Plasmids

[student12]

In genetic engineering they tend to incorporate genes into plamids rather than chromosomes to either clone the genes or synthesise the gene products.

Why insert genes into plasmids rather than chromosomes?
My book suggests that plamids are more often replicated than chromosomes.

But then another question pops up...Why are plasmids frequently replicated?
b/c what i know is that plamids are essential for bacteria under certain conditions eg: to be resistant against certain antibiotics...but how does this relate to replication rate?

And lastly, Why can't plasmids be directly inserted into bacteria?
b/c my book states that approximately one in a million cells take up the plasmid copy.

Can anyone answer these questions and put me out of confussion...thanks

[canalon]

1/ It's harder to do it nicely in chromosome, and you can disrupt important cell functions. But there are system to do that.

2/Because they have an independant origin of replication. some plasmids can be found in ~500 copies in the cell, some only once, depending on the origin of replication. They do that because they are mostly genetic parasites, that can make themselves useful not to be thrown out of the nice comfy bacteria.

3/ relation to replication rate? see above or I don't know what you mean.

4/They are small, and bacteria too so it is hard to take one plasmid in syringe and to insert it into the bacteria... So we have to rely on less accurate techniques. Bu since it is easy to get a few billions bcateria in 1 milliliter of culture that is more than enough to get something.

[herb386]

Also, if you did put a gene in a bacterial chromosome it would be much harder to get it into the cell. Plasmids can enter bacteria by passing through the cell membrane when you use a heat shock procedure or something similar. This only works for small amounts of DNA so you would have to find another method to get a chromosome into the cell.

If you did get another chromosome into a cell there would be a huge increase in the amount of DNA replication needed per cell cycle. This would slow down its rate of reproduction and so slow down the replication rate of your gene.

Hope that answers some of your questions.

[student12]

Thankyou both for answering my questions. Now I am confusion-free so to speak...Thanks

[mkwaje]

Additional info for you:

Many plasmids are already sequenced and because of this its easy to cut, insert and monitor your gene of interest.

Regarding replication rate, you can induce plasmid replication, say you have your gene inserted on a plasmid coding for tetracycline resistance in E coli, then you can add tetracycline in the medium so that the cells will replicate the tet plasmid in large numbers.

hope that helps..