All right, teh girl who sits next to me needs to know what a cosmid is for a test on Thursday. I told her that I’d check if anybody here knew.
Can any of the Teeming Millions fill me in? I know less than she does; apparently, it has something to do with genetics. Search engines and even an online medical dictionary have been tapped out.
Summarizing: Can someone tell me what the hell a cosmid is?
–John
Beautiful, Flyp. You’re a wonderful human being.
–John
Okay, you’ve impressed her, now bore her to tears:
Cosmids were invented because plasmids had a limited capacity to be taken up and propagated in bacteria when you cloned in very large stretches of DNA. This is a problem when making genomic libraries. A library is where you chew up all the DNA in a cell’s nucleus and paste all the pieces into plasmids. You can now manipulate this library in various ways to examine the different genes. Obviously, the larger the fragment that a recombinant plasmid can accomodate, the fewer recombinants you need to encompass the whole genome, and the less work is involved in screening, sequencing, etc. Cosmids can handle very long stretches of cloned DNA, up to about fifty thousand base pairs, bu this application - genomic libraries - hasn’t actually been all that successful with cosmids for various picayune technical reasons. But cosmids have turned out to be useful for - as you might guess - cloning very large eukaryotic genes, including their introns. They’ve also been used for cloning regions of the genome that contain whole gene families, with genes often separated by long stretches of noncoding DNA.
What cosmids are, is plasmids with the bacteriophage lambda cos site in them (hence the name). These sites are recognized by the phage’s enzymes and under the right conditions will lead to the DNA being packaged into an infective viral particle. Then the cosmid (carrying the long stretch of genes) can be used to infect bacteria; the viral particle injects its DNA into the cell and yields bacteria that harbor the very long stretches of DNA you’re after. Replication and amplification of the genes can be accomplished by turning on the viral lytic cycle, or just using a replication competent cell.
Obviously, this is a highly abbreviated version, though it’s probably more than you really wanted to know.
I got most of this from Sambrook, Fritch, and Maniatis’ Molecular Cloning: A Laboratory Manual, 2d ed. Cold Spring Harbor Laboratory Press 1989
Ugh, I hate that book.
You can clone even larger DNA segments by using Yeast Artifical Chromosomes (YACs) and Bacterial Artifical Chromosomes (BACs). In those cases, you’re fooling the organism into thinking your piece of DNA is one of its chromosomes.
Yeah the ladies find that info to be boring, but fortunately my olive oil charm and my huge pectoral muscles make up for it.
I used to rock and roll all night and party every day. Then it was every other day. Now I’m lucky if I can find a half an hour a week in which to get funky.