Exonucleases are enzymes that catalyze the removal of nucleotides in either the 5-prime to 3-prime or the 3-prime to 5-prime direction from the ends of single-stranded and/or double-stranded DNA. Removal of nucleotides is achieved by cleavage of phosphodiester bonds via hydrolysis. These important enzymes can have slightly different activities, and therefore can be used for a wide variety of applications. Most exonucleases digest at nicks in the DNA. Some exonucleases remove one base at a time. Lambda Exonuclease is an example of this and transforms double-stranded DNA into single-stranded DNA by chewing from the free ending containing a 5-prime phosphate, degrading one strand preferentially but not the other. Other examples are Exo I and Exo III. Other exonucleases, such as T5, ExoV or Exo VII remove short oligos. The products of T5 Exo also include individual bases. Exonucleases such as Exo VII and V, digest in both the 5-prime to 3-prime and 3-prime to 5-prime direction, while others, such as Exo T and Exo I, only work in one direction. Some exonucleases, such as Exo I and Exo T only digest single-stranded DNA while leaving behind double-stranded DNA. This can be useful to remove PCR primers, for example. Exonucleases such as T7 Exo digest only double-stranded DNA, while others, such as T5 Exo and Exo V, can digest both single and double-stranded DNA. As you can see, exonucleases work in many different ways and can be used individually or combined to achieve the activity that you require for your specific application. For more information regarding NEB endonucleases, please visit the following resources at www.neb.com.
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