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MCDB 1041 Class 22
Gene expression
Mutations
Learning Goals:
•  Recognize different kinds of mutations (frameshift, insertions,
deletions, point mutations)
•  Predict how different mutations in the DNA affect RNA and protein in
different ways
•  Explain how changes to chromosome structure and presence and absence
of cell-specific transcription factors dictate which genes get
transcribed and ultimately translated
What controls whether or not a gene is
transcribed?
•  Transcription factors (cell-specificnproteins
that turn on transcription by assembling at
the promoter)
•  Chemical modifications of DNA sequences
that can prevent the DNA from being
unwound (thus keeping it “inactive”)
•  Chemical modifications of the Histones (the
proteins that DNA is wound around)
Transcription factors
-proteins that bind to the DNA (usually near or at the
promoter region) and allow RNA polymerase to bind
Transcription
RNA
promoter
factor
poly3’
erase
5’
template region of DNA
5’
3’
•  There are all-purpose TFs that bind to all sequences
•  There are unique transcription factors that are produced
in some cells and not others
These unique transcription factors bind to regions near the
promoter and allow transcription: this determine which
genes will get expressed in which cells
Neuron
Muscle
Which of these cells has the neurogenin gene?
a.  Neurons
b.  Muscles
c.  Both
d.  Neither
Neuronal cells neuron
contain the protein neurogenin Which of these cells has neurogenin
mRNA?
a.  Neurons
b.  Muscles
c.  Both
d.  Neither
Muscle cells contain the protein myosin Alterations to the
chromosomes determine how
easily transcription factors
(TFs) can bind to promoter
regions: chemical modifications
•  Addition of a methyl group
(CH3) make DNA
inaccessible; TFs can’t bind
•  Addition of an acetyl group
(COCH3) to the histone
proteins opens the structure
of the chromosome, allowing
TFs to bind
Changes in protein structure result in the protein
functioning differently or not at all.
If a person has many copies of a particular protein
whose structure is changed from normal, where
did this change most likely originate?
a. 
b. 
c. 
d. 
In
In
In
In
the
the
the
the
DNA sequence of the gene
mRNA sequence
amino acid sequence
protein
How do mutations occur?
• 
Spontaneous (mistakes in replications)
–  Chemical instability of the bases
–  Mismatch of bases, due to repeated sections of DNA,
some of which for loops
•  Mutagens: exposure to radiation, chemicals, etc
–  Some chemicals are structurally similar to DNA bases, and
are used by mistake
–  Radiation can cause chemical changes and can also cause
unusual base pairing
Spontaneous mutation
Average rate: 1 mutation per 100,000 bases
during each round of replication
But, there is no set rate of mutation. The
chance of mutation depends on:
–  Size of the gene
–  Sequence of the gene
–  “Hot spots” –-repeated areas, other
sequences that seem to be prone to mutation
Mutations are countered by DNA Repair
In humans, mistakes in DNA replication that are NOT
REPAIRED occur on average 1 in 100 million bases (not
very often!).
In other words, DNA repairs itself most of the time!
Specialized enzymes cut out the DNA when a mismatch
is detected, and DNA polymerase inserts the correct
matching sequence
A mutation occurs in a skin cell. Another mutation
occurs in a spermatocyte before it undergoes meiosis.
Which of these mutations is it possible for an
offspring of this person to inherit?
a.  The skin cell mutation
b.  The spermatocyte mutation
c.  Both
d.  Neither
Mutations in our DNA lead to changes in proteins
Mutations are random
Some mutations have a negative effect, some
have neutral affects, some have positive effects
How do these mutations affect protein?
Achondroplasia (short stature) is caused by a dominant
mutation in the Fibroblast Growth Factor Receptor Gene 3
(FGFR3), which helps bones grow to an appropriate length
Molly (Amy’s daughter) Part of the FGFR3 gene DNA:
http://tlc.discovery.com/fansites/
lpbw/bios/molly.html
Amy Same part of FGFR3 DNA:
AGCTACGGGGTG
TCGATGCCCCAC This single change in the DNA sequence makes a difference in how their bones grow AGCTACAGGGTG
TCGATGTCCCAC
http://tlc.discovery.com/fansites/
lpbw/bios/amy.html
Achondroplasia results from a “point” mutation
Another example: Sickle Cell Anemia
Sickle cell anemia also results from a “point” mutation.
Point mutations that result in a change in an amino acid are
called “missense” mutations
Although a missense mutation seems like a trivial change, just this single base
change can produce a protein that functions not at all or completely differently
12.1
than Figure
the normal
protein
Normal red blood cell
Sickle shaped blood cell
Chapter 12 Opener
normal A
muta@on A A
A A G
U U U
U U C
phe
phe
What about the mutation shown above? What is the result?
a.  Transcription is stopped by this mutation
b.  Translation is stopped by this mutation
c.  Both a and b
d.  Neither a. nor b. is true, but the amino acid is changed
e.  None of the above: the amino acid is the same
Point mutations can also be
“silent”
Mutations can also be Insertions or Deletions
Frameshift
Non-frameshift
A frameshiB will shiB the reading frame of the codons, thus producing many different amino acids (see above) An inser(on of 3 new DNA bases would have what affect on a protein? a.  It would have a totally new sequence b.  It would have one addi(onal amino acid c.  It would have one too few amino acids d.  It would have no effeect normal muta@on A T A
A T T
U A U
U A A
tyr
Stop What about the mutation shown above? What is the result?
a.  Transcription is stopped by this mutation
b.  Translation is stopped by this mutation
c.  Both a and b
d.  Neither a. nor b. is true, but the amino acid is changed
e.  None of the above: the amino acid is the same
Point mutations can lead to a stop= “nonsense”
= shortened protein
But, mutations do NOT stop transcription
•  Handout Different scenario:
What do you think would happen to the amino acid
sequence coded by the DNA below if the DNA
sequence were shortened by removing the highlighted
nucleotide?
CGG TCG TAC AGG TGA CGC CAG
The amino acid sequence made from the RNA made from
this sequence of DNA would be:
a. Unchanged
b. Completely altered
c.  Different after the deletion