What treatment is given to make a bacterial cell competent to take up $DNA$?

If a recombinant $DNA$ bearing a gene for resistance to Ampicillin is transferred into $E. coli$ cells,the host cells become transformed into Ampicillin-resistant cells. What happens when these $E. coli$ are grown on a medium containing Ampicillin?

$A$ recombinant $DNA$ molecule was created by ligating a gene to a plasmid vector. By mistake,an exonuclease was added to the tube containing the recombinant $DNA$. How does this affect the next step in the experiment,i.e.,bacterial transformation?

What is the first step in the Southern blot technique?

Plasmid $pBR322$ has a $PstI$ restriction enzyme site within the gene $amp^R$ that confers ampicillin resistance. If this enzyme is used for inserting a gene for $\beta$-galactosidase production and the recombinant plasmid is inserted into an $E. coli$ strain:

How can purified $DNA$ be precipitated from a solution after appropriate treatment?