Circular cohesive ends

The DNA of bacteriophage A, is a linear double-strand molecule that has complementary single-strand ends. These molecules can form closed circular molecules when two cohesive ends on the same molecule join, and they can form linear dimers, trimers, or longer molecules when sites on different molecules are joined. 

Bacteriophage X is a genetically complex and extensively studied virus of E. coli. Because it has been the object of molecular genetic research, it was investigated and developed as a vector. The DNA of phage X, in the form in which it is isolated from the phage particle, is a linear duplex molecule of about 45.5 kb pairs. The entire DNA sequence has been determined (SI). At each end are short, single-stranded 5 projections of 12 nucleotides that are complementary in sequence and by which the DNA adopts a circular structure when it is injected into the host cell i.e., X DNA naturally has cohesive termini that associate to form the cos site. 

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