Key Terms

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Key Terms A screen is a search for mutants in
which each candidate is tested for the mutant
phenotype of interest. Selection describes the
use of particular conditions to allow the
preferential growth or survival of organisms with
a particular phenotype. Selection in the
laboratory is usually strong, while natural
selection is usually weak in the short term.
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• Key Terms
• A dominant allele determines the phenotype
  displayed in a
• heterozygote with another (recessive) allele.
• An allele is one of several alternative forms of
  a gene occupying a
• given locus on a chromosome.
• A recessive allele is obscured in the phenotype
  of a heterozygote by the dominant allele,
• often due to inactivity or absence of the product
  of the recessive allele.
• A complementation test determines whether two
  mutations are alleles of the same gene.
• It is accomplished by crossing two different
  recessive mutations that have the same
• phenotype and determining whether the wild-type
  phenotype can be produced.
• If so, the mutations are said to complement each
  other and are probably
• not mutations in the same gene.
• Interallelic complementation describes the
  change in the properties of a
• heteromultimeric protein brought about by the
  interaction of subunits coded
• by two different mutant alleles
• the mixed protein may be more or less active than
  the protein consisting of subunits
• only of one or the other type.

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• Key Terms
• A null mutant has a mutation that completely
  eliminates the function of a gene,
• usually because it has been physically deleted.
• The polymerase chain reaction is a technique in
  which cycles of denaturation,
• annealing with primer, and extension with DNA
  polymerase are used to amplify the number of
• copies of a target DNA sequence by large factors
  such as gt 106 fold.
• An open reading frame is a sequence of DNA
  consisting of triplets that
• can be translated into amino acids starting with
  an initiation codon and ending
• with a termination codon.
• A conditional-lethal mutation kills a cell or
  virus under certain (nonpermissive) conditions,
• but allows it to survive under other
  (permissive) conditions.
• A temperature-sensitive mutation creates a gene
  product (usually a protein)
• that functions at some low temperature but poorly
  or not at all at some high temperature.
• The converse is a cold-sensitive mutation.
• A cold-sensitive mutant is defective at low
  temperature
• but functional at normal temperature.
• The plasmid shuffle is a technique used in yeast
  genetics to screen for
• mutations in an essential gene.

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• Key Terms
• A parental ditype is a tetrad in which two
  genetic markers are segregating,
• and two spores have one parental genotype and two
  spores have the other parental genotype.
• A nonparental ditype is a tetrad in which two
  genetic markers are segregating,
• and two spores have one nonparental genotype and
  two spores have the
• other nonparental genotype.
• Chromatids are the copies of a chromosome
  produced by replication.
• The name is usually used to describe the copies
  in the period before
• they separate at the subsequent cell division.
• The four-strand stage of meiosis is the stage
  after DNA replication,
• prior to meiosis I, when the two pairs of sister
  chromatids are adjacent.
• A tetratype is a tetrad in which two genetic
  markers are segregating.
• Two of the four spores have parental genotypes
  and two have recombinant genotypes

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 Tests to see if a mutant is dominant or
recessive are done in diploids. The mutant is
mated by a wild-type strain. If the diploid has
a wild-type phenotype, the mutation is said to be
recessive. If the diploid has a mutant
phenotype, the mutation is said to be dominant.
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 Complementation tests can only be done on
recessive mutations. One mutant is mated by
another and the diploid is analyzed. If the
diploid has a mutant phenotype, the mutations
fail to complement and are likely in the same
gene. If the diploid has a wild-type phenotype,
the mutations complement and are likely in two
different genes.
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Three classes of plasmids are generally useful
for transformation of S. cerevisiae. These three
classes are known as Integrating, Centromer
ic, and High-copy-number or 2-micron.

• An origin of replication for propagation in E.
  coli
• A selectable marker for transformation of E.
  coli, usually ampicillin resistance and
• A selectable marker for transformation of S.
  cerevisiae,
• usually a gene such as the URA3 that complements
  an auxotrophy.
• The URA3 encodes a protein required for
  pyrimidine biosynthesis.

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Suppressor analysis is a proven method to
identify interacting genes Missense mutations
change a single codon and so as to cause the
replacement of one amino acid by another in a
protein sequence. A high-copy-number
suppressor allows a mutation in one gene to be
suppressed by overexpression of a second gene
that is present on a high-copy-number plasmid.
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A true revertant will be completely linked to the
initial mutation, producing all PD tetrads. An
extragenic suppressor mutation will segregate
from the initial mutation, resulting in PD,
NPD, and TT tetrads.
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 Suppression between interacting proteins. In
the example shown, two proteins, A and B,
normally interact. In the a mutant, this
interaction is impaired and there is a mutant
phenotype. However, a suppressor mutation that
alters B restores the interaction with a,
resulting in a wild-type phenotype.
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In addition to the isolation of suppressor
mutations, a second genetic approach to identify
interacting genes is called a synthetic lethal
screen. This type of screen identifies pairs of
mutations in which both are required to cause
inviability. The rationale for identifying and
studying such genes is that if each of two
mutations impairs an aspect of an essential
process, the double mutant should have a more
severe phenotype than either of the single
mutants. This double-mutant lethality is called
synthetic lethality. Thus, a synthetic lethal
screen is the opposite of a suppressor screen,
in which the second mutation causes a wild-type
phenotype.
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 Two types of interactions are diagrammed. In one
possibility, two genes, TUB1 and TUB3, each
encodes a version of the essential protein
alpha-tubulin, with TUB1 providing most of the
product. Some point mutations in TUB1 that
destabilize the interactions ofresulting protein
are not lethal. However, in combination with a
tub3 null allele there is not enough functional
alpha-tubulin and the result is lethality.
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Several proteins interact in a complex that
carries out an essential function. A mutation in
X, Y, or Z reduces the activity of this
complex, but still allows enough for
viability. However, a double mutant that impairs
two proteins reduces the activity below a
threshold required for viability, resulting in
synthetic lethality.
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