A piece of DNA is 40960 base pairs long…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A piece of DNA is base pairs long. It is cut by three enzymes, one with a site of bases, one with a site of bases, and one with a site of bases. Assume that every base is equally likely at each place, so that a site turns up by chance at an equal chance everywhere.
- How many sites of the -base enzyme do you expect?
- How many sites of the -base enzyme do you expect?
- How many sites of the -base enzyme do you expect?
Need a reminder? The key ideaShow
A restriction endonuclease cuts both strands inside its palindromic recognition sequence. A site of bases turns up by chance about once in bases. A straight DNA with sites gives fragments. A circular DNA with sites gives fragments.
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. Length bp. Sites of , and bases.
Plan. A site of bases turns up about once in bases. The expected number of sites is the length divided by .
Step 1. (a) . sites.
Step 2. (b) . sites.
Step 3. (c) . sites.
Answer: sites; sites; sites.
Check. The longer the site, the fewer the sites. A value below means that the enzyme will often not cut such a piece at all.
Watch out. This is an average. Real DNA is not random, so real counts differ.
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