Microbiology and Immunology

Yeast artificial chromosome (YAC) WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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Another feature of the sexual reproduction process in

yeast is the production of pheromones by the cells. Yeast

cells respond to the presence of the chemicals by changing

their shape. The peanut-like shape they adopt has been

dubbed “shmoos,” after a character in the “Li’l Abner”

comic strip. This shape allows two cells to associate very

closely together.

See also Cell cycle (eukaryotic), genetic regulation of;

Chromosomes, eukaryotic; Economic uses and benefits of

microorganisms; Yeast artificial chromosome; Yeast, infectious

YYeast artificial chromosome (YAC)EAST ARTIFICIAL CHROMOSOME(YAC)

The yeastartificial chromosome, which is often shortened to

YAC, is an artificially constructed system that can undergo

replication. The design of a YAC allows extremely large seg-

ments of genetic material to be inserted. Subsequent rounds of

replication produce many copies of the inserted sequence, in a

genetic procedure known as cloning.

The reason the cloning vector is called a yeast artificial

chromosome has to do with the structure of the vector. The

YAC is constructed using specific regions of the yeast chro-

mosome. Yeast cells contain a number of chromosomes;

organized collections of deoxyribonucleic acid(DNA). For

example, the yeast Saccharomyces cerevisae contains 16

chromosomes that contain varying amounts of DNA. Each

chromosome consists of two arms of DNA that are linked by

a region known as the centromere. As the DNA in each arm

is duplicated, the centromere provides a region of common

linkage. This common area is the region to which compo-

nents of the replication machinery of the cell attach and pull

apart the chromosomes during the cell division process.

Another region of importance is called the telomere. The end

of each chromosome arm contains a region of DNA called

the telomere. The telomere DNA does not code for any prod-

uct, but serves as a border to define the size of the chromo-

some. Finally, each chromosome contains a region known as

the origin of replication. The origin is where a molecule

called DNA polymerase binds and begins to produce a copy

of each strand of DNA in the double helix that makes up the

chromosome.

The YAC was devised and first reported in 1987 by

David Burke, who then also reported the potential to use the

construct as a cloning vehicle for large pieces of DNA.

Almost immediately, YACs were used in large-scale determi-

Light micrograph of baker’s yeast.

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