Final answer:
During mRNA processing in eukaryotes, two structures are added: the 5' methylguanosine cap and the 3' poly-A tail. These structures protect the mRNA from degradation and aid in its export from the nucleus.
Step-by-step explanation:
Eukaryotic pre-mRNAs undergo several processing steps before they can be translated. Two structures added during mRNA processing are the 5' methylguanosine cap and the 3' poly-A tail. The 5' cap is added to the beginning of the mRNA molecule and helps protect it from degradation and export it from the nucleus. The poly-A tail is added to the end of the mRNA molecule and also helps protect it and facilitate its export from the nucleus.