Question

A certain segment of DNA can be used as a molecular clock. Its rate of mutation is one mutation per 10 million years. Examine the DNA segments from two different species:

Species A: CTTAAGCTAGTAAGGACC
Species B: CATAAGTTAGTAAGGTCC

Using this example, explain how this information can be used to determine how long ago these two species shared a common ancestor.

Answer 1

I think the people who wrote the problem want you to count the number of differences between the two DNA strands. If there is one mutation every 10 million years, then the time since divergence = number.differences * 10 million.

That said, I do not like this question. To use that method you would need to know the sequence of the original strand of DNA.

Point 1. The same mutation in the same base of both evolved sequences would mask genuine genetic change.

Point 2. Multiple mutation in the same base are possible. These subsequent mutations could return the base to its initial value, or it could simply replace it with yet a third base.

So while the theory is, loosely, "more difference between sequences, diverged longer ago," the actual computational methods are a bit more interesting.

Answer 2

The approch remains same when we are using computaional method.
scoring matrices are used to calculate divergance. method is calibrated by combining with other methods. Its just complicated.