The mRNA AUG GAA UGG CAU UAA is read from the first base, and the amino acids are added until a…
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The mRNA AUG GAA UGG CAU UAA is read from the first base, and the amino acids are added until a stop codon is reached. The codons stand for: AUG methionine (Met), GAA glutamic acid (Glu), UGG tryptophan (Trp), CAU histidine (His), UAA stop. Further codons: UGA stop, AAU asparagine (Asn), GGC glycine (Gly), AUU isoleucine (Ile).
- How many amino acids are in the normal protein?
- The codon UGG becomes UGA by a point mutation. How many amino acids does the protein have now?
- The first G of the codon GAA is deleted, and the sequence is read again from the start. How many of the amino acids that are read now differ from the normal protein?
Need a reminder? The key idea
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. AUG GAA UGG CAU UAA.
Plan. Read the codons in order, and stop at a stop codon.
Step 1. (a) Met, Glu, Trp and His come before UAA. So the normal protein has amino acids.
Step 2. (b) The new sequence is AUG GAA UGA CAU UAA. Met and Glu are read, and then UGA is a stop codon. The protein has amino acids.
Step 3. (c) After the deletion, the bases are AUG AAU GGC AUU AA. The amino acids are Met, Asn, Gly and Ile. Met is the same as in the normal protein. Asn, Gly and Ile replace Glu, Trp and His, so amino acids differ. The last two bases AA do not make a codon, and no stop codon is read.
Answer: amino acids; amino acids; amino acids.
Check. A nonsense change shortens the protein. A frameshift changes every amino acid after the deletion.
Watch out. A frameshift can also remove the stop codon from the reading, so the protein may become longer.
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