Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Yeast genetics

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nation of genetic sequences, most prominently the Human

Genome Project.

YAC contains the telomere, centromere, and origin of

replication elements. If these elements are spliced into DNA in

the proper location and orientation, then a yeast cell will repli-

cate the artificial chromosome along with the other, natural

chromosomes. The target DNA is flanked by the telomere

regions that mark the ends of the chromosome, and is inter-

spersed with the centromere region that is vital for replication.

Finally, the start site for the copying process is present. In

essence, the yeast is fooled into accepted genetic material that

mimics a chromosome.

The origin of the DNA that is incorporated into a YAC

is varied. DNA from prokaryotic organisms such as bacterial

or from eukaryotessuch a humans can be successfully used.

The power of YACs is best explained by the size of the DNA

that can be copied. Bacteriaare also capable of cloning DNA

from diverse sources, but the length of DNA that a bacterium

can handle is up to 20 times less than that capable of being

cloned using a YAC.

The engineered YAC is put back into a yeast cell by

chemical means that encourage the cell to take up the genetic

material. As the yeast cell undergoes rounds of growth and

division, the artificial chromosome is replicated as if it were a

natural chromosomal constituent of the cell. The result is a

colonyof many genetically identical yeast cells, each contain-

ing a copy of the target DNA. The target DNA has thus been

amplified in content. Through a subsequent series of proce-

dures, DNA can then be isolated from the rest of the DNA

inside the yeast cells.

Use of different regions of DNA in different YACs

allows the rapid determination of the sequence, or order of the

constituents, of the DNA. YACs were invaluable in this

regard in the sequencing of the human genome, which was

completed in preliminary form in 2001 The human genome

was broken into pieces using various enzymes. Each piece

could be used to construct a YAC. Then, sufficient copies of

each piece of the human genome could be generated so that

automatic sequencing machines would have enough material

to sequence the DNA.

Commonly, the cutting enzymes are selected so that the

fragments of DNA that are generated contain overlapping

regions. Once the sequences of all the DNA regions are

obtained the common overlapping regions allow the fragment

sequences to be chemically bonded so that the proper order

and the proper orientation is generated. For example, if no

overlapping regions were present, then one sequence could be

inserted backwards with respect to the orientation of its neigh-

bouring sequence.

See also Chromosomes, prokaryotic; Gene amplification;

Yeast genetics

YEAST, ECONOMIC USES AND BENEFITS•

seeECONOMIC USES AND BENEFITS OF MICROORGANISMS

YYeast geneticsEAST GENETICS

Yeastgenetics provides an excellent model for the study of the

genetics of growth in animal and plant cells. The yeast

Saccharomyces cerevisiaeis similar to animal cells (e.g., sim-

ilar length to the phases of its cell cycle, similarity of the chro-

mosomal structures called telomeres). Another yeast,

Saccharomyces pombeis rather more similar to plant cells

(e.g., similarities in their patterns of division, and in organiza-

tion of their genome).

As well as being a good model system to study the

mechanics of eukaryotic cells, yeast is well suited for genetic

studies. Yeasts are easy to work with in the laboratory. They

have a rapid growth cycle (1.5 to two hours), so that many

cycles can be studied in a day. Yeasts that are not a health

threat are available, so the researcher is usually not in danger

when handling the organisms. Yeasts exist that can be main-

tained with two copies of their genetic material (diploid state)

or one copy (haploid state). Haploid strains can be mated

together to produce a diploid that has genetic traits of both

“parents.” Finally, it is easy to introduce new DNAsequences

into the yeast.

Genetic studies of the yeast cell cycle, the cycle of

growth and reproduction, are particularly valuable. For exam-

ple, the origin of a variety of cancers is a malfunction in some

aspect of the cell cycle. Various strains of Saccharomyces

cerevisiaeand Saccharomyces pombeprovide useful models

of study because they are also defective in some part of their

cell division cycle. In particular, cell division cycle (cdc)

mutantsare detected when the point in the cell cycle is

reached where the particular protein coded for by the defective

geneis active. These points where the function of the protein

is critical have been dubbed the “execution points.” Mutations

that affect the cell division cycle tend to be clustered at two

points in the cycle. One point is at the end of a phase known

as G1. At the end of G1 a yeast cell becomes committed to the

manufacture of DNA in the next phase of the cell cycle (S

phase). The second cluster of mutations occurs at the begin-

ning of a phase called the M phase, where the yeast cell com-

mits to the separation of the chromosomal material in the

process of mitosis.

Lee Hartwell of the University of Washington at Seattle

spearheaded the analysis of the various cdc mutants in the

1960s and 1970s. His detailed examination of the blockage of

the cell cycle at certain points—and the consequences of the

blocks on later events—demonstrated, for example, that the

manufacture of DNA was an absolute prerequisite for division

of the nuclear material. In contrast the formation of the bud

structures by Saccharomyces pombecan occur even when

DNA replication is blocked.

Hartwell also demonstrated that the cell cycle depends

on the completion of a step that was termed “start.” This step

is now known to be a central control point, where the cell

essentially senses materials available to determine whether the

growth rate of the cell will be sufficient to accumulate enough

material to permit cell division to occur. Depending on the

information, a yeast cell either commits to another cycle of

cell growth and division or does not. These events have been

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