Name: _______________________________
Score: _____/25
Mutation Secret Messages
Introduction
Your DNA replicates (copies itself) many thousands of times per day. During replication, DNA polymerase must
match the correct complementary base to about 3 billion nucleotides. Although DNA polymerase is an excellent
proofreader, and although cells have many check points, occasionally a mistake is made. When there is a mistake in
DNA replication, this is called a mutation.
Mutations can be harmful, harmless, or beneficial to the organism. Most mutations are harmless. Either they occur
in non-coding parts of the genome, in DNA that does not directly code for a protein, or the mutation makes no
difference in the type of protein produced. This latter mutation is called a silent mutation. Every so often, though,
a mutation causes a change in a protein, resulting in a trait or disease never before seen in an organism. Mutation is
the basis of all variation in nature. Traits that are common today were once mutations!
Examples of mutations in the human population include blue eyes, left-handedness, red hair, ability to digest lactose
(milk), and non-sensitivity to the compound PTC.
In today’s activity, you will learn about the effects of three types of mutations: substitution mutations, frameshift
mutations, and silent mutations.
Pre-lab Questions:
1.
2.
3.
4.
5.
Are most mutations helpful, harmless, or harmful? Explain your answer.
A mutation that makes no difference in the protein that is made is called what kind of mutation?
All variation observed in nature is originally caused by what?
Give three examples of mutations.
What are the three types of mutations we will learn about in this activity?
Part 1: Normal DNA
Together with your partner, transcribe and translate your assigned DNA message.
Sentence Number: _____
DNA:
mRNA:
Amino Acids:
Message:
Part 2: Substitution Mutation
Now you will make a substitution mutation in your original DNA sequence. A substitution mutation is a
spontaneous change of one nucleotide for another. For example, an adenine might be replaced by a cytosine. Make
your substitution somewhere in the middle of your DNA sequence. Circle the change you make, then transcribe and
translate the message again, with your substitution mutation.
DNA:
mRNA:
Amino Acids:
Message:
How did your message change? Can you still understand what your message should have said?
Part 3: Frameshift Mutations
Once again make a change to your original DNA transcript, but this time, delete a random base (**remember! A
base is just one letter! Note three!) Draw an empty circle where your deleted base should have been. Delete your
base somewhere in the middle of your DNA sequence. Do not delete a base from the first or last codon. Notice that
all of your codons (groups of three bases) will shift after the point of mutation. Transcribe and translate your
message with the deletion mutation.
DNA:
mRNA:
Amino Acids:
Message:
How did your message change? Can you still understand what your message should have said?
At what point did your message change?
Because deletions change the groupings of codons, or to put it another way, because deletions shift the frame in
which codons are read, they are called frameshift mutations.
Again change your original DNA sequence, this time by adding an extra base. Circle the added base, and do not add
a base to the first or last codon. Transcribe and translate your new message.
DNA:
mRNA:
Amino Acids:
Message:
Would an additional mutation be classified as a frameshift mutation? Explain why or why not.
Part 4: Silent Mutations
Transcribe and translate the DNA message below.
AGA ACA TAA CTC TTA ACA CTC TAA AGA CCA GCA CTC CGA TGA
DNA:
mRNA:
Amino Acids:
Message:
Now Mrs Howard will cause a substitution mutation on the tenth codon. Notice that the third letter of the codon has
been changed from an A to a C. The change is underlined in the mutated message below. Transcribe and translate the
mutated message.
DNA:
AGA ACA TAA CTC TTA ACA CTC TAA AGA CCC GCA CTC CGA TGA
mRNA:
Amino Acids:
Message:
How did the substitution mutation affect the message?
Substitution mutations that change a codon in such a way that it still codes for the same amino acid have no effect on
the final protein. A mutation has occurred, but there’s no overall effect. This type of mutation is called a silent
mutation.
Conclusion Questions
1. Which is usually more serious for the organism: a point mutation or a frameshift mutation? Explain your
reasoning.
2. Recall that all proteins begin with the mRNA start codon AUG and therefore the amino acid Methionine. What
would happen if a substitution mutation occurred in the start codon? What would be the effect on the protein?
3. Recall that all proteins end with one of the three STOP codons. What would happen if a substitution mutation
occurred in the STOP codon? What would be the effect on the protein?