Exploring Genes

Recombinant DNA technology has revolutionized biochemistry since it came into being in the 1970s. The genetic
endowment of organisms can now be precisely changed in designed ways. Recombinant DNA technology is a fruit of
several decades of basic research on DNA, RNA, and viruses. It depends, first, on having enzymes that can cut, join, and
replicate DNA and reverse transcribe RNA. Restriction enzymes cut very long DNA molecules into specific fragments
that can be manipulated; DNA ligases join the fragments together. The availability of many kinds of restriction enzymes
and DNA ligases makes it feasible to treat DNA sequences as modules that can be moved at will from one DNA
molecule to another. Thus, recombinant DNA technology is based on nucleic acid enzymology.
A second foundation is the base-pairing language that allows complementary sequences to recognize and bind to each
other. Hybridization with complementary DNA or RNA probes is a sensitive and powerful means of detecting specific
nucleotide sequences. In recombinant DNA technology, base-pairing is used to construct new combinations of DNA as
well as to detect and amplify particular sequences. This revolutionary technology is also critically dependent on our
understanding of viruses, the ultimate parasites. Viruses efficiently deliver their own DNA (or RNA) into hosts,
subverting them either to replicate the viral genome and produce viral proteins or to incorporate viral DNA into the host
genome. Likewise, plasmids, which are accessory chromosomes found in bacteria, have been indispensable in
recombinant DNA technology.
These new methods have wide-ranging benefits. Entire genomes, including the human genome, are being deciphered.
New insights are emerging, for example, into the regulation of gene expression in cancer and development and the
evolutionary history of proteins as well as organisms. New proteins can be created by altering genes in specific ways to
provide detailed views into protein function. Clinically useful proteins, such as hormones, are now synthesized by
recombinant DNA techniques. Crops are being generated to resist pests and harsh conditions. The new opportunities
opened by recombinant DNA technology promise to have broad effects.
The Basic Tools of Gene Exploration
The rapid progress in biotechnology indeed its very existence is a result of a relatively few techniques.
1. Restriction-enzyme analysis. Restriction enzymes are precise, molecular scalpels that allow the investigator to
manipulate DNA segments.
2. Blotting techniques. The Southern and Northern blots are used to separate and characterize DNA and RNA,
respectively. The Western blot, which uses antibodies to characterize proteins, was described in Section 4.3.4.
3. DNA sequencing. The precise nucleotide sequence of a molecule of DNA can be determined. Sequencing has yielded a
wealth of information concerning gene architecture, the control of gene expression, and protein structure.
4. Solid-phase synthesis of nucleic acids. Precise sequences of nucleic acids can be synthesized de novo and used to
identify or amplify other nucleic acids.
5. The polymerase chain reaction (PCR). The polymerase chain reaction leads to a billionfold amplification of a segment
of DNA. One molecule of DNA can be amplified to quantities that permit characterization and manipulation. This
powerful technique is being used to detect pathogens and genetic diseases, to determine the source of a hair left at the
scene of a crime, and to resurrect genes from fossils.
A final tool, the use of which will be highlighted in the next chapter, is the computer. Without the computer, it would be
impossible to catalog, access, and characterize the abundant information, especially DNA sequence information, that the techniques just outlined are rapidly generating.