DNA Sequence

One of the major goals of the Human Genome Project is to learn the nucleotide sequences of our genes. The most frequently used method was designed by Sanger in the 1960s. Basically what he did was make a mixture of DNA, DNA polymerase, the four basic nucleotides and a modified nucleotide, say "ddATP". Then he would heat the DNA to separate the strands, and as the mixture cooled, a complementary strand of DNA would be formed. What was different was that the complementary strand contained the modified ddATP instead of ATP randomly inserted. Now it gets interesting. Because the modified nucleotide (ddATP) cannot join with with other nucleotides, the result is a fragment of DNA of different lengths. Now if one takes these fragments of different sizes and uses gel electrophoresis, a scientist can obtain a gel with bands of different sized fragments each containing a specific modified nucleotide. Now you can determine the sequence of the DNA. If you haven't figured it out, scientists do know that DNA polymerase builds a sequence from 5' end of the molecule so we now know which end to start from. In the early 1970s it would have taken a year to determine the sequence of a DNA fragment 25 nucleotides long. Now it takes less than one day.