SOLVED PROBLEM 1. Using Figure 9-5, show the consequences on subsequent translation of the addition of an adenine base to the beginning of the following coding sequence:

Solution

With the addition of A at the beginning of the coding sequence, the reading frame shifts, and a different set of amino acids is specified by the sequence, as shown here (note that a set of nonsense codons is encountered, which results in chain termination):

–ACG–AUC–GGA–ACC–ACG–UGA–UAA–GCA–

– Thr – Ile – Gly – Thr – Thr – stop – stop

SOLVED PROBLEM 2. A single nucleotide addition followed by a single nucleotide deletion approximately 20 bp apart in DNA causes a change in the protein sequence from

–His–Thr–Glu–Asp–Trp–Leu–His–Gln–Asp–

to

–His–Asp–Arg–Gly–Leu–Ala–Thr–Ser–Asp–

Which nucleotide has been added and which nucleotide has been deleted? What are the original and the new mRNA sequences? (Hint: Consult Figure 9-5.)

Solution

We can draw the mRNA sequence for the original protein sequence (with the inherent ambiguities at this stage):

Because the protein-sequence change given to us at the beginning of the problem begins after the first amino acid (His) owing to a single nucleotide addition, we can deduce that a Thr codon must change to an Asp codon. This change must result from the addition of a G directly before the Thr codon (indicated by a box), which shifts the reading frame, as shown here:

Additionally, because a deletion of a nucleotide must restore the final Asp codon to the correct reading frame, an A or G must have been deleted from the end of the original next-to-last codon, as shown by the arrow. The original protein sequence permits us to draw the mRNA with a number of ambiguities. However, the protein sequence resulting from the frameshift allows us to determine which nucleotide was in the original mRNA at most of these points of ambiguity. Nucleotides that could have appeared in the original sequence are circled. In only a few cases does the ambiguity remain.