Horizontal gene transfer and viruses

[Numsgil]

What I want to know is how organisms decide what genes to transfer in non-viral gene transfer.

For instance, how does a bacteria know that the gene for drug resistance is the one it wants to transfer and not the one for glycolisis?

I can imagine conjugation resulting in the transfer of a random segment of DNA from the donor, but is there a better way than that?  I mean, that's alot of DNA to go randomly stumbling through.

And how is the DNA inserted so that it doesn't disrupt existing genes (say, the ones required for actually existing or reproducing).  Or doesn't it...  But then that would mean horizontal gene transfers is a particularly risky business...

And then Endy's question.  Why aren't DNA strands hella longer from constant insertion?

[shvarz]

How does dna fight them?

Why isn't our dna even longer with continous addition of new viruses? If the germline kept increasing in size you'd think there would be more problems.

There is no way to fight them.

As to your second question :)  guess what - more than half of our genome consists of pieces of retroviral DNA, that's 500,000,000 bases for you :)  Retrovirses are our longest "friends", they've been around for ages...  They do cause problems...  On the other hand, some people beleive (without much evidence IMO) that it was retroviruses that turned monkeys into humans :)

[shvarz]

Nums: We know very little about horizontal gene transfer.  In fact, up until several years ago we thought it almost never happens.  Now people find it everywhere they look.  But the mechanism is still not clear.

The best example of known horizontal gene transfer is antibiotic resistance in bacteria.  These genes exist on plasmids - rather small circular self-replicating pieces of DNA.  The main bacterial DNA is on chromosome - large circular DNA of about 10^7 bases.  The plasmids are not longer than 10^4 bases.  But there is a lot of them.  And they are fairly stable, so that even if bactria dies, the plasmid may stick around for quite some time.  Bacteria swap plasmids with each other very often.

If we want to reproduce that we need two things: multiple strands of DNA (chromosomes, but notice that plasmids are not actually chromosomes, they are just separate pieces of DNA) and particles carrying DNA (what we call viruses now, but we need to make them more general).

[Numsgil]

Hmm, I'm going to need to think about this for a while.  This may be tricky...