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 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.
Distinguishing Between
Bacterial Chromosomes and Plasmids Analogy
How do Restriction Enzymes
Work Analogy
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