Each gene codes for a protein, and transcription is the first step of gene expression. Most protein synthesis occurs in organelles known as ribosomes, which are located outside of the nucleus where DNA is stored. To relay information to a ribosome, the cell makes a copy of the relevant gene from DNA and sends that copy out of the nucleus. The copy is called a messenger ribonucleic acid, or mRNA. Like DNA, mRNA is made of the same nucleobases, except for one: it does not contain thymine [T], but instead contains uracil [U]. That means that the complement of [A] in mRNA is [U]. As such, the rules of complementation in mRNA are as follows: [A] becomes [U]
[T] becomes [A]
[C] becomes [G]
[G] becomes [C]

Requried:
Create a function that gives the complement of a base, and then write another function that uses it iteratively over a whole strand.