Final answer:
Mature siRNA annealed to its target usually leads to translational repression when the siRNA is perfectly complementary to the target mRNA, allowing for cleavage by the RISC complex.
Step-by-step explanation:
Mature siRNA annealed to its target usually leads to translational repression primarily when the siRNA is perfectly complementary to the target mRNA sequence. This perfect complementarity allows the RNA-induced silencing complex (RISC) to bind and cleave the mRNA, preventing its translation into protein. In contrast, microRNAs (miRNAs), which are often only partially complementary to their target mRNAs, generally lead to translational repression without degradation of the mRNA.
In the absence of complete complementarity, such as with miRNAs or imperfectly matched siRNAs, the target mRNA may not get cleaved but can still experience translational repression. This is due to the fact that, even without perfect complementarity, the formation of partial double-stranded RNA structures can interfere with the translation process. However, for the case of siRNAs, where perfect complementarity is expected, it's this precise matching that usually triggers the degradation of the target mRNA by the RISC complex.
It's noteworthy that the presence of a 5' cap or an Argonaute binding site on the target mRNA often play roles in the efficiency of RNA interference, but the chosen answer focuses on the degree of complementarity between the siRNA and its mRNA target, which is a critical factor determining the outcome of the interaction.