A gel has a ladder of 8000, 4000, 2000, 1000 and 500 base pairs…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A gel has a ladder of , , , and base pairs. The bands lie at , , , and mm from the well. Two unknown bands, U1 and U2, lie at mm and mm.
- Fit a straight line of against the distance, and find the size of U1.
- Find the size of U2.
- Why is a straight line of the size itself against the distance not a good fit?
Need a reminder? The key ideaShow
In a gel, the size of a fragment falls in a straight line against the distance on a log scale. A ring cut at sites gives pieces, and a straight DNA gives . In PCR, copies are made from one molecule in cycles. The Wallace rule is in degrees Celsius.
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 ladder, and the two distances.
Plan. Use the line of the logarithm of the size.
Step 1. (a) The best line through the ladder is . At mm: , so the size is base pairs.
Step 2. (b) At mm: , so the size is base pairs.
Step 3. (c) The size changes by the same factor for the same step in distance. That is an exponential curve, and it is a straight line only on a log scale.
Answer: bp; bp; the size is exponential in the distance.
Check. U1 lies between the 8000 and the 4000 bands and U2 between the 1000 and 500 bands, and the answers lie in those ranges.
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