6.3 dna mutations · single-gene mutations involve changes in the nucleotide sequence of one gene...
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6.3 DNA Mutations
SBI4U
Ms. Ho-Lau
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DNA Mutations
Gene expression can be affected by
errors that occur during DNA replication.
Some errors are repaired, but others can become mutations (changes in the nucleotide sequence of a cell’s DNA)
All cells undergo spontaneous mutation
Mutagens: substances or events that increase the rate of mutation
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DNA Mutations
in reproductive cells can affect the next generation
in somatic cells can affect daughter cells
can be neutral or harmful
in rarer cases, they are beneficial
important in terms of species change and adaptation
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DNA Mutations
2 categories:
• single-gene mutations
• chromosome mutations
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Single-gene Mutations
involve changes in the nucleotide sequence of one gene
categorized according to how they affect the amino acid sequence:
1. Silent mutations
2. Missense mutations
3. Nonsense mutation
categorized according to how they affect the nucleotide sequence:
1. Point mutations
2. Frameshift mutations
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1. Silent Mutations
one nucleotide is changed
But does not change the
amino acid sequence of
the protein
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2. Missense Mutations
one nucleotide is changed
changes the amino acid sequence of the protein
usually harmful
Occasionally, produce
a protein that helps with
survival
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3. Nonsense Mutations
Introduced a premature stop codon
Shorten the mRNA
Shorten the protein
Usually harmful
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Single-gene Mutations
involve changes in the nucleotide sequence of one gene
categorized according to how they affect the amino acid sequence:
1. Silent mutations
2. Missense mutations
3. Nonsense mutation
categorized according to how they affect the nucleotide sequence:
1. Point mutations
2. Frameshift mutations
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1. Point Mutations
result from a change in a single base pair within a
DNA sequence
can involve substitution, addition, or deletion of one
nucleotide
A substitution may not affect the protein due to the
redundancy built into the genetic code.
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2. Frameshift Mutations
result from the insertion or
deletion of nucleotides not
divisible by three
causes a change in the
reading frame
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DNA Mutations
2 categories:
• single-gene mutations
• chromosome mutations
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Chromosome Mutations
Changes in chromosomes and many genes
1. Deletion: a series of nucleotides are deleted from the chromosome
2. Duplication: a series of nucleotides are duplicated on the same
chromosome
3. Inversion: a chromosome segment is broken and re-inserted in the
opposite direction
4. Reciprocal Translocation: a group of nucleotides from one
chromosome are exchanged with the
nucleotides of a different chromosome
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Chromosome Mutations
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Causes of Mutations
Mutations may be spontaneous or induced.
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Causes of Spontaneous Mutations
• the result of normal molecular interactions
• incorrect base pairing by DNA polymerase during replication
• transposition, in which specific DNA sequences (called
transposons) move within and between chromosomes
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Causes of Induced Mutations
1. physical mutagens
• an event or substance such as high-energy radiation (X rays
and UV rays) that physically change the structure of DNA
• example, UV radiation distorts the DNA molecule at adjacent
C and T bases, causing replication interference.
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Causes of Induced Mutations
2. chemical mutagens
• molecules that can enter the nucleus of a cell and induce
mutations by reacting with DNA
• Example: nitrites and chemicals in cigarette smoke
• Some mutagens have a structure similar to DNA nucleotides
and can incorporate into a DNA strand, causing incorrect
base pairing during replication.
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DNA Repair
DNA polymerases and mismatch repair mechanisms
help to repair replication errors
Two additional types of DNA repair mechanisms are:
• specific repair mechanisms: tailored to fix certain
types of damage (eg. photorepair)
• non-specific repair mechanisms: can correct different
forms of damage (eg. excision repair)
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Specific Repair
Photorepair is a specific
mechanism to repair damage to
DNA caused by exposure to UV
radiation.
A photolyase enzyme recognizes
the damage, binds to the dimer,
and uses visible light to cleave
the dimer.
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Non-specific Repair
Excision repair is a non-specific
mechanism of DNA repair
because it can fix a variety of
damage.