Question

Which of the following best explains why only pre-mRNA is recognized and receives a 5′ cap?

1) The enzyme that initiates the capping step is known to associate with RNA polymerase II, which generate mRNAs.
2) Only pre-mRNAs contain proper sequence for the cap to be added on.
3) The tail of the pre-mRNA can recruit the right combination of enzymes for capping.
4) Nuclear pore complex only recognize pre-mRNA to be allowed out to the cytoplasm for capping process to begin.
5) rRNA and tRNAs do not exit the nucleus to receive the cap via enzyme sin the cytoplasm.

Answer 1

Only pre-mRNA receives a 5′ cap because the capping enzyme associates with RNA polymerase II, distinct from enzymes that process other types of RNA. The cap protects mRNA from degradation and is essential for ribosomal recognition during translation.

Step-by-step explanation:

The reason that only pre-mRNA is recognized and receives a 5′ cap is primarily due to the association of the capping enzyme with RNA polymerase II, which is responsible for synthesizing mRNAs. This process occurs co-transcriptionally, meaning as the pre-mRNA is still being synthesized. Other RNAs, like pre-rRNAs and pre-tRNAs, are processed differently and do not receive a 5' cap. The 5' cap provides protection against degradation and also serves as a recognition point for the ribosome to initiate translation. Once elongation is completed, the pre-mRNA undergoes further processing, like 3' polyadenylation and splicing, before being considered mature mRNA and transported out of the nucleus to participate in protein synthesis.