Wanted to clarify some stuff about transcription Units

Both the 5' cap and polyA tail share the same dual function. Both are recognized by a complex of proteins that protects the mRNA from nucleolytic degradation and promotes binding of the mRNA by ribosomes.

The 5' GTP cap and the poly-A tail that are added to immature mRNA during processing, before the mRNA leaves the nucleus. They serve three major purposes:

(A) The 5' cap protects the mRNA from degradation by RNase, and enzyme that degrades unprotected mRNA.

(B) The 5' cap also serves as a point of attachment to the ribosome during translation.

(C) The 3' poly A tail offers protection from exonucleases (enzymes that degrade nucleotide sequences) and assists the mRNA strand in leaving the nucleus.

Without either of these structures added to mRNA, it would have difficulty leaving the nucleus and would be attacked and degraded by enzymes in short order. In other words, genes would never be translated and eukaryotic life as we know it would be impossible.

5' cap helps protect mRNA from degradation by hydrolytic enzymes and after mRNA reaches the cytoplasm, the 5' cap functions as part of an "attach here" sign for ribosomes. The poly-A-tail inhibits degradation of RNA and helps ribosomes attach and facilitates export of mRNA from the nucleus.

They both function in protecting the mRNA and also helping the ribosome find the right place to begin translation. The 5' cap consists of a modified guanosine triphosphate. When the mRNA leaves the nucleus, the 5' cap is like an identification that tells the nucleus the mRNA is able to leave the nucleus. It also prevents the work of enzymes that degrade mRNA. As for the Poly A tail, it gradually is degraded in the cytosol, but if it is long enough and the mRNA can reach the ribosome, the designated protein should be translated. Thus, the poly A tail acts kind of like a wall; enzymes gradually wear away at it trying to reach the rest of the mRNA. The poly A tail also helps the ribosome orient itself with regards to the mRNA during translation.

The 5' cap serves several functions: it aids in recognition of the mRNA by the ribosome and protects the mRNA from degradation by exonucleases. mRNA's without the cap are regarded as foreign and are degraded. Influenza steals caps from cellular mRNA's so that they are not degraded.

The poly-A tail prevents degradation by exonucleases and is involved in termination of transcription. The tail also has roles in translation.