Final answer:
The two main types of posttranscriptional modifications in eukaryotic mRNA are splicing, and 5' capping coupled with 3' polyadenylation, which are essential for mRNA stability and translation.
Step-by-step explanation:
The two main types of posttranscriptional modifications that take place in the mRNA of eukaryotes are splicing and 5' capping and 3' polyadenylation. After the primary mRNA transcript is synthesized, it must undergo several modifications before it becomes mature mRNA that can be translated into proteins. The splicing process involves the removal of non-coding sequences known as introns; the remaining coding sequences, called exons, are then joined together. The addition of the 5' cap, which is a modified guanine nucleotide, protects the mRNA from degradation and assists with ribosome binding during translation. Polyadenylation, the addition of a poly-A tail at the 3' end of the mRNA, also aids in protecting the mRNA molecule and regulates its export from the nucleus and its lifespan in the cytoplasm.