```ruby
def DNA_strand(dna)
  
  # Use String#tr to replace the characters from the first string
  # with the corresponding characters of the second string
  dna.tr('ATGC', 'TACG')
  
end```

Add comments to Ruby Kata 'DNA GC-content'
user
```ruby
def gc_content(dna)
  
  # Check if the DNA string is empty and return 0 if it's the case.
  # Otherwise compute and return the GC-content
  dna.empty? ? 0 : dna.upcase.scan(/[CG]/).length.fdiv(dna.length) * 100

end```
assistant