A coding sequence has 36 bases. It is read from the first base in groups of three…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A coding sequence has bases. It is read from the first base in groups of three.
- How many codons does the sequence have?
- Three bases are inserted near the start. How many complete codons are there now?
- One base is deleted near the start. How many bases are left over after the last complete codon?
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. bases.
Plan. Divide the number of bases by . The remainder is what is left over.
Step 1. (a) codons.
Step 2. (b) After inserting bases there are bases. complete codons. The frame is kept.
Step 3. (c) After deleting base there are bases. , so there are complete codons and bases are left over. The frame has shifted.
Answer: codons; codons; bases.
Check. An insertion of bases adds one codon, as the sums show.
Watch out. A shifted frame changes every codon after the mutation, not only the one near it.
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