Download Learned about mutations

When Good DNA Goes Bad:
A Look at Mutations
Name:
_______________________________
Date: ____________________ Per: _________
INTRODUCTION: Each time a cell divides, it must make a copy of its DNA during a process known as DNA replication.
Sometimes during replication, an error is made that causes changes in the mRNA and proteins that are made using that DNA.
These errors or changes are called mutations. A mutation is defined as a change in the DNA sequence. Some mutations are
bad, some are good, and some have no effect. It all depends on the final protein made by the cell following the new DNA
instructions. If the protein doesn’t function as well as the original, the mutation would be bad. If the protein functions better
than the original, the mutation would be good. If the same protein is made as the original, the mutation would have no effect.
In this activity you will be using magnets to build sections of DNA, transcribing the DNA to mRNA and translating the mRNA
into amino acids. You will also be experimenting with various types of mutations to determine which causes the greatest change.
Step 1: The nucleotide magnets represent the base pairs that make up DNA. The four DNA nucleotides are adenine, thymine,
cytosine, and guanine (A, T, C, and G). The DNA sequence below represents a portion of a gene. A real gene would be many more
bases long. With your partner, make the DNA sequence below using the magnets and the whiteboard. Check your accuracy!
T
A
C
C
A
C
G
T
A
G
A
C
A
T
C
Step 2: DNA is double stranded, but when a protein is made, only one side is used. The side you built on the board is the side
we are using. Imagine the complementary side of DNA is still there, just puffed out so it’s not in the way. Use the nucleotide
magnets and make the mRNA sequence that would be created off this DNA strand. Notice that you can connect the base pairs
together like puzzle pieces to represent the hydrogen bonds that form. Remember, in RNA there is no T. Use the “U”
nucleotide wherever you would want to put a “T”.
Step 3: Write the mRNA sequence below from the DNA template in Step 1.
Answer this question: What step of protein synthesis did you just do when you made the mRNA based on the DNA template?
Step 4: Codons are groups of three mRNA bases. Write the mRNA from step 3 as codons below and then use the codon chart
to write the amino acids requested by each mRNA codon. You can use the 3 letter abbreviation.
mRNA Codon
Amino Acid
Answer this question: What is the last amino acid you wrote? What do you think this amino acid means? What amino acid do you
think ends all proteins?
Step 5: Point mutations are changes in only 1 or a few nucleotides of DNA. Point mutations include substitutions in which one
base is changed to another. In your original DNA strand, change the 6th base from a C to a T. Since that is the only change,
you can leave your magnet mRNA the same, except you need to change the mRNA base you paired at the mutation location. Fix
it. Write the newly mutated mRNA in the space below by codons and translate the new mRNA into a polypeptide sequence. Do
you have to look up all the mRNA? No – you just need to look up the one you mutated because the others are the same.
mRNA Codon
Amino Acid
Answer this question: What did you discover about your new polypeptide sequence (compared to the original)?
Sometimes when a mutation occurs the resultant polypeptide is not changed because you still code for the same amino acid, just
a different way. These are referred to as silent mutations. Was the mutation you just modeled silent? Circle: Yes or No
Step 6: Return the DNA and mRNA back to original sequence from Steps 1 and 2. Now, do this mutation. Replace the 8th base
on the DNA sequence from a “T” to a “C”. Since that is the only change, you can leave your magnet mRNA the same, except you
need to change the mRNA base you paired at the mutation location. Fix it. Write the newly mutated mRNA in the space below
by codons and translate the new mRNA into a polypeptide sequence. Do you have to look up all the mRNA? No – you just need to
look up the one you mutated because the others are the same.
mRNA Codon
Amino Acid
Answer this question: What did you discover about your new polypeptide sequence (compared to the original)?
Sometimes when a mutation occurs the resulting polypeptide is changed. These mutations are called one of two names. A
missense mutation is when one amino acid in the sequence is changed. A nonsense mutation is when the mutation leads to a stop
codon being anywhere other than the end. What type of mutation did you just model? Circle: Missense or Nonsense
Answer this question: What do you think happens to the size of the protein if a stop codon is put anywhere but at the end?
Step 7: Return the DNA and mRNA back to original sequence from Steps 1 and 2. Another type of mutation occurs when one
base is either deleted or a new base is inserted into a DNA sequence. This causes the entire sequence of codons to shift over
by one base and is called a frameshift mutation. In your original DNA strand, add one extra base (C) immediately after the
START codon (TAC) in your DNA sequence. Notice how you have to shift the magnets to get it to fit? Use the magnets to fix
your mRNA strand to correctly match the mutated DNA.
Write the newly mutated DNA in the space below in 3’s. Transcribe and translate this into a new amino acid sequence
(polypeptide). You will notice an extra base at the end. You can’t do anything with this base because you need three bases to
code for an amino acid. Just leave it be.
DNA
mRNA
Amino Acids
Answer this question: What did you discover about your new polypeptide sequence (compared to the original)?
Answer this question: What would have happened to the polypeptide if you had deleted a single base instead of inserting a base
at the same location?
Answer this question: What would have been the result if the insertion or deletion of a base had happened near the end of the
sequence?
FINAL QUESTION: Summarize what you have learned about mutations and their effect on the resulting polypeptide by
addressing the following questions. Which type of mutation would have the least effect on the resultant polypeptide? The
greatest? Does a mutation always have an effect on the polypeptide? Would the effect be as significant if you delete or insert
3 bases?