Ch. 12.4 Mutations
Section Objectives:
•Categorize the different kinds of
mutations that can occur in DNA.
•Compare the effects of different
kinds of mutations on cells and
organisms.
Mutations
• Any change in DNA sequence is called a mutation.
– Mutations can be caused by errors in replication,
transcription, cell division, or by external agents.
• If mutation occurs in gametes (sex cells) it will
be passed on to offspring
– A mutation may produceno change, a new trait, or it may
result in a protein that does not work correctly.
– If the mutation results in a protein that is
nonfunctional, and the embryo may not survive.
– In some rare cases a gene mutation may have
positive effects.
Mutations
• If mutation takes place in a somatic cell, it is
not passed on to organism’s offspring
– Damage to a gene may impair the function of the cell
– When that cell divides, the new cells also will have the
same mutation
– Some mutations of DNA in body cells affect genes that
control cell division.
– This can result in the cells growing and dividing rapidly,
producing cancer.
Characteristics of Mutations
• One of the main sources of genetic variation.
– Unique traits are thought to originate through mutations
that are passed on
• Occur at random
• sometimes a mistake in base pairing during DNA
replication.
• many mutations are caused by factors in the
environment
• Can be classified as either:
– Gene Mutations
– Chromosomal Mutations
Gene Mutations
Changes that affect the nucleotide sequence of a
gene
–
–
–
–
change the DNA
changes the mRNA
may change protein
may change trait
DNA
TACGCACATTTACGTACG
mRNA
AUGCGUGUAAAUGCAUGC
protein
aa aa aa aa aa aa aa
trait
Types of gene mutations
• Changes to the letters (A,C,T,G bases) in the
DNA
– point (or substitution) mutation
• change to ONE letter (base) in the DNA
• may cause change to protein, may not
– frameshift mutation
• addition of a new letter (base) in the DNA sequence
• deletion of a letter (base) in the DNA
• both of these shift the DNA so it changes how the
codons are read
• big changes to protein!
Point Mutations
• One base change
– can change the meaning of the whole protein
THEFATCATANDTHEREDRATRAN
THEFATCARANDTHEREDRATRAN
OR
THEFATCATENDTHEREDRATRAN
Point Mutations
• Substitution mutation = may change amino
acid
AUGCGUGUAUACGCAUGCGAGUGA
MetArgValTyrAlaCysGluStop
AUGCGUGUAUACGUAUGCGAGUGA
MetArgValTyrValCysGluStop
Sickle cell anemia- a single substitution
• Hemoglobin protein in red blood cells
– strikes 1 out of 400 African Americans
– limits activity, painful & may die young
Sickle Cell Anemia- substitution
Does convey resistance to malarial infections when heterozygous
Point Mutations
• Substitution mutation = may be no change to
protein
AUGCGUGUAUACGCAUGCGAGUGA
MetArgValTyrAlaCysGluStop
AUGCGUGUAUACGCUUGCGAGUGA
MetArgValTyrAlaCysGluStop
Point Mutations
• Substitution mutation = may change to STOP
AUGCGUGUAUACGCAUGCGAGUGA
MetArgValTyrAlaCysGluStop
AUGCGUGUAUAAGCAUGCGAGUGA
MetArgValStop
Frameshift Mutations
• Add or delete one or more bases
– changes the meaning of the whole protein as it
changes how the mRNA codons are read. This change
causes all remaining codons to be incorrectly grouped.
The change in “reading frame” causes all resulting
proteins to be made improperly.
THEFATCATANDTHEREDRATRAN
THEFATCANTANDTHEREDRATRAN
OR
THEFATCAANDTHEREDRATRAN
Frameshift Mutations
• Insertion = add one or more basesAUGCGUGUAUACGCAUGCGAGUGA
MetArgValTyrAlaCysGluStop
AUGCGUGUAUACGUCAUGCGAGUGA
MetArgValTyrValMetArgValA
Frameshift Mutations
• Deletion = lose one or more bases
AUGCGUGUAUACGCAUGCGAGUGA
MetArgValTyrAlaCysGluStop
AUGCGUGUAUACGAUGCGAGUGA
MetArgValTyrAspAlaSerGA
Chromosome mutations
• Chromosome mutations- changes in the
structure of a chromosome or the loss of a
chromosome. Often occur during cell division.
– Deletion- loss of a piece of a chromosome due to
breakage. Information carried by the missing piece
is lost.
– Inversion- a chromosomal segment breaks off and
reattaches in the reverse orientation on the same
chromosome.
Chromosome mutations cont.
– Translocation- a chromosomal segment breaks off
and attaches to another, non-homologous
chromosome.
– Nondisjunction- results from the failure of a
chromosome to separate from its homologue
during meiosis. During nondisjunction, one
gamete receives an extra copy, and the other
gamete is missing the chromosome entirely.
Patterns of Inheritance
• Nondisjunction- failure of the chromosomes
to separate correctly during cell division.
– Monosomy- and individual is missing one of a pair
of particular chromosomes (the total number
would be 45 for humans)
• Turner syndrome- individual only has 1 X chromosome;
results in a female with immature physical
development, sterility, and a webbed neck.
Chromosomes Don’t Separate Properly
during Meiosis
Nondisjunction Also Can Change the
Number of Sex Chromosomes
• Turner syndrome
– Missing an X chromosome; XO
– Female; webbed neck; no secondary
sexual traits at puberty; sterile; may
age prematurely
• XXX females
– Develop normally
Turner Syndrome
Nondisjunctions cont.
• Trisomy- an individual has 3 of a particular
chromosome. (total number would be 47 for
humans)
– Down syndrome- extra chromosome at number
21- individual has mental deficiencies, folded skin
above the eyes, weak muscles
– Kleinfelter’s syndrome- genotype XXY male- has
mental retardation, and low fertility.
In Down Syndrome There Are Three
Copies of Chromosome 21
Nondisjunction Also Can Change the
Number of Sex Chromosomes
• Klinefelter syndrome (XXY)
– Low fertility; mental retardation; small testes;
sparse body hair; enlarged breasts
– Testosterone injections may reverse the
phenotype
• XYY condition
– Taller
– Normal male phenotype
Kleinfelter’s Syndrome
Induced Mutations
• Mutagen- environmental factor that damages
DNA.
–
–
–
–
–
–
–
–
Sunlight (UV light) causing skin cancer.
Radiation causing infertility or cancer
X-rays
Chemicals like pesticides and herbicides
Nuclear Radiation
Asbestos
Formaldehyde
Pathogens- frogs with extra legs