Practice question

Cosmid vectors combine plasmid origin and lambda phage cos site required for packaging into phage heads. Cos sites are 12 base sticky cohesive ends that direct concatemer cleavage during lambda packaging. Incorporating cos sequence enables in vitro packaging of recombinant DNA flanked by cos sites into infectious lambda particles, allowing transduction into Escherichia coli with high efficiency. This permits cloning of 37-52 kb inserts larger than standard plasmids but smaller than BACs. Cosmid retains plasmid replication and antibiotic selection, merging phage delivery with plasmid maintenance, widely used for genomic library construction and large insert screening.