DNA ligase forms phosphodiester bonds between the sugar and phosphate groups on the two strands of DNA.

This joins the sticky ends of the isolated DNA fragment to the sticky ends of the DNA in the plasmid vector.

Using the same restriction enzyme ensures that the sticky ends are complementary so the isolated DNA fragment and the DNA of the vector can be joined together.

Plasmids or bacteriophages.

Transformation involves introducing vectors with recombinant DNA into host cells, transforming these cells. 

Calcium ions and changes in temperature can make bacterial membranes more permeable to plasmids.

Electroporation is a method of transformation during which an electrical current makes bacterial membranes porous allowing the plasmids to move into bacterial cells.

Electroporation can also be used to transform DNA fragments into eukaryotic cells.

1. A marker gene that causes them to exhibit a specific trait, like antibiotic resistance, may be incorporated into the vector
2. A marker gene that codes for a fluorescent protein that can be seen under UV light, like green fluorescent protein (GFP), may be incorporated into the vector
3. A marker gene may be inserted within the GFP gene, preventing fluorescence if it is successfully incorporated
4. A marker gene may be incorporated that codes for enzymes that cause a colour change to a particular substrate