NUCLEIC ACIDS 61
the double helix. Next, the vial is cooled to 55 ° C so that the primers will bind
(anneal) to the DNA strands. This process takes about 20 seconds. In the fi nal
step, the DNA polymerase (usually the Taq polymerase from the hot springs
bacteriumThermophilis aquaticus , which works best around 75 ° C) starts
making complete copies of the DNA template. The complete sequence of steps
for the PCR process takes about two minutes, and a million or so copies of
the desired DNA can be produced in one or two hours. The PCR method has
found applications in clinical medicine, genetic disease diagnostics, forensic
science, and evolutionary biology. For example, very small samples of DNA,
perhaps from a crime scene or from an ancient mummy, can produce suffi cient
copies to carry out forensic tests such as DNA profi ling.
2.3.6 Genomics and the Human Genome,
Each cell in an organism contains its complete genome; but depending on the
cell type, only the genes necessary for conducting the work of that cell type
are expressed. The human genome sequence consists of an ordered listing of
the adenine, cytosine, guanine, and thymine bases found on the 46 human
chromosomes. Only about 1% of the genome sequence codes for proteins
necessary for human life. Most of the rest of the genome consists of large
repetitive noncoding regions whose function is not well understood. It is
known, however, that critical clues to diseases such as cancer, diabetes, and
osteoporosis lie in areas of the genome that do not code for protein. About
one - fourth of the genome contains long, gene - free segments, whereas other
regions contain much higher gene concentrations. The number of protein
coding regions (genes) in the human genome, estimated at 30,000 – 40,000, is
surprisingly small, given that the fruit fl y has 13,000 genes and the thale cress
plant has 26,000.^19
Francis Collins, director of The National Human Genome Research Insti-
tute, and Ari Patrinos, head of the Offi ce of Biological and Environmental
Research at the Department of Energy ’ s (DOE) Offi ce of Science, managed
and coordinated the sequencing of the human genome program, initiated in
- A substantially complete version of the 2.9 billion base - pair human
genome sequence was published in 2001.^20 Reference 20 reported the results
of an international collaboration to produce and make freely available a draft
sequence of the human genome, presented an initial analysis of the draft
sequence data, and described some of the insights that could be gleaned from
the sequence. In addition to the draft sequence, pages 745 – 964 of this February
2001 issue ofNature contained accompanying discussions from many scientists
on diverse topics in evolutionary, medical, and biological sciences that would
benefi t from this accomplishment.
The Human Genome Project used the shotgun sequencing method in which
enzymes cut DNA into hundreds or thousands of random bits that were then
sent to automated sequencing machines capable of handling DNA fragments
up to 500 bases long. After sequencing, the fragments were pieced back
