After a frameshift, the codons that are read are almost random, and each one has a chance of 3 in…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
After a frameshift, the codons that are read are almost random, and each one has a chance of 3 in 64 to be a stop codon.
- On average, after how many codons does the first stop codon come, counting the stop codon itself?
- What is the chance that none of the first 30 codons is a stop codon?
- What is the chance that a stop codon appears within the first 30 codons?
- A frameshift happens near the start of a gene of 300 codons. Is the polypeptide likely to be as long as the normal one?
Need a reminder? The key ideaShow
The repressor works in trans, on every operator in the cell. An operator or a promoter works in cis, on the genes next to it only. A change of one base can be silent, missense or nonsense. The third base is silent in about 69 per cent of the changes, and the second base in none. After a frameshift, a stop codon comes after about codons on average.
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. A chance of for each codon.
Plan. Use the waiting time for a chance, and the power of a chance.
Step 1. (a) codons.
Step 2. (b) .
Step 3. (c) .
Step 4. (d) Probably not. The first stop is expected after about 21 codons, so the polypeptide would end far before codon 300.
Answer: ; ; ; no, it would be short.
Check. The answer to (c) is large, so a short polypeptide is the usual result.
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