Download Genes and mutations

Round and wrinkled peas
Fig. A, page 22
1
Mutations: Primary tools of genetic
analysis

Mutations are heritable changes in base
sequences that modify the information
content of DNA
Forward mutation – changes wild-type to
different allele
 Reverse mutation – causes novel mutation to
revert back to wild-type (reversion)

2
General observations of mutation
rates

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Mutations affecting phenotype occur very
rarely
Different genes mutate at different rates
Rate of forward mutation is almost always
higher than rate of reverse mutation
3
Mutations that cause a phenotypic
change are very rare
Fig. 7.3
4
Classification of mutations by effect
on DNA molecule
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Substitution – base is replaced by one of the other
three bases
Deletion – block of one or more DNA pairs is lost
Insertion – block of one or more DNA pairs is
added
Inversion – 180 degree rotation of piece of DNA
Reciprocal translocation – parts of
nonhomologous chromosomes change places
Chromosomal rearrangements – affect many
genes at one time
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Fig. 7.2
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Unequal crossing over creates one
homologous chromosome with a duplication
and the other with a deletion
7.10 a
7
Transposable elements move around the genome
and are not susceptible to excision or mismatch
repair
Fig. 7.10 e
8
Trinucleotide instability causes
mutations


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FMR-1 genes in
unaffected people
have fewer than
50 CGG repeats.
Unstable
premutation
alleles have
between 50 and
200 repeats.
Disease causing
alleles have > 200
CGG repeats.
Fig. B(1) Genetics and Society
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Trinucleotide repeat in people with
fragile X syndrome
Fig. A, B(2) Genetics and
Society
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Are mutations spontaneous or
induced?

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Most mutations are spontaneous.
Luria and Delbruck fluctuation experiments and replica
plating - simple ways to tell if mutations are spontaneous
or if they are induced by a mutagenic agent
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Fig. 7.4
12
Interpretation of Luria-Delbruck
fluctuation experiments
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Bacterial resistance arises from mutations
that exist before exposure to bacteriocide
The bacteriocide is a selective agent killing
the nonresistant cells, allowing only the
preexisting mutant cells to survive.
Mutations do not arise as a direct response
to environmental change
Mutations occur randomly at any time
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Replica plating verifies preexisting
mutations
Fig. 7.5 a
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Fig. 7.5b
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Chemical and physical agents cause
mutations

Hydrolysis of a purine
base, A or G occurs 1000
times an hour in every cell

Deamination removes –
NH2 group. Can change
C to U, inducing a
substitution to an A-T
base pair after replication
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

X rays break the
DNA backbone
UV light produces
thymine dimers
Fig. 7.6 c, d
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Oxidation from free radicals formed by irradiation
damages individual bases
Fig. 7.6 e
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Repair enzymes fix errors created by
mutation
Excision repair
enzymes
release
damaged
regions of
DNA. Repair
is then
completed by
DNA
polymerase
and DNA ligase
Fig. 7.7a
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DNA polymerase proofreading
Fig. 7.8
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Mutagens increase mutation rate
using different mechanisms
Fig. 7.12a
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Fig. 7.12 b
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Fig. 7.12 c
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Consequences of mutations

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Germ line mutations – affect the evolution of
species
Somatic mutations – affect the survival of an
individual
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Cell cycle mutations may lead to cancer
Conditional mutations – produce changes in
phenotype under one set of conditions but not
under another

Conditional lethal mutations
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The Ames test
for carcinogens
using hismutants of
Salmonella
typhimurium
Fig. 7.13
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What mutations tell us about
gene structure
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Complementation testing - are two
mutations in the same or different genes?
Benzer’s experiments - genes are linear
sequences of nucleotide pairs
Some regions of chromosomes mutate at a
higher rate than others – hot spots
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Complementation testing
Fig. 7.15 a
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Fig. 7.15 b,c
Five complementation groups (different genes) for eye color.
Recombination mapping demonstrates distance between genes and alleles.
30
Recombination within a gene can
generate wild-type
Fig. 7.16
31
A gene is a linear sequence of
nucleotide pairs

Seymour Benzer mid 1950s – 1960s
If a gene is a linear set of nucleotides,
recombination between homologous
chromosomes carrying different mutations
within the same gene should generate wild-type
 T4 phage as an experimental system

Can examine a large number of progeny to detect
rare mutation events
 Could allow only recombinant phage to proliferate
while parental phages died
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Benzer’s experimental procedure
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Generated 1612 spontaneous point mutations and
some deletions
Mapped location of deletions relative to one
another using recombination
Found approximate location of individual point
mutations by deletion mapping
Performed recombination tests between all point
mutations known to lie in the same small region of
the chromosome
Result – fine structure map of the rII gene locus
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Working with T4 phage
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Phenotypic properties of T4 phage
Fig. 7.17 b
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Complementation test for mutations
in different genes
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Detecting recombination between
two mutations in the same gene
Fig. 7.17 d
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Deletions for rapid mapping of point
mutations to a region of the chromosome
Fig. 7.18 a
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Recombination
mapping to identify
the location of each
point mutation
within a small
region
Fig. 7.18 b
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Fine structure map of rII gene
region
Fig. 7.18 c
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What mutations tell us about
gene function

One gene, one enzyme hypothesis - a gene contains
the information for producing a specific enzyme

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Beadle and Tatum used auxotrophic and prototrophic
strains of Neurospora to test hypothesis
Genes specify the identity and order of amino
acids in a polypeptide chain
The sequence of amino acids in a protein
determines its three-dimensional shape and
function
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Beadle and Tatum – One gene, one
enzyme
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1940s – isolated mutants that disrupted synthesis
of arginine
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Auxotroph – needs supplement to grow on minimal
media
Prototroph –needs no supplement
Recombination analysis - mutations in four
distinct regions of genome
Complementation tests - each of four regions
correlated with a different complementation group
(4 different genes)
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Fig. 7.20 a
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Fig. 7.20 b
44
Interpretation of Beadle and Tatum
experiments

Each gene controls the synthesis of an
enzyme involved in catalyzing the
conversion of an intermediate into arginine
45
Some proteins are multimeric, containing subunits
composed of more than one polypeptide
Fig. 7.24
46
How do genotypes and phenotypes
correlate?
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Alteration of amino acid composition of a
protein
Alteration of the amount of normal protein
produced
Changes in different amino acids at
different positions have different effects
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Summary of dominance
relationships
Fig. 3.2
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Codominant blood group alleles
Fig. 3.4b
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Dominant or recessive alleles depend on the
relationship between protein function and
phenotype
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Recessive – alleles that produce nonfunctional proteins (loss-of
function)
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Null mutations – no synthesis of protein or promote synthesis of protein
incapable of carrying out any function
Hypomorphic mutations – produce much less protein or a protein with
weak function
Incomplete dominance – phenotype varies in proportion to
amount of protein
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Recessive mutations
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Incomplete dominance
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Dominant or recessive alleles depend on the
relationship between protein function and
phenotype
– reflect several different occurrences (generally gainof-function)
Dominant
mutations – produces more protein or same amount of a
more effective protein
Haploinsufficiency –one allele does not provide enough protein
Dominant negative – mutant protein negatively affects the normal
product
Neomorphic mutations – generate a novel phenotype
Hypermorphic
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Dominant mutations
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