A DNA sequence can be represented as a string of the letters

A DNA sequence can be represented as a string of the letters ACTG (short for adenine, cytosine, guanine, and thymine).

(a) How many DNA sequences are exactly 20 letters long?

(b) Given a DNA sequence of length 20, how many single letter mutations are possible?

(c) Given a DNA sequence of length 20, how many double letter mutations are possible?

Solution

(a) DNA sequences are exactly 20 letters long = 4 *20 = 80

(b) DNA sequence of length 20, no. of single letter mutations are possible = 20( 20 -1) = 380

(c) DNA sequence of length 20, no. of double letter mutations are possible = (20 * (20-1) /2) * 9

= 1710