EasyBiologyHigh School (11–12)Heredity and Evolution › Principles of Inheritance and VariationNumericalAPA-LevelIB
The mRNA codon GAG stands for glutamic acid…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
The mRNA codon GAG stands for glutamic acid.
- How many different single-base substitutions are possible in one codon?
- How many of the nine changes of GAG are silent?
- How many of the nine changes of GAG are missense?
Only GAA and GAG stand for glutamic acid. The stop codons are UAA, UAG and UGA. Every other codon stands for a different amino acid.
Need a reminder? The key ideaShow
A codon has 3 bases, and each base can change to any of 3 others, so one codon has 9 possible substitutions. A change is silent if the amino acid stays the same, missense if it changes, and nonsense if the new codon is a stop codon. The insertion or the deletion of a number of bases that is not a multiple of 3 shifts the reading frame.
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. The codon GAG, glutamic acid.
Plan. List the changes place by place, and sort them.
Step 1. (a) places new bases changes.
Step 2. The list. First place: AAG (Lys), CAG (Gln), UAG (stop). Second place: GCG (Ala), GGG (Gly), GUG (Val). Third place: GAA (Glu), GAC (Asp), GAU (Asp).
Step 3. (b) Only GAA codes for the same amino acid, so change is silent.
Step 4. (c) Of the other , UAG is a stop codon (nonsense). The remaining give a different amino acid, so changes are missense.
Answer: changes; silent; missense.
Check. (silent, missense, nonsense).
Watch out. The silent change is in the third place. The third base is the one that most often changes nothing.
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