Lesson 12: Mutations
Point Mutations:
 A small-scale change in the nitrogenous
base sequence of DNA. A point mutation
is a failure by the replicating cell to copy
the genetic information accurately. Point
mutations may be beneficial, harmful, or
neutral (having no effect on the
organism). There are three major point
mutations.
1) Base Pair Substitution:
 In a base-pair substitution, one
nitrogenous base is accidentally replaced
with a different base.
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2) Insertions:
 In an insertion, one or more nitrogenous
bases are inserted during the copying
process.
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3) Deletions:
 In a deletion, one or more nitrogenous
bases are deleted during the copying
process.
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Chromosome Mutations:
An error that involves an entire chromosome
or a large part of a chromosome are known
as chromosome mutations. An example of a
chromosome mutation is non-disjunction.
Non-disjunction occurs when sister
chromatids fail to separate during meiosis
which results in the production of gametes
that contain too many or too few
chromosomes.
Non-disjunction leading to chromosomal
mutations
Inheritance of Mutations:
 Most mutations occur in parts of the DNA
that are 'non-coding'. Many mutations can
be corrected as the replication process
continues, but as an organism gets older,
the rate of mutations becomes more
frequent potentially leading to cancer.
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Inheritance of Mutations:
Mutations that occur in somatic cells (body
cells) will not get passed on to offspring, but
mutations in the gametes (sex cells) do.
Mutations that are dominant (such as
Huntington's disease) will show up in the first
generation of offspring, but mutations that
are recessive may take several generations
to show up phenotypically, and that too, only
if by chance two individuals with the same
mutation produce offspring.
Sickle Cell Anemia:
 Individuals with sickle cell anemia have
inherited a mutated gene in which a single
adenine base is substituted for a thymine.
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Are all mutation harmful?
Lactose Tolerance and Lactose
Intolerance:
 About 75% of the adults worldwide are
lactose intolerant. Those adults that can
tolerate lactose can do so because of a
genetic mutation. Individuals who have
inherited the mutated gene for lactose
tolerance continue to produce the
enzymes that breakdown lactose.
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Are all mutations harmful?
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Spontaneous or Induced Mutations:
Spontaneous mutations occur naturally and
randomly whereas induced mutations are caused by
external, environmental factors. Induced mutations
are a result of chemical or physical agents such as
radiation or cigarette smoke.
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"Jumping Genes" or Transposons:
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Point mutations are mutations that occur within a
gene in fixed locations. Barbara McClintock's
work in genetics back in the 1940s, showed that
genes do not always remain in fixed locations.
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Since each kernel is an individual corn plant
embryo, one cob produces a large sample of
offspring; making it an ideal species for genetic
research.
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http://www.youtube.com/watch?v=91vRFKBMT4
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McClintock's research showed that an
organism's genome is not static, but
rather there are segments of DNA that
can move as a unit from one location to
another. These moveable segments are
called transposons (or 'jumping genes').
Transposons are specific segments of DNA
that can move along or between the
chromosomes through the process of
transposition.
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McClintock learned that these transposons can
affect the colour of the corn kernels. If a
transposon is inserted into the gene for purple
kernels, the gene is disrupted and purple
pigment cannot be produced. Instead, the
resulting kernel is white.
Hemophilia and Transposons:
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Hemophilia is an inherited X-linked
disease that is caused by genetic
mutation of the blood-clotting factor gene.
In rare cases, hemophilia can also be
caused by transposons. If a transposon
inserts itself into a normal blood factor
VIII gene, then the individual (if male)
will be affected by hemophilia.
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Microarray Technology
Microarray technology simplifies the search
for disease causing genes. A microarray
consists of a small membrane or glass slide
that contains samples of hundreds, or even
thousands of DNA fragments arranged in a
regular pattern. Each fragment corresponds
to a particular gene. Samples with genetic
material to be tested are spread over the
microarray chip and interact with these gene
fragments.
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http://www.dnalc.org/resources/3d/26microarray.html
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http://www.youtube.com/watch?v=VNsTh
MNjKhM