How does the 5' cap and poly-A tail modify mRNA?

The 5' cap and poly-A tail modify mRNA by protecting it from degradation, aiding in nuclear export, and promoting translation.

The 5' cap and poly-A tail are crucial modifications that occur during the processing of pre-mRNA into mature mRNA in eukaryotic cells. These modifications play a significant role in the stability, nuclear export, and translation of the mRNA.

The 5' cap is added to the 5' end of the mRNA molecule during transcription. This cap is a modified guanine nucleotide and it protects the mRNA from degradation by exonucleases, enzymes that degrade RNA from its ends. The 5' cap also aids in the export of the mRNA from the nucleus to the cytoplasm, and it is involved in the initiation of translation, the process by which the genetic code in the mRNA is read to synthesise a protein.

The poly-A tail is added to the 3' end of the mRNA molecule after transcription. This tail consists of a string of adenine nucleotides, typically several hundred bases long. The poly-A tail protects the mRNA molecule from enzymatic degradation in the cytoplasm. It also enhances the translation efficiency of the mRNA by promoting its circularisation, which facilitates the re-initiation of translation.

In addition to these roles, the length of the poly-A tail can influence the stability and translation efficiency of the mRNA. A longer poly-A tail generally increases both the stability and the translation efficiency of the mRNA. However, the poly-A tail can be gradually shortened over time in a process called deadenylation, which can lead to the degradation of the mRNA or a decrease in its translation efficiency.

In summary, the 5' cap and poly-A tail are essential modifications of mRNA that protect it from degradation, aid in its export from the nucleus to the cytoplasm, and promote its translation into protein. These modifications therefore play a crucial role in the regulation of gene expression in eukaryotic cells.