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Gene splicing is just what it sounds like: cutting
the DNA of a gene to add base pairs. Contrary to
the immediate image, however, no sharp instruments
are involved; rather, everything is done chemically.
Chemicals called restriction enzymes act as the
scissors to cut the DNA. Thousands of varieties
of restriction enzymes exist, each recognizing only
a single nucleotide sequence. Once it finds that
sequence in a strand of DNA, it attacks it and splits
the base pairs apart, leaving single helix strands
at the end of two double helixes. Scientists are
then free to add any genetic sequences they wish
into the broken chain and, afterwards, the chain
is repaired (as a longer chain with the added DNA)
with another enzyme called ligase. Hence, any form
of genetic material can be spliced together; bacteria
and chicken DNA can, and have been, combined. More
often, though, splicing is used for important efforts
such as the production of insulin and growth hormone
to cure human maladies. In the past, insulin was
only obtainable from the pancreas of cadavers (and
it required 50 cadavers to yield one dose!). With
modern splicing techniques, enough insulin can be
produced for all diabetics. The insulin-producing
genes from human DNA are spliced into plasmid DNA;
the plasmids are then allowed to infect bacteria,
and, as the bacteria multiply, large amounts of
harvestable insulin are produced. Splicing has other
practical medicinal uses, too. In July of 1996,
a 68-year-old woman became the first patient to
be treated for arthritis (a disease which affects
an estimated 2.1 million Americans) via gene therapy.
At the University of Pittsburgh, therapeutic DNA
that blocks the production of a specific protein
(IL-1) that causes arthritis pain was injected into
two of her knuckles.
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