CHAPTER 14: Genes in Action
Essential Questions:
1.What is a Mutation?
2. What Causes Mutation?
3. What is the origin of genetic differences among organisms?
4. What types of mutations are possible?
5. How can genetic change occur on a larger scale?
6. How will Modern Technology in DNA affect us?
Gene Mutations
1. A Mutation is a change in the sequence of DNA
bases within a gene
2. What Causes Mutations:
• Mutations can be spontaneous or caused by
environmental influences called mutagens.
• Mutagens include radiation (X-rays, UV
radiation), and organic chemicals (in
cigarette smoke and pesticides).
Origins of Genetic Difference
3. MUTATION is the origin differences among
organisms
For the most part, genetic differences among
organisms originate as some kind of mutation.
A genetic mutant is an individual whose DNA or
chromosomes differ from some previous or normal
state.
Every unique allele of every gene began as a
mutation of an existing gene.
In genetics, a mutation is a change in the structure
or amount of the protein produced
Effects of Mutations
–Because of the way DNA is translated, a mutation
can have many possible effects.
–A mutation may have no effect, or may harm or
help in some way.
–Mutations are noticed when they cause an unusual
trait or disease, such as sickle cell anemia.
–Many mutations may go unnoticed.
Mutations
• Changes to DNA are called mutations
– 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 mutations
4. Changes to the letters (A,C,T,G bases) in the
DNA can result in 2 types of mutation
– Point 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 shift the DNA so it changes how the codons are
read
• BIG changes to protein
Types of Point Mutations
1. Silent mutation - the change in the codon results in
the same amino acid
Ex: UAU  UAC both code for tyrosine
2. Nonsense mutation - a codon is changed to a stop
codon; resulting protein may be too short to function
Ex: UAC  UAG (a stop codon)
3. Missense mutation - involves the substitution of a
different amino acid, the result may be a protein that
cannot reach its final shape
Ex: Hbs which causes sickle-cell disease
Sickle cell anemia
• Hemoglobin protein in red blood cells
– strikes 1 out of 400 African Americans
– limits activity, painful & may die young
Normal
round cells
Misshapen
sickle cells
Only 1 out of
146 amino acids
Point Mutations
• One base change
– can change the meaning of the whole protein
THEFATCATANDTHEREDRATRAN
THEFATCARANDTHEREDRATRAN
OR
THEFATCATENDTHEREDRATRAN
Does this change
the sentence?
A LITTLE!
Point Mutations
• Silent mutation = no change to protein
AUGCGUGUAUACGCAUGCGAGUGA
MetArgValTyrAlaCysGluStop
AUGCGUGUAUACGCUUGCGAGUGA
MetArgValTyrAlaCysGluStop
The code
Does
this has
change
the protein?
repeats
in it!
Why not?
Mutations as Changes in Results of
Genes
–Changes in a DNA sequence may affect
the results of genes in many ways.
–A mutation is silent when it has no effect on a gene’s
function.
–Point mutations are often silent because the genetic code is
redundant (each amino acid has multiple codons).
–A missense or replacement mutation results when a codon is
changed such that the new codon codes for a different amino
acid.
Point Mutations
• Missense mutation = changes amino acid
AUGCGUGUAUACGCAUGCGAGUGA
MetArgValTyrAlaCysGluStop
AUGCGUGUAUACGUAUGCGAGUGA
MetArgValTyrValCysGluStop
Does
this change
the protein?
DEPENDS…
Point Mutations
• Nonsense mutation = change to STOP
AUGCGUGUAUACGCAUGCGAGUGA
MetArgValTyrAlaCysGluStop
Really destroyed
that protein!
AUGCGUGUAUAAGCAUGCGAGUGA
MetArgValStop
Frameshift Mutations
The reading frame of a sequence depends on the
starting point for reading. An insertion or deletion
can shift the reading frame, or cause a frameshift.
In frameshift mutations, the remaining sequence
may be “read” as different codons.
An insertion or deletion of many codons is likely to
disrupt the resulting protein’s structure and function.
Frameshift Mutations
• Add or delete one or more bases
– changes the meaning of the whole protein
THEFATCATANDTHEREDRATRAN
Delete
Add one!
one!
Does this change
the sentence?
A LOT!
THEFATCANTANDTHEREDRATRAN
OR
THEFATCAANDTHEREDRATRAN
Frameshift Mutations
• Addition = add one or more bases
AUGCGUGUAUACGCAUGCGAGUGA
MetArgValTyrAlaCysGluStop
AUGCGUGUAUACGUCAUGCGAGUGA
MetArgValTyrValMetArgValA
Does this change
the protein?
A LOT!
Frameshift Mutations
• Deletion = lose one or more bases
AUGCGUGUAUACGCAUGCGAGUGA
MetArgValTyrAlaCysGluStop
AUGCGUGUAUACGAUGCGAGUGA
MetArgValTyrAspAlaSerGA
Does this change
the protein?
A LOT!
Deletion Mutation:Cystic Fibrosis
deletion
Loss of one
amino acid!
LARGE-SCALE GENETIC CHANGE
5. How can genetic change occur on a larger
scale?
Very large-scale genetic change can occur by
misplacement, recombination, or multiplication
of entire chromosomes.
Chromosomal Mutations
During meiosis, chromosomes pair up and may
undergo crossover. Usually, the result is an equal
exchange of alleles between homologous
chromosomes.
Errors in the exchange can cause chromosomal
mutations.
Changes in Chromosome Structure
Chromosome mutation - a change in chromosome
structure
4 Types of chromosomal mutations:
Inversion
Translocation
Deletion
Duplication
Chromosomal Deletion: Williams
Syndrome
• Deletions occur when a
single break causes a lost
end piece, or two breaks
result in a loss in the
interior.
• Children have a pixie look
(turned-up noses, wide mouth,
small chin, large ears)
• Poor academic skills, good
verbal and musical abilities.
Skin ages prematurely from
lack of the gene that controls
the production of elastin (also
affects cardiovascular health).
Chromosomal Duplication: Dup15
• Duplication results in a
chromosome segment being
repeated in the same
chromosome
• An inverted duplication in
chromosome 15 causes Dup
15 syndrome
– Poor muscle tone, mental
retardation, seizures,
curved spine, and autistic
characteristics
Chromosome Translocation :
Alagille Syndrome
• Translocation is exchange of
chromosomal segments between
two, non-homologous
chromosomes.
– Ex: Alagille syndrome results
from a deletion of chromosome
20 or a translocation that
disrupts an allele on
chromosome 20.
– Distinctive face, abnormalities
of eyes & internal organs, and
severe itching.
Chromosome Inversion
• Inversion involves a segment of a chromosome being turned
180 degrees
• The reverse sequence of alleles can alter gene activity.
• Crossing-over between inverted and normal chromosomes
can cause duplications and deletions
Changes in Chromosome Number
• Nondisjunction – occurs when:
– In meiosis I, homologous pair both go into the same
daughter cell or in meiosis II, the sister chromatids both go
into the same gamete.
• Example: Trisomy 21 Down syndrome is a genetic disorder
caused by the presence of all or part of a third copy of
chromosome 21. It is associated with growth delays, and mild
to moderate intellectual disability.
EFFECTS OF GENETIC CHANGE
•What are the possible effects of mutations?
The results of genetic change may be harmful, beneficial,
or neutral; most changes are neutral and may not be
passed on to offspring.
Most genetic changes will cause no change in the
appearance or function of organisms.
•Mutations that occur in gametes can be passed on to
offspring, but mutations in body
cells affect only the individual in which they occur.
Some Common Genetic Diseases
Huntington’s Disease: Autosomal dominant
Neuromuscular disease; degeneration of muscle, onset in early 30’s.
Cystic Fibrosis: Autosomal Recessive;
Defective Protein is made that creates excess mucus; clogs lungs.
Color Blindness: Sex-linked recessive
Inablity to distinguish colors (8% of male population)
Hemophilia: Sex-linked recessive
Inablity of blood to clot
Muscular Dystrophy: Sex-linked recessive
Loss of muscle that begins in early childhood
Tay-Sach’s Disease : Autosomal Recessive
Degeneration of Central Nervous System; infant mortality
DNA Technology and Modern
Medicine
• The Human Genome Project Decoded an entire
length of DNA in 2003
• The Genetic Sequencing allows for improvements
in the detection and treatment of genetic disease
and cancer.
• Gene Therapy is a technology in which good
copies of a gene are introduced into those
individuals affected by genetic disease