[NARRATOR:] A common technique
in genetic engineering is to insert a new gene into
a loop of bacterial DNA called a plasmid. The molecular tool
used to cut DNA is a restriction
enzyme, such as EcoRI. The enzyme has a
precise shape that allows it to run along the
groove of the double helix, scanning in the case of EcoRI
for the base letter sequence GAATTC. The enzyme then cuts the
plasmid at this specific point, allowing a new piece
of DNA to be inserted. When it cuts, EcoRI
leaves a sticky end. This helps the new
gene to attach. The joins are then
stitched together by another enzyme
called DNA ligase. The genetically
engineered bacteria is grown in a culture medium. Very quickly, large
numbers of the bacteria can be produced, each with
a copy of the inserted gene. The bacteria duly
manufacture whatever protein the gene codes for, and
so the desired product is made.