You know that a particular stretch of DNA changes at the rate of 1 base per 10 million years.You sequence this length of DNA in two species and find that they differ by 4 bases: AATGCGATCG and AATGTTATGC.Based on these data,how long has it been since these two species diverged? (Hint: Ancestral sequence is AATGCTATGG.)

Question

Quizplus · Accepted Answer

The two species differ by 4 bases out of a sequence length of 10. Therefore, the rate of change is 4/10 = 0.4 substitutions per site. 
Since the rate of change is 1 substitution per 10 million years, then the time it takes for 0.4 substitutions is 0.4 * 10 million years = 4 million years.
To find the time since these two species diverged, we need to calculate the total number of substitutions from the ancestral sequence to each of the two species. 
From the ancestral sequence AATGCTATGG, the first species has 1 substitution (CG to T) and the second species has 3 substitutions (CG to T, A to T, and T to G). 
The total number of substitutions from the ancestral sequence is therefore 1 + 3 = 4. 
Since each substitution takes 4 million years, the time since these two species diverged is 4 * 4 = 16 million years.
Therefore, the best choice is B, 20 million years.

You Know That a Particular Stretch of DNA Changes at the Rate