Difference
Between
3'
to
5'
and
5'
to
3'
Exonuclease
Activity
| 3' -->5' Activity | 5'-->3' Activity |
1. Removal of nucleotides (hydrolysis of DNA) takes place in the 3'-->5' direction. |
l. Hydrolysis is in the 5'-->3' direction. |
| 2. Hydrolysis begins only at the 3'-OH terminus | 2. Hydrolysis begins at the terminal phosphodiester bond or at a bond several residues away from the 5' end |
| 3. The nucleotide removed must have a free 3'-OH terminus. | 3. Cleavage takes place irrespective of whether the 5' end is 5'hydroxyl or 5'-phosphorus. |
| 4. The products of hydrolysis are exclusively mononueleotides. | 4. The products may be 5' -mono- nucleotides, dinucleotides or oligonucleotides. |
5. The cleaved bond must be in the single stranded region. . |
5. The cleaved bond must be in the double stranded region |
| 6. The large fragment (MW 75,000) of polymerase I has 3'-->5' exonuclease activity and also polymerizing activity | 6. The small fragment (MW 36,000) of polymerase I has 5'-->3' activity. |
| 7. Has a 'proofreading' and 'editing' function in the initial stages of replication. Polymerase I removes mismatched nucleotide pairs and follows this up by resynthesis. | 7. Has a role in the excision of pyrimidine dimers during repair replication. (The dimers are formed when DNA is exposed to UV radiation). |
| 8. All the three polymerases (I, II and III) show 3'--> 5' exonuclease activity | 8. Polymerase I shows 5'-+3' exonuclease activity. This activity is reported to be absent in Pol 11 and Pol IIV although some works indicate that it may be present in Pol III. |