Download Mutation Notes

MUTATIONS
How mistakes are made…
Mutations
Mutations are defined as “a sudden
genetic change in the DNA sequence
that affects genetic information”.
They can occur at the molecular level
(genes) and change a single gene, or at
the chromosome level and affect many
genes.
Things that can cause mutations are
called “mutagens”.
Known mutagens are ultraviolet light,
cigarette smoking, some viruses, certain
chemicals like PCB’s.
Mutagens often result in cancer.
Inheritability of Mutations
It depends on where it occurs
i. Germ mutations – occur in
gametes. Inheritable
(colorblindness, hemophilia)
ii. Somatic mutations – affect body
cell, not inheritable (CAN
RESULT IN cancer)
Effects of Mutations
Not all mutations have effects.
Silent mutations - have no effect on the
expression of the gene.
Causes for this type of mutation:
 It is in a non-coding region
 It does not change the amino acid sequence
 The change does not affect the folding of the
protein
Types of Mutations
a) Point Mutations – effects a single gene
i. Substitution
ii. Frameshift
a. deletion
b. insertions
b) Chromosomal Mutations – most drastic,
change in structure or # of chromosomes
(affects many genes)
III. Point Mutations
a)
Substitution – one base exchanges for another,
affects 1 amino acid
(Ex. GCA-TCA  GCT-TCA
The Effects of Point Mutations
►A point mutation is a change in a single base pair in DNA.
 A change in a single nitrogenous base can change the entire
structure of a protein because a change in a single amino acid
can affect the shape of the protein.
Normal
mRNA
Protein
Replace G with A
Point
mutation
Stop
mRNA
Protein
Stop
Effects of substitution mutations
Missense mutations – causes a change in
the amino acid coded for
Nonsense mutations – causes a stop codon
to occur prematurely
Sickle cell anemia
Tay-Sachs
Frameshift Mutations
►What would happen if a single base were lost from a
DNA strand?
►A mutation in which a single base is added or deleted
from DNA is called a frameshift mutation because it shifts
the reading of codons by one base.
 As a result, every codon after the deleted base would
be different.
Deletion of U
mRNA
Protein
Frameshift – affects several amino
acids
-Insertion – 1 base is inserted,
affects several amino acids
Ex. (GCA-TCA  GCA-GTC-A
-Deletion – base is removed,
affects several amino acids
Ex. (GCA-TCA  GCT-CA
X
Cystic fibrosis
Chromosome mutations
Nondisjunction – incomplete
chromosome division during
meiosis; results in gametes with too
many or too few chromosomes.
Ex: Down syndrome is caused by an
extra #21 chromosome
Polyploidy
Aneuploidy
Down’s Syndrome
•Trisomy 21
•1 in 700 births
•Mental retardation
•Vision problems
•Heart problems
Likelihood of chromosomal mutations
1 in 1700 for mothers under 20.
1 in 1400 for mothers between 20
and 30.
1 in 750 for mothers between 30
and 35.
1 in 16 for mothers over 45.
Sex chromosome disorders –
occurs as a result of
nondisjunction of the sex
chromosomes
Ex: Turner’s syndrome – X-;
Klinefelter’s syndrome – XXY.
Klinefelter’s Syndrome
•XXY
•1 in 1,000
•Usually sterile because of low
sperm count
•Tall, sparse body hair
•Suffer from gynecomastia- male
breast tissue
•Testosterone treatments
Turner’s Syndrome
•XO genotype—Monosomy X
•1 in 2,500 births
•Short, sterile
•75% result in non-disjunction
from the father
XYY-Jacob’s Syndrome a.k.a. “Super
Males”
• 1 in 1,000 men
•Normal appearance,
very tall
•Low IQ, prone to
violence
Patau’s Syndrome
•1 out of 6,000 births
•Trisomy 13
•80-90% do not survive past 1 yr old
Edward’s Syndrome
•Trisomy 18
•Second most common
trisomy after down’s
syndrome
•Only 5% live to age 1
•1 out of 8,000 births
•Severely retarded, many
die from malformed heart
•Polydactyly or syndactyly
Other Chromosomal Mutations
Affect many different genes
Caused by errors in meiosis or
environmental disturbances.
Translocation – occurs when a piece
of one chromosome breaks off and
attaches to a nonhomologous
chromosome
Inversion – occurs when a piece of
one chromosome breaks off, flips,
and reattaches to the same
chromosome
Deletion – occurs when a piece
of a chromosome breaks off and
is lost
Duplication – occurs when a
segment of a chromosome is
repeated
Cri du Chat
“Cry of the Cat”
•1 in 50,000 births
•A piece of chromosome 5
is missing
•Severe mental retardation,
nonverbal
•Does not run in families