The enzyme that is used to join complementary dna nucleotides together is

DNA polymerase joins complementary DNA nucleotides together while DNA ligase joins together the gaps left behind from the Okazaki fragments.

The last thing that the scientist would do in producing transgenic bacteria for generating insulin would be to extract the insulin from the bacterial culture. The proper sequence of the steps to be done by the scientist should be:

Cut out the insulin gene from human DNA. 

Insert the human insulin gene into a plasmid

Transform bacteria with the recombinant plasmid. 

Extract the insulin from the bacterial culture.