The YACs are Here!
Yeast Artificial Chromosomes (YACs)
National Center for Human Genome Research, National Institutes
of Health. "New Tools for Tomorrow's Health Research." Bethesda, MD:
Department of Health and Human Services, 1992.
DNA is a very large molecule that contains in its chemical structure
the information cells need to synthesize the 100,000 or so proteins
the human body requires to function normally. Not only does DNA
command the synthesis of these important substances, it also transfers
genetic traits from parents to their children and tells a cell how and
when to reproduce itself during growth, or to mend wounds.
A technique using modified yeast chromosomes, known as "YACs," for
yeast artificial chromosome, enables scientists to borrow the DNA
duplication machinery of cells, including human cells.
Before YACs were developed, scientists relied on the duplication
machinery of a common bacterium known as Escherichia coli to make
millions of copies, or "clones," of a piece of DNA they were
interested in studying. But the size of a DNA piece that can be
reproduced in YACs is 10 times greater than that which can be
reproduced in bacteria. Because of this capacity, YACs enable
scientists to isolate and study large portions of the genetic
endowment of organisms whose chromosomes are big and complex, such as
those of humans.
But to use yeast cells for reproducing DNA, scientists first had to
know which parts of the yeast chromosome were necessary to direct the
duplication process. Only a fraction of 1 percent of a yeast cell's
total DNA is necessary for replication, including the center of the
chromosome (the centromere), the ends of the chromosome (telomeres),
and another short stretch of DNA called the autonomous replication
sequences (ars). As long as these genetic parts are attached to DNA
from humans or other organisms, the yeast cell is tricked into making
copies of that DNA during its own cell division.
The development of YACs has made it possible to put very large pieces
of DNA into yeast cells and reproduce them in large quantities. No
other cloning procedure used to date has been capable of making copies
of such large pieces of DNA. The length of a strand of DNA is usually
measured in units called nucleotides - the subunit molecules that make
up DNA. The longest piece of DNA that can be cloned in yeast is
between 0.5 to 1 million nucleotides, compared to only about 50,000 in
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Cloning into a Yeast Artificial Chromosome
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