Mutations
What Are Mutations?
• Changes in the nucleotide
sequence of DNA
• May occur in somatic cells
(aren’t passed to
offspring, only to
descendant cells)
• May occur in germ-line
cells (eggs & sperm) and
be passed to offspring
Are Mutations Helpful or
Harmful?
• Mutations happen
regularly
• Almost all mutations are
neutral
• Chemicals & UV
radiation cause
mutations
• Many mutations are
repaired by enzymes
Types of Mutations
Point Mutations
• Change in the
nucleotide sequence
of a one or a few
base pairs
• May be due to
copying errors,
chemicals, viruses,
etc.
Types of Point Mutations
• Include:
– Substitutions
– Frame-shift
Mutations
(Insertion,
Deletion)
Substitution
• One or more
nucleotides are
changed to a
different base.
• AAC GCC AGC
• AAG GCC AGC
• Can cause an amino
acid to be
changed…or not!
Frame-shift Mutation
• Inserting or deleting
one or more
nucleotides
• Changes the “reading
frame” like changing a
sentence
• Proteins built
incorrectly
Frameshift Mutation
• Original:
– The fat cat ate the wee
rat.
• Frame Shift (“a” added):
– The fat caa tet hew
eer at.
Amino Acid Sequence
Changed
Gene Mutation
Animation
How Would the Phenotype Change?
Missense Substitution
• Sickle Cell
disease is the
result of one
nucleotide
substitution
• Occurs in the
hemoglobin gene
Nonsense Substitution
• Thallasemia is the
result of a stop
codon substitution
that shortens the
gene
• Occurs in the
hemoglobin gene
Silent Mutation
• The Genetic Code is
repetitive – there are
64 codons that code
for 20 amino acids.
• A silent mutation
makes no change in
amino acid sequence:
Deletion
• Cystic fibrosis
(mucus in the
lungs) is the
result of one
nucleotide
deletion
• Causes a frameshift!
Insertion
• Huntington’s
Disease (nerve
cell deterioration)
is the result of
many nucleotide
insertions (CAG
repeats)
Chromosome Mutations
• May Involve:
– Changing the
structure of a
chromosome
– The loss or
gain of part of
a chromosome
Chromosome Mutations
• Five types exist:
– Deletion/Insertion
– Duplication
– Inversion
– Translocation
– Nondisjunction
Deletion
• Due to breakage
• A piece of a
chromosome is lost
Insertion
An additional
piece of
chromosome is
added
Deletion
• Cri-du-Chat is caused by a
deletion in one of the
chromosomes.
• Causes an abnormal larynx
Duplication
• Occurs when a
gene sequence is
repeated
Inversion
• Chromosome segment
breaks off
• Segment flips around
backwards
• Segment reattaches
Translocation
• Involves two
chromosomes that
aren’t homologous
• Part of one
chromosome is
transferred to
another chromosomes
Translocation
Nondisjunction
• Failure of chromosomes to
separate during meiosis
• Causes gamete to have too many
or too few chromosomes
• Disorders:
– Down Syndrome – three 21st
chromosomes
– Turner Syndrome – single X chromosome
– Klinefelter’s Syndrome – XXY
chromosomes
Chromosome Mutation
Animation
Normal Male
30
Normal Female
31
Male, Trisomy 21 (Down’s)
2n = 47 32
Female Down’s Syndrome
2n = 47 33
Turner’s Syndrome
2n = 45
34
Turner’s Syndrome
35
Klinefelter’s Syndrome
36
Klinefelter’s Syndrome
37