What is added to the 5' end of eukaryotic mRNA to facilitate its export from the nucleus?

The addition of a 5' cap to eukaryotic mRNA is a crucial modification that occurs during the transcription process. This cap consists of a modified guanine nucleotide that is added to the 5' end of the mRNA molecule shortly after transcription begins. The presence of the cap serves several important functions.

Firstly, the 5' cap protects the mRNA from degradation by exonucleases, which are enzymes that can break down RNA molecules. Without this protective cap, the mRNA would be more susceptible to being degraded in the nucleus before it even reaches the cytoplasm.

Secondly, the 5' cap is essential for the proper export of mRNA from the nucleus. It is recognized by the nuclear export machinery, ensuring that the mature mRNA is transported efficiently through the nuclear pore complexes into the cytoplasm, where translation can occur.

Lastly, the 5' cap plays a vital role in the initiation of translation. Ribosomes recognize and bind to the 5' cap, facilitating the recruitment of the translation machinery needed to begin protein synthesis.

In contrast, other possible answers do not serve the same purpose in mRNA processing and export. For example, an adenine tail (often referred to as a poly-A tail) is