Final answer:
Eukaryotic pre-mRNAs are modified by adding a 5' methylguanosine cap and a 3' poly-A tail for protection and nuclear export, and by splicing out introns to join exons into the mature mRNA sequence.
Step-by-step explanation:
Eukaryotic pre-mRNAs undergo several critical modifications before being exported to the cytoplasm. The three major modifications are the addition of a 5' cap, the addition of a 3' poly-A tail, and splicing of introns.
The first modification is the addition of a 5' methylguanosine cap, which occurs shortly after the initiation of transcription and is important for mRNA stability and proper exit from the nucleus. The second major modification is the addition of a poly-A tail at the 3' end of the mRNA, which is a chain of adenine nucleotides that further protects the mRNA from degradation and assists in its export from the nucleus. Lastly, splicing occurs where non-coding sequences, known as introns, are removed from the pre-mRNA, and the coding sequences, known as exons, are joined together to form the mature mRNA sequence that will be translated into protein. The final mRNA transcript is then exported from the nucleus to the cytoplasm for translation into protein.