Mutualism is where both animal help each other to benefit form each other. Win-Win . Commensalism is where nothing happen neither animal benefits from it .
Because for the strands to form a double helix the base pairs have to match perfectly (adenine with thymine and cytosine with guanine) in DNA , and if a strand of RNA is paired with a split strand of DNA it goes adenine with uracil and cytosine with guanine (uracil replaces thymine)