collect the molecules 
separately as they pass 
out of the column. If 
necessary, additional 
methods of purifying the 
protein can then be used 
and its biological activity 
can be examined in detail. 
Another technique, 
called gel electrophore- 
sis, uses electrical 
currents to cause protein 
or nucleic acid molecules 
to travel through a semi- 
solid gel according to 
then - size and charge. 
The separated molecules 
can then be transferred 
onto a sheet of special 
paper, where various 
methods can be used to 
detect each molecule or 
fragment. Electrophoresis 
is especially useful for the 
analysis and comparison of 
samples containing many 
different nucleic acid 
fragments or proteins. 
In addition to the above 
techniques, cell biology 
has been revolutionized 
by recombinant DNA 
technology. Often called 
genetic engineering, 
recombinant DNA tech- 
nology enables scientists 
to grow large quantities 
of cells that have been 
genetically altered to 
make a particular pro- 
tein (usually a protein 
normally found in 
another organism) and 
then to harvest the pro- 
tein from the cells. 
Often, bacteria and 
yeast cells are used to 
produce these proteins. 
Through the use of 
recombinant DNA tech- 
nology, scientists have 
discovered many new 
classes of genes and 
proteins and are able to 
compare the genetic 
material of species as 
different as humans and 
bacteria. This helps 
them determine the func- 
tions of proteins and 
regions within proteins 
and to piece together 
their roles in complex 
systems. Recombinant 
DNA technology has 
been augmented by 
another technique, 
called PCR (polymerase 
chain reaction), which 
allows researchers to 
make many copies of 
DNA segments without 
first having to grow these 
segments in bacteria or 
other organisms. 
