually have the virus cut into parts… which is simply one of nature’s ingenious inventions.
June 5th, 2008 at 13:58
1. This indeed very interesting. Is the same mechanism also works with RNA (you didn’t mention)? and I thought that all viruses have RNA only.
2. When you say “no acceleration”, you probably mean “no momentum”? Surely when something is at a stand still at one moment and moving in the next moment - it was accelerating in between?
3. The virus with separate genome parts is even more interesting - how does it work ? does a virus have to get all separate parts into the cell for an infection to occur (something like a “distributed attack”)? In that case I would say that for such an attack to work the virus has to make sure that there are many many more copies around for a sufficient infection then a virus with the entire genome in a single fragment.
June 7th, 2008 at 7:36
All these conferences… looks like you r having really hard time…