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Mutations
Natural and Artificial Mutations
Mutations
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There are 2 classes of mutations
Nucleotide mutations occur when 1-4
nucleotides are altered, added or removed as
a result of damage or errors in replication
Transpositions occur when entire sections of
DNA “jump” to a different location in the DNA,
disrupting genes
Examples of nucleotide mutations
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Point Mutations – one nucleotide is altered
Silent Mutation: CTC to CTT – no change since both
codons code for glutamic acid
Missense Mutation: CTC to CTA replaces glutamic
acid with aspartic acid in hemoglobin
since they are functionally similar amino acids, the
protein is not greatly affected and the mutation
introduces a variation in the species
CTC to CAC replaces glutamic acid with valine
Valine is hydrophobic and results in clumping of
hemoglobin, resulting in sickle cell anemia
Sickle Cell Anemia
Nucleotide Mutations
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A point mutation could also result in the
production of a stop codon in the middle of a
gene
If this occurs in an essential protein, such as
hemoglobin, the mutation is lethal and is
called a nonsense mutation
Frameshift mutations are also normally lethal
– the insertion or deletion of a nucleotide
shifts the entire reading frame and every
codon is altered
Transposons
Transposons
Transposons
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Approximately 50 % of the human genome is
made up of transposons
They can “jump” from one location to another,
or they can copy themselves first, and the
copy jumps
Transposons can cause mutations by
inserting themselves into exons, or by taking
exons with them; “shuffling” the genetic deck
Transposons can jump to a promoter region
and either turn off or turn up transcription
Transposons
Artificial Mutation
UV Radiation
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UV radiation produces covalent bonds between
adjacent thymine base pairs
These dimers block replication by DNA polymerase
Cells can repair the damage by removing the
damaged section on one side of the helix
DNA polymerase and DNA ligase complete the
repair
If the repair is done incorrectly, a mutation results
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http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter11/animation_quiz_5.html
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Chemical Mutagens
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Base analogs
Chemicals that have a
very similar structure to
thymine, uracil,
adenine, cytosine or
guanine
Example: 5’bromouracil
Base analogs generally
result in point mutations
Chemical Mutagens
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Acridine dyes
These chemicals have
a positive charge so
they bind to the
negatively charged
DNA
They insert between
base pairs and cause
frameshift mutations
Examples; nitrous acid
and hydrazine
Chemical Mutagens
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Alkylating agents
Can transfer methyl groups,
ethyl groups, bond with
phosphate groups
This can result in any type
of mutation, including the
lethal breakage of the DNA
sugar-phosphate backbone
They are actually used as
chemotherapy in the
treatment of cancer,
destroying the cancerous
cells’ DNA
Image from Science Daily