Download cookie-aseSHO

Cookie-ase
Excerpted and Adapted by Drs. Jennifer Doherty and Ingrid Waldron, University of Pennsylvania, 20111
From “Cookie-ases: Interactive Models for Teaching Genotype-Phenotype Relationships”
By Rebecca L. Seipelt, the American Biology Teacher, Online Publication, May 2006
Today you will simulate the changes in a protein that can result from mutations in a gene.
A mutation is a permanent change in the DNA of a gene. A mutated gene may result in
the production of a protein that is less able to do its job. The protein you will simulate is a
hypothetical enzyme, cookie-ase, which opens sandwich cookies.
★ What is a gene?
★ Complete the following flowchart to show how a gene can influence the structure and
function of a protein enzyme. Use the terms: DNA, protein, RNA.
nucleotide sequence in the _________ of a gene
 nucleotide sequence in messenger ___________
transcription
 amino acid sequence in ____________
translation
 structure and function of the protein enzyme
One type of mutation is a change in one or more nucleotides in the nucleotide sequence of
a gene. Changes in the nucleotide sequence of a gene can result in the placement of the
wrong amino acid in a protein when the protein is made. This error can make the protein
defective so it cannot do its job as well or at all.
Mutations result in different versions of a gene. Each different version of a gene is a
different allele.
1
Teachers are encouraged to copy this student handout for classroom use. A Word file (which can be used to prepare a modified version if desired) and
teacher notes are available on the molecular biology page of https://sites.google.com/site/biologypd/home .
1
The figures below show the nucleotide sequence in the DNA of a gene for a hypothetical very short
protein.2 The resulting mRNA and protein are also shown. Remember that each group of three
nucleotides makes up a triplet codon. Each triplet codon is then translated into a single amino acid.
Translation of a piece of mRNA begins when the ribosome recognizes the start codon AUG. AUG also
codes for the amino acid methionine. Translation stops when the ribosome recognizes one of three stop
codons that do not code for an amino acid: UGA, UAG, UAA.
Normal allele = A
T A C T T C A A A C C G A T T
DNA
mRNA
This allele is able to generate a fully functional protein.
A U G A A G U U U G G C U A A
protein
Met
Lys
Phe
Gly
Stop
Met
★ Circle the triplet codon in the mRNA that codes for the amino acid phenylalanine (Phe).
Point Mutation  a1 allele A point mutation changes a single nucleotide in a strand of DNA and
changes one triplet codon in mRNA which is translated into a different amino acid in the protein.
Sometimes an important area of the protein is changed so that it cannot function properly.
C replaced with T
T A C T T C A A A T C G A T T
DNA
mRNA
protein
A U G A A G U U U A G C U A A
Met
Met
Lys
Phe
Ser
Stop
★ Circle the codon that has been changed by the point mutation.
Insertion Mutation  a2 allele
An insertion mutation inserts an extra nucleotide into a strand of
DNA. The DNA shown below has an insertion mutation.
Extra A
DNA
T A C A T T C A A A C C G A T T
C
mRNA_____________________________________
protein_____________________________________
★ Write in the results of transcription for this DNA. Mark off each codon in the mRNA.
Compare the codons in this mRNA to the codons in the mRNA for the normal allele shown above.
Notice that the insertion mutation results in a change in every codon after the insertion. An insertion
mutation has the potential to change every amino acid in the protein.
★ Write in the results of translation for the mRNA. Remember that the codon UAA is a stop codon.
2
Real proteins typically have 50-2000 amino acids, and even a short polypeptide has more amino acids than our hypothetical protein.
2
As you saw, an insertion mutation can result in a stop codon instead of a codon for an amino acid. This
results in a much shortened protein which may be extremely defective.
★ In general, do you expect point mutations or insertion mutations to result in the most severe defects?
Explain why.
To help you understand why an insertion mutation typically has such a big effect, you can imagine that
when a ribosome makes a protein, the ribosome reads a strand of mRNA as you would read a sentence on
a piece of paper. Look at the three sentences below.
Normal sentence
The big cat ate the fat rat.
Point mutation sentence
i replaced with o
The bog cat ate the fat rat.
Insertion mutation sentence
a inserted
The bai gca tat eth efa tra.
★ Which mutated sentence is more difficult to read?
★ Write a deletion mutation sentence which has the letter i deleted from the word big.
Procedure for simulation
In your simulation, you will work in groups of two or three. One of you will be the enzyme cookie-ase
that breaks down the substrate (sandwich cookies) into product molecules (sandwich cookie halves). The
table on the next page explains the different ways you will use your hands to simulate the normal enzyme
and the enzymes produced by the gene with the point mutation or the insertion mutation.
1. Get a stack of job cards, a stop watch, a package of cookies, and three paper plates.
2. Each of you should select a job card. All jobs should be taken. If necessary, the recorder can also be
the timekeeper.
a. Enzyme: The enzyme will try to break down as many substrate molecules (sandwich cookies)
into product molecules (sandwich cookie halves) as possible in 10 seconds. The two halves of the
sandwich cookie must be completely separated to count (broken cookie halves don’t count).
Make sure you wash your hands thoroughly before you start.
3
b. Recorder: The recorder records the numbers of cookies split in 10 seconds for each allele in the
table below and makes sure the enzyme does not cheat and use fingers that are not allowed.
c. Timekeeper: The timekeeper starts and stops the enzyme at the correct time.
3. The simulation is run three times: first with the normal genotype AA, then with the point mutation
genotype a1a1, then with the insertion mutation genotype a2a2.
Genotype
How you will simulate this enzyme type
Cookies split in
10 seconds
You can use all the fingers on both of your hands to
AA
(Normal)
pick up and open a cookie.
You can only use your pointer and middle fingers to
a1a1
(Point Mutation)
pick up and open a cookie.
You can only use your fists
a2a2
(Insertion Mutation)
(thumb inside the other four fingers).
Cookies split in 10 s
(phenotype)
Questions
1. Create a bar graph of your data in the figure below. Be sure to label your Y axis.
AA
a1a1
Genotype
a2a2
2. Did the point mutation or insertion mutation results in the most severe defect? Are the results of
your simulation in agreement with your predictions at the top of page 3?
Next, we will apply what you have learned to understanding the genetic basis for two human
characteristics: albinism and muscular dystrophy.
4
Albinism
Albinism is a genetic disorder that results in very pale skin and hair color. One form of albinism is caused
by mutations in the gene which codes for the protein enzyme tyrosinase.
Tyrosinase (together with other enzymes) acts on the substrate tyrosine to make melanin, the pigment
found in the hair, skin, and eyes.
When the enzyme tyrosinase is defective, not enough melanin is made. The resulting deficiency in
melanin causes the characteristic pale hair and skin of albinos. The lack of pigment may also cause
visual problems.
In a person who is homozygous for the gene that codes for the enzyme tyrosinase, both copies of the
gene have the same allele. Tushana and Raymond are both homozygous for a mutated tyrosinase allele,
but they are homozygous for different alleles and they look different. Raymond is very pale, but he does
have a slight yellow color in his hair, eyes, and skin. Tushana is extremely pale with white skin and
hair.
1. How might the differences between Tushana and Raymond be explained? (Hint: Think about the
different mutations for the cookie-ase gene.)
In a person who is heterozygous for the tyrosinase gene, with one allele coding for a normal tyrosinase
enzyme and one allele coding for a defective tyrosinase, enough normal tyrosinase enzyme is produced
by the one normal tyrosinase allele to make enough melanin to give normal skin and hair color. So, the
heterozygous individual has a normal phenotype.
2. Based on this information, which allele is dominant -- the allele for normal tyrosinase or the allele for
defective tyrosinase? Explain your reasoning.
5
Muscular Dystrophy
You can use the information you have learned so far to understand the genetic basis for two different
types of muscular dystrophy. Muscular dystrophy is caused by harmful alleles of the gene that codes for
the muscle cell protein, dystrophin. If dystrophin is defective or missing, muscle cells gradually break
down so the child with muscular dystrophy becomes weaker and loses the ability to walk. Eventually
the muscles in the internal organs also fail so the person dies.
Duchenne muscular dystrophy is more severe. A child with Duchenne muscular dystrophy begins
showing symptoms of loss of muscle function by about age 3 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.
1. Complete the following table 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
2. Explain your reasoning.
3. In a heterozygous individual, the single copy of the normal allele results in enough normal dystrophin
protein to prevent muscular dystrophy. Which allele is recessive: the allele for normal dystrophin or the
allele for defective dystrophin?
4. Muscular dystrophy is an X-linked disorder; the gene for dystrophin is on the X chromosome.
Explain why boys are much more likely than girls to have muscular dystrophy.
6