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Mutations
• Mutations are changes in DNA that may
or may not affect phenotype.
• Some mutations affect a single gene,
while others affect an entire
chromosome.
•
•
https://www.youtube.com/watch?v=5ia5zdfoou0
https://www.youtube.com/watch?v=6zMIl7x2WSY
Single Gene Mutations
• A substitution (point) mutation substitutes
one nucleotide for another.
– a single point in the sequence is changed
• For instance a C is changed to an A.
• Compare this to reading a sentence: Cats
eat big rat. --> point mutation --> Cats eat
big hat
– The sentence is still almost recognizable, the
protein changed in this case might still function.
Types and Effects of Substitution
Mutations
• Missense mutation: causes a single amino acid change
• Silent mutation: causes no amino acid change
• Nonsense mutation: causes amino acid chain to be
terminated early
mutated
base
Frameshift Mutations
• A frameshift mutation is a point mutation that
involves an addition or deletion
– results in a shift in the reading frame
– The reading frame is the way in which the
codons are read – start with AUG and go from
there
Frameshift Mutations
– Cats eat big rat --> add an additional letter A,
and the reading frame changes
– Aca tse atb igr at - this protein does not resemble
the original and will probably not be functional
Mutations may or may not affect
phenotype.
• Some gene mutations change phenotype.
– A mutation may cause a premature stop codon.
– A mutation may change protein shape or the active
site.
– A mutation may change gene regulation.
blockage
no blockage
• Some gene mutations do not affect
phenotype.
– A mutation may be silent.
– A mutation may occur in a noncoding region.
– A mutation may not affect protein folding or the
active site.
Chromosomal Mutations
• Chromosomal mutations affect many genes.
• Chromosomal mutations tend to have a bigger
effect than single gene mutations.
• Chromosomal mutations may occur during crossing
over
• Gene duplication or deletion results from
unequal crossing over between homologous
chromosomes.
• Deletion – a segment of a chromosome has been
deleted
• Duplication –
a segment of a
chromosome has
been copied and
added to the
chromosome
more than once
• Inversion – a segment of a chromosome has been
reversed
• Translocation results from the exchange of DNA
segments between nonhomologous chromosomes.
• Translocation– a segment of a chromosome has
been moved to a different chromosome
Mutation Effects
• Mutations in body cells do not affect
offspring.
• Mutations in sex cells can be harmful or
beneficial to offspring.
• Natural selection often removes mutant
alleles from a population when they are less
adaptive.
Mutations can be caused by several
factors.
• Replication errors can cause
mutations.
• Unequal crossing over leads
to chromosomal mutations.
• Mutagens, such as UV ray
and chemicals, can cause
mutations.
• Some cancer drugs use
mutagenic properties to kill
cancer cells.
•
https://www.youtube.com/watch?v=ShKOZBgd4gk
Red blood cell phenotypes and blood flow in normal
people and people with sickle cell anemia
RBC smear from normal person
Sickle-shaped blood
cells can occlude
(block) capillaries.
This leads to pain.
RBC smear from person with
sickle cell anemia
http://www.wadsworth.org/chemheme/heme/microscope
Hemoglobin Beta (HBB) gene
Gene Location:
Human chromosome 11p15.5
*Chromosome 11 is an autosome
Gene Structure:
Three exons spanning 1,606 genomic base
pairs
Polypeptide Size: 147 amino acids
http://www.ornl.gov/sci/techresources/Human_Genome/posters/chromosome/hbb.shtml
What is Hemoglobin?
• Hemoglobin is a protein found in red blood cells
• Hemoglobin is made up of 4 polypeptide subunits
(2 alpha, 2 beta)- tetramer
– HBB codes for β-globin (aka hemoglobin beta)
– Hemoglobin carries oxygen which is needed in body
tissues
Are all mutations harmful?
• Missense mutation: causes a single amino
acid change
• Silent mutation: causes no amino acid
change
• Nonsense mutation: causes amino acid
chain to be terminated early
Hb A ALLELE (normal allele)
Coding
Template
5’-…CAT CTG ACT CCT GAG GAG AAG TCT GCC GTT…-3’
3’-…GTA GAC TGA GGA CTC CTC TTC AGA CGG CAA…-5’
TRANSCRIPTION
mRNA
5’-…CAU CUG ACU CCU GAG GAG AAG UCU GCC GUU…-3’
TRANSLATION
Protein
N-…His Leu Thr Pro Glu Glu Lys Ser Ala Val…-C
Hb S ALLELE (mutant allele)
Coding
Template
5’-…CAT CTG ACT CCT GTG GAG AAG TCT GCC GTT…-3’
3’-…GTA GAC TGA GGA CAC CTC TTC AGA CGG CAA…-5’
TRANSCRIPTION
mRNA
5’-…CAU CUG ACU CCU GUG GAG AAG UCU GCC GUU…-3’
TRANSLATION
Protein
N-…His Leu Thr Pro
?
Va
Glu Lys Ser Ala Val…-C
(Missense mutat
Hb A ALLELE (normal allele)
Coding
Template
5’-…CAT CTG ACT CCT GAG GAG AAG TCT GCC GTT…-3’
3’-…GTA GAC TGA GGA CTC CTC TTC AGA CGG CAA…-5’
TRANSCRIPTION
mRNA
5’-…CAU CUG ACU CCU GAG GAG AAG UCU GCC GUU…-3’
TRANSLATION
Protein
N-…His Leu Thr Pro Glu Glu Lys Ser Ala Val…-C
New mutant allele…
Coding
Template
5’-…CAT CTG ACT CCT GAA GAG AAG TCT GCC GTT…-3’
3’-…GTA GAC TGA GGA CTT CTC TTC AGA CGG CAA…-5’
TRANSCRIPTION
mRNA
5’-…CAU CUG ACU CCU GAA GAG AAG UCU GCC GUU…-3’
TRANSLATION
Protein
N-…His Leu Thr Pro
?
Glu Lys Ser Ala Val…-C
Glu
(Silent mutati
Hb A ALLELE (normal allele)
Coding
Template
5’-…CAT CTG ACT CCT GAG GAG AAG TCT GCC GTT…-3’
3’-…GTA GAC TGA GGA CTC CTC TTC AGA CGG CAA…-5’
TRANSCRIPTION
mRNA
5’-…CAU CUG ACU CCU GAG GAG AAG UCU GCC GUU…-3’
TRANSLATION
Protein
N-…His Leu Thr Pro Glu Glu Lys Ser Ala Val…-C
Another new mutant allele…
Coding
Template
5’-…CAT CTG ACT CCT TAG GAG AAG TCT GCC GTT…-3’
3’-…GTA GAC TGA GGA ATC CTC TTC AGA CGG CAA…-5’
TRANSCRIPTION
mRNA
5’-…CAU CUG ACU CCU TAG GAG AAG UCU GCC GUU…-3’
TRANSLATION
Protein
N-…His Leu Thr Pro
?
-
Glu Lys Ser Ala Val…-C
(Nonsense mutati
What happens when the HBB gene is mutated?
• Most commonly, people with sickle cell anemia have two defective
hemoglobin beta HbS alleles (instead of normal HbA alleles).
– The HbS allele has a substitution of a single base pair
– This mutation results in a glutamic acid to valine change in the protein
– Valine is a hydrophobic amino acid
• Homozygous expression of the HbS allele results in hemoglobin
tetramers with two mutant beta hemoglobin subunits. (view picture)
• The problem is that the hemoglobin tetramers made with HbS betaglobin are “sticky” and clump to form long fibers.
• These long fibers cause the red blood cell to have a sickle shape.
Is the structure of hemoglobin tetramers in normal people
and people with sickle cell anemia different?
Hb tetramer in normal people
http://www.rcsb.org/pdb/cgi/explore.cgi?pid=16791063394243
&page=0&pdbId=4HHB
Hb tetramer in people
with sickle cell
anemia
http://www.rcsb.org/pdb/cgi/explore.cgi?pid=18121063394293
&page=0&pdbId=2HBS
Hemoglobin tetramers in people with the
HbS/HbS genotype stick together!
http://www.rcsb.org/pdb/explore/images.do?structureId=2HBS
How does the
HbS/HbS
genotype in
people with
sickle cell
anemia cause
sickle-shaped
red blood
cells?
Hartwell et al. 2008. Genetics: Genes to Genomes, McGraw-Hill
Genotype
Phenotype
Hemoglobin
protein
Red blood cell
Person
Genotype
(HbA/HbA)
Phenotype
Hemoglobin
protein
Red blood cell
Person
Normal
tetramers
Normal in
shape (donut)
and number
Normal
Genotype
(HbS/HbS)
Phenotype
Hemoglobin
protein
Red blood cell
(RBC)
Person
Tetramers stick
together and
form long fibers
Abnormal
shape (sickle),
fewer RBCs
Sickle cell anemia
How is sickle cell anemia treated?
• Blood transfusions
– Build-up of iron is a problem
• Hydroxyurea medication (causes switch to
expression of fetal hemoglobin)
– Can be toxic
Is sickle cell anemia found more often in
certain ethnic groups?
Ashley-Koch A, Yang Q, Olney RS. (2000) Sickle Hemoglobin (HbS) Allele and
Sickle Cell Disease: A HuGE Review. Am J Epidemiol Vol. 151, No. 9.
Malaria Incidence, 2009
Cibulskis RE, Aregawi M, Williams R, Otten M, Dye C (2011)
Worldwide Incidence of Malaria in 2009: Estimates, Time Trends, and a Critique of Methods.
PLoS Med 8(12): e1001142. doi:10.1371/journal.pmed.1001142