Discussion
Which of these is a cloning vector commonly used to carry large DNA inserts?
Answer: B
Correct answer: Bacterial artificial chromosome
A bacterial artificial chromosome, or BAC, is a vector based on the F plasmid replicon and able to carry inserts of 100 to 300 kilobases - far more than an ordinary plasmid. Its stability made it the principal vector of the Human Genome Project. The vectors form a series by capacity, and the order is worth knowing. A plasmid takes up to about 10 kb; a phage lambda vector about 25 kb; a cosmid, which combines plasmid replication with the lambda packaging signal, about 45 kb; a BAC up to 300 kb; and a YAC, a yeast artificial chromosome, over a megabase - though YACs are prone to instability and chimerism, which is why BACs were preferred for genome sequencing. A useful vector needs three things: an origin of replication, a selectable marker such as an antibiotic resistance gene, and a multiple cloning site with unique restriction sites. The other options are not vectors at all.
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