Genetic engineering is mostly three parts.
Cutting the DNA, inserting the DNA and 
attaching the DNA. To cut DNA geneticists
use restriction enzymes, which "seeks out"
a certain spot on a DNA strand to cut. For
example one enzyme could cut at "AATG" and
another could seek out "ATC". DNA is cut 
irregularly so one helix is longer than the
other. The longer side is called the 
"sticky end",  because the "sticky end"
attaches easily with another strand of DNA
with a corresponding set genes.                     
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|  |  |  |  |  |  |  |  |  |  |  |  |<-sticky end
sticky end->|  |  |  |  |  |  |  |  |  |  |  |  |
            _____________________________________   
	
The new DNA is inserted into the cell after
it has been cut. This is quite difficult 
because cells naturally do not let DNA into
the cell. There are many ways to get your new
DNA into the cell. One, you could use 
microinjection which uses a tiny, glass needle
to poke through the cell wall and inject the 
DNA. Two, surrounding your DNA in lipids. 
Lipids are fatty molecules which are let through
the cell wall like food. Three, electroporation
which sounds like what it does, jolting the 
cell with electricity forcing it to open pores
in the cell wall, letting your DNA through. 
Lastly a gene gun can be deployed to shoot 
small, metal fragments covered with DNA through
the cell wall. This works great in prokaryotic
cells, which have only one chromosome and no 
nucleus. For eukaryotic cells you need a better
way of injecting DNA.Something natural was used
to produce results in eukaryotic cells, a 
retrovirus. What retroviruses do is they enter
the cell and inject the nucleus with their own
DNA. DNA from the retrovirus is assimilated 
into the cells original DNA, this causes the 
retroviral disease. Geneticists use the 
retrovirus to genetically engineer cells 
indirectly. First they change the prokaryotic
retrovirus then they inject the new retrovirus 
to change the eukaryotic cells.    

How Genetic Engineering Work Links