Download Mutations

Introduction
 A mutation is a change in
the normal DNA sequence.
 They are usually neutral,
having no effect on the
fitness of the organism.
 Sometimes, beneficial
mutations occur, which
cause the “mutants” to
better survive in their
current environment.
 This is the basis for the
theory of Evolution by
Natural Selection.
Introduction
 Mutations are usually
thought of as being
negative.
 They are responsible for
many disorders.
 Mutations are caused by
mutagenic agents, such
as X-Rays, UV radiation,
harsh chemicals, etc…
which affect/change the
nucleotides in DNA.
Point mutations
 A point mutation is a change in one base in a DNA
sequence.
 Usually occur due to an uncorrected mistake in DNA
replication.
 Often called a substitution mutation because one
base pair is switched for another.
Point mutations
 Transition point
mutations exchange a
purine for a purine (i.e.
AG) or a pyrimidime for
a pyrimidime (i.e. CT).
 Transversion point
mutations exchange a
purine for a pyrimidime or
vice versa (i.e. AC)
Point mutations
 A point mutation is not a
problem if it occurs in an
intron section of DNA.
 However a point mutation
in an exon region of DNA
may have issues, since
these areas of DNA code
for protein.
 When a point mutation in
an exon occurs, it falls into
one of three categories
depending on how it
affects the reading frame:
Point mutations
1) Silent Mutation: the
change codes for the
same amino acid
Point mutations
2) Missense Mutation: the
change codes for a
different amino acid
Point mutations
3) Nonsense Mutation:
the change creates a
stop codon.
Frameshift Mutations
 A frameshift mutation is
a mutation that affects
how the coding region
for a protein is read, and
therefore affects the
production and structure
of that protein.
 This type of mutation is
caused by insertions or
deletions.
Insertions
 An insertion is the addition of nucleotides into the DNA.
 Can be caused by transposable elements (“jumping
genes” as coined by Barbara McClintock---Nobel Prize
1983) which are pieces of DNA that often change their
place in the genome throughout the life of the organism.
 Can be reversed by exonucleases.
Deletions
 A deletion is the removal of nucleotides from the DNA
sequence.
 These are rarely reversible.
Chromosomal Mutations
 Mutations can occur on
the scale of a
chromosome and not
just a small sequence of
DNA on a chromosome.
 These mutations are very
harmful to the organism.
Chromosome Mutations
 Both deletions and
insertions can happen on
the scale of the
chromosome.
 This either removes the
genes in those sections
(deletions) or affects how
they are
transcribed/translated
(insertions).
 Sometimes the inserted
portion comes from
another chromosome.
Chromosome Mutations
 Chromosomes can also
be duplicated, making
multiple copies of the
same chromosome OR
multiple portions of the
same chromosome.
 If this occurred in an egg
or sperm cell, disorders
like Down Syndrome
(Trisomy 21) can occur in
the offspring.
Chromosome Mutations
 A translocation is the
trading of chromosomal
segments between two
different chromosomes.
 They are usually not the
same size segment being
traded.
 Some forms of cancer are
caused by translocations
(i.e. leukemia).
Chromosome Mutations
 An inversion is the
reversal of a segment of
DNA in a chromosome.
 Since DNA is read in the
5’ to 3’ direction, this can
affect protein synthesis
as codons will be
affected.
Conclusion
 Mutations are usually neutral and have no effect on the
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organism.
However, when mutations are negative, they can have lethal
effects.
Point mutations affect one nucleotide and can be silent (no
harm), missense (potentially bad) or nonsense (really bad).
Frameshift mutations are caused by insertions or deletions of
nucleotides and affect the size of the protein which is always bad.
Chromosomes can have deletions, insertions, duplications,
translocations and inversions… all of which affect protein
production and can lead to genetic disorders.
Stay away from mutagenic agents as much as possible if you want
to minimize your chances of accumulating mutations.