python find last occurrence in string of "DNA" code example

Example: how to find the last occurrence of a character in a string in python

original_string = "sentence one. sentence two. sentence three"

last_char_index = original_string.rfind(".")
new_string = original_string[:last_char_index] + "," + original_string
print(new_string)
"sentence one. sententce two, sentence three"