Clarify expected i/o of Molecules Cells

subjects/molecules-cells/README.md

@@ -4,8 +4,8 @@
 
 Write two functions:
 
-- `RNA` that given a DNA strand it must return is complement RNA
-- `DNA` that given a RNA strand it must return is complement DNA (invert RNA)
+- `RNA` that given a DNA strand it must return is complement RNA strand
+- `DNA` that given a RNA strand it must return is complement DNA strand (invert RNA)
 
 Since both strands are a sequence of nucleotides. It will be given the four nucleotides and its complements
 
@@ -16,3 +16,5 @@ DNA | RNA
  T  -  A
  A  -  U
 ```
+
+Each strand must be represented as upper case string, without spaces, ex: `'ATCG'` is a valid DNA strand.