There are 61 codons that code for amino acids, and each can change in 3 × 3 ways by a change of one…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
There are 61 codons that code for amino acids, and each can change in ways by a change of one base. The list gives the results of a count of all of them.
- First base: 8 silent, 166 missense, 9 nonsense.
- Second base: 0 silent, 176 missense, 7 nonsense.
- Third base: 126 silent, 50 missense, 7 nonsense.
- How many single-base changes are there in all?
- What per cent of them are silent?
- What per cent of the changes of the third base are silent?
- A mutagen makes 1000 random single-base changes in sense codons. How many nonsense changes do you expect?
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. The table of counts.
Plan. Add, and then find shares.
Step 1. (a) Each row adds up to . So changes. Also .
Step 2. (b) Silent: . The share is .
Step 3. (c) .
Step 4. (d) Nonsense: . The share is . In 1000 changes, .
Answer: ; ; ; .
Check. The silent, missense and nonsense counts, , add up to .
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