1. In python, Write a function called 'is_dna' that takes a single character string as an argument and returns 'True' if the argument is a valid dna nucleotide and 'False' otherwise. 2. Write a function called 'is_valid_dna' that takes a string as an argument and returns 'True' if the string is valid DNA and 'False' otherwise. For example, if the string is 'tagtacta' the function should return 'True', but if the string is 'tagtaxta' the function should return 'False'. This function should call the 'is_dna' function you created for problem 2. Write a program to prompt the user to enter a string and respond back whether or not the string is valid DNA.