What Is Biotechnology? - Biotechnology and Your Health: Pharmaceutical Applications - page 6

a G. This rule of base pairing means that when the strands separate
before a cell divides into two, the DNA is copied precisely, because
a T in one strand lines up with an A for the new strand, an A lines
up with a T, and so on. The bases A and T, as well as C and G, are
called complementary bases because they form pairs easily.
In the 1960s, scientists developed methods that allowed them to
figure out the order, or sequence, of bases in a DNA molecule. The
methods have been modified over the years so that machines can
now do much of the work, but the principle remains the same. The
sequencing method requires:
• The piece of DNA to be sequenced;
• DNA polymerase , a protein that can make a copy of a
strand of DNA in a test tube;
• the four different bases;
• chemically modified versions of bases that, when added
to the strand, stop more bases from being added; and
• a short piece of DNA with a base sequence that
complements the sequence at the beginning of the piece
of DNA to be sequenced. (This bit of DNA is called a
primer , and it tells the DNA polymerase where to start
adding bases.)
The DNA, the primer, and the four bases are placed in four test
tubes. The primer is chemically tagged with dye. Then, a chemically
modified version of one base—A, T, C, or G—is added to each test
tube. The reaction is started with the addition of DNA polymerase.
In each tube, sets of DNA strands, complementary to the DNA to be
sequenced, are made. Synthesis of a strand stops when, at random,
the modified form of the base is added. There are millions of copies
of the DNA to be sequenced in each tube, and at the end of an
hour or so, there are millions of copies of strands, each stopped