Exonucleases are enzymes that work by cleaving nucleotides one at a time from the end (exo) of a polynucleotide chain. A hydrolyzing reaction that breaks phosphodiester bonds at either the 3′ or the 5′ end occurs. Its close relative is the endonuclease, which cleaves phosphodiester bonds in the middle (endo) of a polynucleotide chain. Eukaryotes and prokaryotes have three types of exonucleases involved in the normal turnover of mRNA: 5′ to 3′ exonuclease (Xrn1), which is a dependent decapping protein; 3′ to 5′ exonuclease, an independent protein; and poly(A)-specific 3′ to 5′ exonuclease.

1. EXONUCLEASE
Presented by,
Amarthya Siddhartha
Shryli K S
VIIth Sem
Molecular Biology
Yuvaraja’s College (Autonomous),
Mysuru

2. Contents
• Introduction
• The DNA Exonuclease
• Exonuclease of E. coli
• Significance to polymerase
• Exonuclease of Human
• Deficiencies related to exonuclease
• Conclusion
• References

3. Introduction
Enzymes as tools in Genetic Engineering.
a) Nucleases: DNases, RNases, exonucleases, endonucleases,
restriction endonucleases, isoschizomers.
b) DNA polymerase, klenow fragment, reversetransriptase, ligases,
S1 nuclease.
c) DNA modifying enzymes: polynucleotide kinase, terminal
transferase, phosphorylase &
phosphatase.Glycosylases,ribonuclease inhibitors,
topoisomerases.
What is Genetic Engineering?

4. The DNA Exonuclease

5. DNA-degrading enzymes
Enzymes that digest DNA can be divided into the endonucleases and
exonucleases. Endonucleases can degrade DNA from the sugar–
phosphate backbone to yield nicks in double-stranded DNA. In
principle, this can be general or sequence specific (for example,
restriction endonucleases). Moreover, some endonucleases are specific
for cutting single-stranded DNA.
Exonucleases, on the other hand, can degrade DNA from its end —
either from the 3′ terminus (3′–5′ direction) or from the 5′ terminus (5′–
3′ direction). The power of the 3′–5′ proofreading exonucleases lies in
their capacity to remove the sugar–phosphate backbone from the 3′ end
when the bases are not properly paired according to the Watson–Crick
base-pair rules (A–T and G–C), when bases are damaged, when they
are missing, or even, in some situations, when nucleotides are correctly
base-paired.

6. Exonuclease of E. coli

7. Fig.2

8. Significance to Polymerase

9. Exonuclease of Human

10. Deficiencies related to Exonuclease
3

11. Conclusion

12. References
• Lehman IR, Nussbaum AL (August 1964). "The deoxyribonucleases
of Escherichia Coli. V. on the specificity of exonuclease I
(Phosphodiesterase)". J. Biol. Chem. 239 (8): 2628–36
• Paul D. Boyer (1952). The Enzymes (1st ed.). Academic Press.
p. 211.
• Rogers SG, Weiss B (1980). "Exonuclease III of Escherichia coli K-12,
an AP endonuclease". Meth. Enzymol. Methods in Enzymology.
• West S, Gromak N, Proudfoot NJ (November 2004). "Human 5' → 3'
exonuclease Xrn2 promotes transcription termination at co-
transcriptional cleavage sites". Nature. 432
• Igor V. Shevelev and Ulrich Hübscher
THE 3′–5′ EXONUCLEASES
Nature Publishing Group VOLUME 3 MAY 2002