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The use of Cosmids

Genetics as it applies to evolution, molecular biology, and medical aspects.

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The use of Cosmids

Postby Lucanus cervus » Mon Jan 10, 2011 10:07 pm

Hi,


Cosmids are useful because they can be used to clone larger segments of foreign DNA than normal plasmids. This is an advantage in ordered clone sequencing.

But what I don't understand is what role the cos sequence (of the lambda phage) has to do with that?

How come introducing the cos sequence in a plasmid makes it able to be used with larger inserts of foreign DNA?


Thanks in advance,

Sam
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Postby JackBean » Wed Jan 26, 2011 1:44 pm

Cos sequences are ~200 base pairs long and essential for packaging. They contain a cosN site where DNA is nicked at each strand, 12bp apart, by terminase. This causes linearization of the circular cosmid with two "cohesive" or "sticky ends" of 12bp. (The DNA must be linear to fit into a phage head.) The cosB site holds the terminase while it is nicking and separating the strands. The cosQ site of next cosmid (as rolling circle replication often results in linear concatemers) is held by the terminase after the previous cosmid has been packaged, to prevent degradation by cellular DNases

http://en.wikipedia.org/wiki/Cosmid
http://www.biolib.cz/en/main/

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Postby Lucanus cervus » Wed Jan 26, 2011 3:30 pm

Thanks!
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