Sticky and Blunt Ends - Variations in Double-stranded Molecules - Overhangs and Sticky Ends

Overhangs and Sticky Ends

Non-blunt ends are created by various overhangs. An overhang is a stretch of unpaired nucleotides in the end of a DNA molecule. These unpaired nucleotides can be in either strand, creating either 3' or 5' overhangs. These overhangs are in most cases palindromic.

The simplest case of an overhang is a single nucleotide. This is most often adenosine and is created as a 3' overhang by some DNA polymerases. Most commonly this is used in cloning PCR products created by such an enzyme. The product is joined with a linear DNA molecule with 3' thymine overhangs. Since adenine and thymine form a base pair, this facilitates the joining of the two molecules by a ligase, yielding a circular molecule. Here is an example of an A-overhang:

5'-ATCTGACTA-3' 3'-TAGACTGA-5'

Longer overhangs are called cohesive ends or sticky ends. They are most often created by restriction endonucleases when they cut DNA. Very often they cut the two DNA strands four base pairs from each other, creating a four-base 5' overhang in one molecule and a complementary 5' overhang in the other. These ends are called cohesive since they are easily joined back together by a ligase. Also, since different restriction endonucleases usually create different overhangs, it is possible to cut a piece of DNA with two different enzymes and then join it with another DNA molecule with ends created by the same enzymes. Since the overhangs have to be complementary in order for the ligase to work, the two molecules can only join in one orientation. This is often highly desirable in molecular biology.

For example, these two "sticky" ends are compatible:

5'-ATCTGACT + GATGCGTATGCT-3' 3'-TAGACTGACTACG CATACGA-5'

They can form complementary base pairs in the overhang region:

GATGCGTATGCT-3' 5'-ATCTGACT CATACGA-5' 3'-TAGACTGACTACG

Read more about this topic:  Sticky And Blunt Ends, Variations in Double-stranded Molecules

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