Final answer:
Before mRNA leaves the nucleus, it must be modified by adding a 5' cap and a 3' poly-A tail, and by splicing to remove introns. These modifications are essential for mRNA stability, export, and protein synthesis.
Step-by-step explanation:
Before messenger RNA (mRNA) can leave the nucleus, it must be properly processed and modified. This process includes three main steps: the addition of a 5' methylguanosine cap, the addition of a 3' poly-A tail, and splicing, where introns are removed and exons are reconnected. The 5' cap and poly-A tail are critical for the stability and export of mRNA from the nucleus, as well as for subsequent translation. Splicing ensures that only the necessary coding sequences, or exons, are included in the mature mRNA molecule.
Splicing distinguishes introns, which are non-coding regions removed from pre-mRNA, from exons, which are coding regions retained in the final mRNA product. The remaining exons can be reconnected in different sequences in a phenomenon known as alternative gene splicing, which allows a single gene to code for multiple proteins. In summary, mRNA must remove certain bases (introns) through splicing before it can exit the nucleus and participate in protein synthesis.