Final answer:
The 7-methylguanosine cap is necessary for an mRNA to leave the nucleus and perform protein synthesis.
Step-by-step explanation:
The 7-methylguanosine cap is necessary for an mRNA to leave the nucleus. It is added to the 5' end of pre-mRNAs to protect the mRNA from degradation and assist in translation. The cap helps in the attachment of mRNA with the ribosome for translation in the cytoplasm. Without the 7-methylguanosine cap, the mRNA molecule would not be able to efficiently leave the nucleus and perform its function in protein synthesis.