The normal (wild-type) version of a particular protein includes a stretch of ami

Question

The normal (wild-type) version of a particular protein includes a stretch of amino acids as follows:

MET-TRP-VAL-ASP

A mutant arose in this strain as a result of treatment with a mutagen known to change, delete or add single bases. A new amino acid sequence of the mutant is now:

MET-MET-GLY-STOP

Answer the following questions by referring to the attached genetic code

. a) Show the probable sequence(s) for mRNA which codes for this sequence of amino acids in the wild-type.

b) Show the new mRNA sequence in the mutant.

c) State specifically what mutational event the mutagen caused.

Explanation / Answer

So from the genetic code we can read off which codons could be involved ..............................AUG--UGG--GUU--GAU GUC GAC GUA GUG Mutant stretch is.....................................................MET--MET--GLY--STOP So its codons could be any of...................................AUG--AUG--GGU--UGA GGC UAA GGA UAG Comparing the two we see that the first change is the addition of a new 4th base - an A - in the mutant. This shifts the reading frame along. As all of the 3 stop codons begin with a U we know that the wildtype VAL codon must therefore have been GUU not the other 3 possibilities and that the STOP codon in the mutant is UGA ANSWERS A. Thus the wild-type stretch was coded by..............AUG--UGG--GUU--GA(U or C) B. The mutant stretch was coded by........................AUG--AUG--GGU--UGA C. An addition of an A base after the 59th codon [enda] 2. The transcribed portion of a gene that codes for a structural protein is 1462 nucleotides in length. The hnRNA molecule that is produced contains i) a 5' untranslated sequence of 18 nucleotides; ii) 3 introns,of 250,100 and 330 nucleotides length; and iii) an untranslated 3' region of 92 nucleotides. A poly-A tail of 250 nucleotides is added to the mRNA (assume it is added to the end of the molecule).

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