You are going to experiment (on paper) with the somatastatin protein, and see what
happens when mutations occur in the nucleotide sequence coding for it. First, you
need to figure out the DNA code for somatostatin. Fill in this table, using the table of
mRNA codons given in Lab 10, Activity B. (Where there is a choice of codons for a
particular amino acid, choose any one you wish.) The first two have been done for
you. Fill in the blanks labled DNA, mRNA, and Amino Acid for 3-14.
DNA
mRNA
Amino Acid
1 (example) GCT
CGA
GCU ala
2 (example) GGA
CCT
GGA gly
3. Cys
TGC
ACG
UGC Cys
4. Lys
AAA
UUU
AAA Lys
5. Asn
AAT
UUA
AAU Asn
6. Phe
TTT
AAA
UUU Phe
7. Phe
TTT
AAA
UUU Phe
8. Trp
TGG
ACC
UGG Trp
9. Lys
AAA
UUU
AAA Lys
10. Thr
ACT
UGA
ACU Thr
11. Phe
TTT
AAA
UUU Phe
12. Thr
ACT
UGA
ACU Thr
13. Ser
AGT
UCA
AGU Ser
14. Cys
TGC
ACG
UGC Cys
NOTE: The DNA strand on the right-hand side is transcribed into RNA.
Remember: the amino acids are laid out on your screen in a straight line, to make
them easier to work with. In reality, somatostatin has a three-dimensional structure,
so a change in an amino acid will also result in a change in physical shape.
Now proceed to mutate your molecule, as instructed below.
Mutation 1
Sometimes mistakes occur in nucleotide pairing in DNA, caused by chemical
mutagens (pesticides or inhaled smoke, for example). An A-T pair may become
reversed to T-A, or a C-G pair might be substituted for an A-T pair.
Assume that something like this has happened in the last DNA triplet (position 14)
in the code for somatostatin.
Alter the DNA triplet at random and then determine what happens to the mRNA and
thus to the amino acid sequence.
1.What do you think will happen to the somatostatin as a result of your
mutation?
If the codon is changed or mutated such that a C-G pair is substituted for the correct
A-T pair that is normally found in the third position, the degeneracy of the code will
prevent any change in the overall resulting protein. The codon UGC will also code
for Cys and the protein will not be changed.
2.What happens to the mRNA and thus to the amino acid sequence?
Again, the protein will not be changed or altered in any way because two different
codons will code for the same amino acid.
3.What kind of mutation is illustrated by the experiment? Explain your
answer.
A point mutation results from the substitution of one nucleotide for another.
Mutation 2
Another kind of mutation that can occur results from damage to DNA (from highspeed radiation, for example). When the damaged DNA is repaired, an extra base
pair (A-T or G-C) may have been added to the molecule.
Add an extra pair (A-T or G-C) following asparagine in somatostatin.
1.What happens to the remaining amino acids downstream (to the right) of
the mutation?
They will all be changed by a base shift of one amino acid.
2.What do you think will happen to the somatostatin as a result of your
mutation?
A base shift is a more serious mutation in many cases than a simple point mutation,
as all the following amino acids will be affected by the change in shift. This may
result in a premature stop codon that terminates the protein or otherwise alters the
ultimate conformation of he protein.
3.What kind of a mutation have you illustrated here? Explain your answer.
This is an example of a base shift mutation, where the reading frame is shifted by
one base pair.
Critical Thinking Questions
1.At transcription, only one of the strands of DNA is involved. Why do you
think there is a need for a double helix? Wouldn't a single chain (as in mRNA)
be enough?
Two strands of DNA are needed in double helix to conform to the semi-conservative
rules of replication; that is, one strand acts as the template for the new strand so
that it may be copied from the other.
The four 'bases' are needed for humans to survive so for the nucleotides to be kept
safe, the DNA needs a good barrier; it uses the double helix to keep the consent
bombardment of particles from destroying the DNA. If the DNA was using only one
strand then it would be easier to destroy, having the double helix allows the DNA to
have less surface area to be targeted by mutagens. The double helix protects the
nucleotides leaving less space to be mutated or otherwise affected.
2.Mutations lead to the production of aberrant proteins. Why is this a
problem?
The new protein may not perform the same or similar function as the protein that
should have been in its place.
3.What is a mutagen? Give at least two specific examples of the different types
of mutagens.
A mutagen is a chemical or other material that changes the genetic material (DNA).
Cigarette smoke is one powerful mutagen, as is ultraviolet light (sunlight).
4.Are mutations always deleterious? How do you know this?
In many cases, the answer is yes, because mutagens are often also carcinogens;
however, mutations also lead to the diversity seen in all life as we know it. The
mutations lead to small changes that over time may lead to new and likely improved
species. We, as humans are but one example of this, as we are evolved from a simian
ancestor.
5.What is the difference between the effects of a somatic cell mutation and a
germ cell mutation?
Mutations in germ cells will likely be passed on to future generations. In contrast,
somatic mutations generally only affect the current generation. Nevertheless,
mutations in cancer genes may occur in patients when there are mutations in protooncogenes leading to carcinogenesis.