Complement & Reverse Complement

Two Strands, One Molecule

DNA is double-stranded. The two strands are complementary — each base pairs with its partner:

• A pairs with T
• T pairs with A
• G pairs with C
• C pairs with G

The complement of ATCG is TAGC.

But DNA strands run in opposite directions (antiparallel). To get the sequence of the other strand in the conventional 5'→3' direction, you take the complement and reverse it — the reverse complement:

def complement(seq):
    table = {"A": "T", "T": "A", "G": "C", "C": "G"}
    return "".join(table[b] for b in seq)

def reverse_complement(seq):
    return complement(seq)[::-1]

print(complement("ATCG"))          # TAGC
print(reverse_complement("ATCG"))  # CGAT

When AlphaGenome receives a DNA input, it analyzes both strands — the sequence and its reverse complement. Regulatory elements can sit on either strand.

Your Task

Implement complement(seq) and reverse_complement(seq).