The YACs are Here!

Yeast Artificial Chromosomes (YACs)

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 bacteria.

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