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Biology 9
Genetic Mutations
Part I: Preview Questions
1. Based on your current knowledge, how would you define a mutation? What does this word make you think
of? (2 pts)
2. Unlike popular misconceptions about people with green skin or extra body parts, a mutation is simply a change
in the nucleotide sequence, or base pair sequence, of DNA. Most mutations are either neutral (they have no
effect) or harmful, but occasionally mutations can actually cause a helpful change. Some mutations change only
a single base in the DNA sequence – these are called point mutations. Other mutations can be larger-scale
changes in a chromosome, such as cutting out, copying, or switching around a long chunk of DNA.
Based on the paragraph above, write a one-sentence definition for a mutation. Then explain how this is
similar or different to what you thought a mutation was (see your answer to #1). (2 pts)
3. This packet focuses on point mutations – changing a single nucleotide in a DNA sequence. There are three
ways to create a point mutation:

substitution (switch one DNA nucleotide for another)

insertion (add an extra nucleotide into the DNA sequence)

deletion (leave out one nucleotide in the DNA sequence)
Make a prediction: which of these changes do you think will have the biggest effect the amino acid
sequence encoded by the DNA? Which will have the smallest effect? Why? (3 pts)
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Part II: Types of Mutations
Below is the original DNA sequence we will work with. Your first task is to transcribe and translate it into an
amino acid sequence for a protein (2 pts)
Original Gene Sequence
DNA
TAC
mRNA
A.A.s
TTA
TAG
TTA
AAC
CCA
CAA
ACT
Substitution Mutations
In a substitution mutation, one nucleotide is switched for another. We will look at two examples. In example
#1, the third base of the 5th codon has been switched from C to A.
Transcribe and translate the gene again to see how this affects the protein. (2 pts)
Example #1: Substitution Mutation
DNA
TAC
TTA
TAG
mRNA
A.A.s
TTA
 used to be C in the original gene
AAA
CCA
CAA
ACT
Compare the new amino acid sequence with the original. How and why is it different? (2 pt)
Now we’ll look at another example of a substitution mutation. We’ll change the same nucleotide, but this time
we’ll change it from C to T. This example is in a category called “silent mutations.” Transcribe and translate
the gene to figure out why: (2 pts)
Example #2: Silent Mutation
DNA
TAC
TTA
TAG
mRNA
A.A.s
TTA
 used to be C in the original gene
AAT
CCA
CAA
ACT
Compare this amino acid sequence with the original (not with example #1). Why do you think this is
called a “silent” mutation? (2 pts)
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Frame-Shift Mutations
Another category of mutations is called frame-shift mutations. This includes inserting or deleting a nucleotide
from the DNA sequence. In the example below, we will delete the first nucleotide in the 4th codon. Transcribe
and translate to see what happens: (2 pts)
Example #3: Deletion Mutation
DNA
TAC
TTA
TAG
mRNA
A.A.s
 T was deleted from here
TAA ACC
CAC
AAA
CT
Compare this amino acid sequence with the original. How and why is it different? (2 pts)
For the final example, we will insert an extra nucleotide into the 4th codon. Transcribe and translate: (2 pts)
Example #4: Insertion Mutation
DNA
TAC
TTA
TAG
mRNA
A.A.s
 this A was added in
ATT
AAA
CCC
ACA
AACT
Compare this amino acid sequence with the original. How and why is it different? (2 pts)
Analysis Questions for Part II:
1. Think about the phrase “frame shift.” Why do you think adding or deleting a nucleotide is called a “frame
shift” mutation? (1 pt)
2. Which type of mutation tends to have a bigger effect on the amino acid sequence of a protein – a
substitution, or a frame shift mutation? Why? (2 pts)
3. Describe the type of mutation that you think would be most likely to be harmful to an organism. Then
describe the type of mutation that would be least likely to be harmful. Explain your reasoning. (3 pts)
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