5. The amino acid sequences below come from the beginning of a protein. Several
ID: 266860 • Letter: 5
Question
5. The amino acid sequences below come from the beginning of a protein. Several different mutants have been identified with changes in this protein.
Codon
1
2
3
4
5
6
7
8
9
Wild type
Phe
Leu
Pro
Thr
Val
Thr
Thr
Arg
Trp
Mutant 1
Phe
Leu
Pro
Thr
Val
Thr
Thr
Arg
Mutant 2
Phe
Leu
Pro
Ser
Val
Thr
Thr
Arg
Trp
Mutant 3
Phe
Pro
Pro
Arg
For each of the mutants (1-3), state what the effect of the mutation will be at the polypeptide level (frameshift, missense nonsense, etc), and in which codon the mutation occurred. Use the genetic code to help you. (3pts)
For each mutant, state what exact change has occurred at the DNA level (what nucleotide has been added, substituted, etc). Specify the exact nucleotide change. (3pts)
6. What differences would you see between a gene that is epigenetically silenced, compared to a gene that is not expressed because it has a mutation? (2pts)
Codon
1
2
3
4
5
6
7
8
9
Wild type
Phe
Leu
Pro
Thr
Val
Thr
Thr
Arg
Trp
Mutant 1
Phe
Leu
Pro
Thr
Val
Thr
Thr
Arg
Mutant 2
Phe
Leu
Pro
Ser
Val
Thr
Thr
Arg
Trp
Mutant 3
Phe
Pro
Pro
Arg
Explanation / Answer
In mutant 1, the 9th codon is mutated to a stop codon.
TGG to TGA
It leads to premature termination of translation.
In mutant 2, the 4th codon has a missense mutation.
ACC to AGC
It leads to the replacement of Thr with Ser.
In mutant 3, there is deletion at the 2nd base position of the 4th codon.
ACG-GUA-ACG ----> AGG-UAG
It leads to the replacement of Thr with Arg and premature termination of translation.
Epigenetic modifications: These modifications alter the recruitment of RNA polymerase to the gene locus by changing the compactness of the chromatin. Ex: Histone modifications and DNA methylation.
Epigenetic modifications do not cause sequence changes in the DNA.
On the other hand, a mutated gene sequence differs from the WT DNA sequence.
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