Answer :

The extra step of dissolving the nucleus is needed during DNA replication in a eukaryotic cell, whereas bacteria are prokaryotic and therefore don't have a nucleus so they don't need this extra step.

It is basically because Euckaryotic cells are more complex than Bacteria, thus more stages are to be completed when it comes to DNA and Molecular Genetics dynamics.

Eukayotes carry membrane-bound organelles and a nucleus whether they are single celled or multi cellular organisms. In Bacteria on the other hand membrane-bound organelles and/or nucleus are not available; making them simpler organisms; so when the DNA replication process happens it's easier to produce a replica of a Bacteria than it is for an Eukariotic cell.