Download The Molecular Biology of Mutations and Muscular Dystrophy

The Molecular Biology of Mutations and Muscular Dystrophy
Modified from original by Dr. Ingrid Waldron, Department of Biology, University of Pennsylvania, 2011
1. The table below shows five different mutations of a small section of a gene. Use the base-pairing
rules to complete the 2nd column of mRNA codons. (Do not pay attention to underlined DNA
nucleotides in the first column right now.)
2. In the 2nd mRNA codon column, highlight in red any of the three stop codons: UAA, UAG or UGA.
3. Using the mRNA codon chart below, identify the correct amino acid that goes with each mRNA
codon, writing your answer in the 3rd column, Polypeptide. Remember that if you have a stop codon,
there are no additional amino acids after that, even if there is more mRNA! Instead, use a dash (-) to
indicate any amino acids that will be missing as a result of stop codons.
4. In the 2nd column, make bold any mRNA codons that were changed as a result of the mutation.
5. In the 3rd column, highlight in yellow any amino acids that were changed as a result of the mutation.
DNA (Template Strand)
mRNA codon
Polypeptide
CGU UCA UGG ACU
arg – ser – trp – thr
Mutation =
GCCAGTACCTGA
(nucleotide change underlined)
____ ____ ____ ____
___ ___ ___ ___
Mutation =
GCACGTACCTGA
(nucleotide change underlined)
____ ____ ____ ____
___ ___ ___ ___
Mutation =
GCAAGTACTTGA
(nucleotide change underlined)
____ ____ ____ ____
___ ___ ___ ___
Mutation =
GAAGTACCTGA
(first C deleted)
____ ____ ____ ____
___ ___ ___ ___
Mutation =
GCAAGTACTGA
(second C deleted)
____ ____ ____ ____
___ ___ ___ ___
Original DNA =
GCAAGTACCTGA
Analysis Questions
1. Explain why deleting a single nucleotide in a gene generally results in more severe defects in the
polypeptide than a point mutation that changes a single nucleotide.
Muscular dystrophy includes several genetic diseases which cause increasing weakness of a person's
muscles. Two different types of muscular dystrophy are caused by different types of mutations in the
gene for a protein that helps to prevent muscle cells from dying prematurely.
Duchenne muscular dystrophy is more severe. A child with Duchenne muscular dystrophy
begins to show symptoms of loss of muscle function by about age three, and needs to use a
wheelchair by about age 10. A person with Duchenne muscular dystrophy typically dies as a
young adult, due to failure of the muscles in the internal organs.
Becker muscular dystrophy is milder. Symptoms do not begin until age 12 or later, and the
person can live into their 40s or 50s.
2. The table shows the main kinds of mutations that are responsible for these two types of muscular
dystrophy. Complete the second column to indicate which type of muscular dystrophy you think
would be caused by each type of mutation.
Type of Mutation
Type of Muscular Dystrophy
Deletion Mutation 
# nucleotides deleted from mRNA is a multiple of 3
Deletion Mutation 
# nucleotides deleted from mRNA is not a multiple of 3
Point Mutation  stop codon
3. Explain your reasoning for your answers in the above table.