def gc_content(seq: str) -> float:
    """计算 DNA 序列的 GC 含量（百分比）。"""
    seq = seq.upper()
    gc = sum(1 for base in seq if base in "GC")
    return gc / len(seq) * 100


if __name__ == "__main__":
    example = "ATGCGCGATTACGCGGCTA"
    result = gc_content(example)
    print(f"序列: {example}")
    print(f"长度: {len(example)}")
    print(f"GC 含量: {result:.2f}%")