Answer
$4^n$
Work Step by Step
One element of the DNA can be 4 different base. (T,C,A or G)
Therefore there are $4^n=2^{2n}$ different n-element DNA chains
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.