Did a quick play around with Exercism and some reading.
Here's my solution for RNA Transcription in Python:
def to_rna(dna):
dna_rna_trans = {'G': 'C', 'C': 'G', 'T': 'A', 'A': 'U'}
rna_strand = []
dna_list = list(dna)
for dna_nucleotide in dna_list:
if dna_nucleotide not in dna_rna_trans:
return ''
else:
rna_strand.append(dna_rna_trans[dna_nucleotide])
return ''.join(rna_strand)