In general, mRNAs targeted by siRNAs are cleaved by RISC because:

• A. the siRNA binds to the mRNA and recruits DNA polymerase for degradation
• B. the siRNA is perfectly complementary to the target mRNA, allowing RISC to cleave the mRNA
• C. the siRNA causes the mRNA to be exported from the nucleus and degraded in the cytoplasm
• D. the siRNA modifies the mRNA cap structure, leading to its cleavage

Answer: (B)

Verified step by step guidance

1. Understand the role of siRNA (small interfering RNA) in gene silencing: siRNAs guide the RNA-induced silencing complex (RISC) to target specific mRNA molecules for degradation.
2. Recall that RISC uses the siRNA as a guide strand to find complementary sequences on the target mRNA.
3. Recognize that the key to mRNA cleavage by RISC is the perfect or near-perfect base pairing between the siRNA and the target mRNA, which allows RISC to precisely cut the mRNA.
4. Note that DNA polymerase is not involved in this process, as it functions in DNA replication, not RNA degradation.
5. Conclude that the mechanism involves siRNA binding perfectly to the mRNA, enabling RISC to cleave the mRNA directly, leading to its degradation and preventing translation.