Question

What do u mean by "cos" sites in the case of lambda phages??
The context is:-
Cosmids are formed by the combination of plasmids and "cos" sites of phage lambda.

Answer 1

Cos sites are sticky ends.

The genome of lambda phage contains double-stranded, linear DNA, with 12-base single-strand segments at both 5' ends. These two single-stranded segments are the "sticky ends" (cos site). The cos site circularizes the DNA in the host cytoplasm.The lambda genome can be inserted into the E. coli chromosome and is then called a prophage. Cosmids are formed by the combination of plasmids and cos sites of lambda phage.