Transcribe and translate EACH of the three following DNA gene sequences. Turn th

Question

Transcribe and translate EACH of the three following DNA gene sequences. Turn these in using the graphic organizer. The letters represent the DNA N-base sequences of the genes.

The first is the original gene. The next two are mutations of the original. Note that the changes (mutations) are shown in red.

Original Gene sequence 3'-T A C C C T T T A G T A G C C A C T-5

Mutated gene sequence 1 3’-T A C G C T T T A G T A G C C A T T-5'

Mutated gene sequence 2 3’-T A A C C T T T A C T A G G C A C T-5’

Also, answer the following questions:

What is the significance of the first and last codons of an mRNA transcript?
What meaning do these mRNA codons have for protein synthesis?
Did the mutations result in a change in the final proteins? If so, describe the change.
In general, why might a change in amino acid sequence affect protein function?

Explanation / Answer

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Original gene:

duplex DNA:
5'-A T G G G A A A T C A T C G G T G A-3'
3'-T A C C C T T T A G T A G C C A C T-5'

mRNA:
5'-A U G G G A A A U C A U C G G U G A-3'

Protein:
Met-Gly-Asn-His-Arg-stop(opal)



Mutated gene 1:

duplex DNA:
5'-A T G C G A A A T C A T C G G T A A-3'
3'-T A C G C T T T A G T A G C C A T T-5'

mRNA:
5'-A U G C G A A A U C A U C G G U A A-3'

Protein:
Met-Arg-Asn-His-Arg-stop(ochre)



Mutated gene 2:

duplex DNA:
5'-A T T G G A A A T G A T C C G T G A-3'
3'-T A A C C T T T A C T A G G C A C T-5'

mRNA:
5'-A U U C C A A A U G A U C C G U G A-3'

------> No AUG start codon <------
Protein (assuming there is a start codon upstream of this sequence):
Ile-Pro-Asn-Asp-Pro-stop(opal)
If there is no AUG upstream of this sequence, then there is no protein synthesized.

 

 

What is the significance of the first and last codons of an mRNA transcript?
-- The first codon must be AUG; it signals the start of translation. The last codon has to be a stop codon, either UAA, UAG or UGA that signals the ribosome to get off the mRNA and thus stop the translation. If there is no stop codon, the ribosome will not dissociate from the mRNA, thus stalling translation.

What meaning do these mRNA codons have for protein synthesis?
-- The start codon is used with the recruitment of the methionine tRNA initial codon that recruits the ribosomal small subunit leading to the start of protein synthesis. The stop codons break the protein synthesis thus terminate translation that leads to the release of the synthesized protein.

Did the mutations result in a change in the final proteins? If so, describe the change.
-- The protein sequences are different at a few locations. The protein sequence of mutant #1 has an Arginine residue at position #2 and a different stop codon. Although the stop codon is different, the translation will still stop as there are a total of three possible stop codons.
-- The protein sequence of mutant #2 has mutations at residues 1,2,4 and 5. So this is a different protein. Most important is the initiation codon is no longer AUG. Missing this initiation codon leads to NO PROTEIN synthesis. To obtain a protein, there will have to be an AUG codon upstream of this mRNA sequence; otherwise, there will be no protein.

In general, why might a change in amino acid sequence affect protein function?
-- A change in the amino acid change may lead both to a different protein conformation and to the presentation of inappropriate functional groups for proper function of the protein. As a result, the protein may no longer function properly. The sequence determines both the fold and the chemical functional groups distributed in this fold. These chemical groups provide the lining of the active site of an enzyme; the binding sites; and the other biologically relevant areas of the protein. 

 

/update #1 -- corrected typos

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