People often says that plasmids become unstable if you use them to clone long DNA fragments (over 20 kb), and that for this purpose is better to use cosmids (can carry DNA fragments up to 50 kb). However, as far as I know the only difference between both vectors is the presence of cos sites in the latter, and that those cos sites are important for DNA packaging into viral particles (not involved in vector replication). Considering that, how can we explain that cosmids remain stable with larger DNA segments?. To bring up an example, pHC79 is a cosmid derived from pBR322 plasmid (they are exactly the same except for the presence of lambda cos sites). pHC79 has been successfully used to clone DNA segments up to 50 kb. The question is: Can we expect that pBR322 works as well as pHC79?
Johannes is correct, the problem isn't stability of the large plasmid but that you can not efficiently transform E. coli with plasmids of 50kb in size. The transformation rate will be extremely so. Because cosmids are packaged you circumvent the transformation barrier using that approach.
The second issue is whether large cosmids/plasmids are less stable. The answer is that to some degree they are less stable but that is not the same as being impossibly unstable. But you do need to be more careful with large plasmids.
The major difference between colE1-derived plasmids like pBR322 or similar and most cosmids is not stability of plasmids or inserts after the initial cloning process, but the way of inserting the DNA into the vector. It is much easier to go through the Lambda packaging procedure than to try the adventure to insert fragment lenghts in the 45 kb range into a non-cos-plasmid. Packaging allows tremendously high transformation rates.
Johannes is correct, the problem isn't stability of the large plasmid but that you can not efficiently transform E. coli with plasmids of 50kb in size. The transformation rate will be extremely so. Because cosmids are packaged you circumvent the transformation barrier using that approach.
The second issue is whether large cosmids/plasmids are less stable. The answer is that to some degree they are less stable but that is not the same as being impossibly unstable. But you do need to be more careful with large plasmids.
I agree with your comments regarding the advantages of packaging/infection of larger vectors versus electroporation. However, apart from the transformation barrier, it seems that cosmids and plasmids have a similar capacity to carry DNA fragments (once inside the bacterial cells). Furthermore (regarding the first commentary of Zoia), as I mentioned before, pHC79 has been succesfully used to clone 40-50 kb DNA fragments, and it has the same replication origin of plasmid pBR322 (same copy number). So, copy number should not make a big difference between DNA size capacity of plasmids vs. cosmids.
Right Andres. Once they are in cell, there is no fundamental difference between cosmids and other pBR-type plasmids.
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