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DNA stores the genetic blueprint that powers cellular functions. In eukaryotes, where DNA resides inside a nucleus, the instructions must be transferred to the cytoplasm via messenger RNA (mRNA). Once transcribed, the nascent mRNA undergoes a series of enzymatic edits that add essential features, signaling that the molecule is ready for translation.
The first modification common to all eukaryotic mRNAs is the 5′ cap. As RNA polymerase III synthesizes the transcript, the 5′ end is subsequently modified by a trio of enzymes that attach a 7‑methylguanylate group. This cap not only protects the RNA from exonucleases but also serves as a recognition signal for ribosomes and export machinery.
At the opposite end, the 3′ terminus is appended with a poly‑A tail by poly(A) polymerase. Typically, 100–250 adenosine residues are added, a feature that enhances mRNA stability and facilitates its export from the nucleus.
While bacterial mRNAs lack both a 5′ cap and a poly‑A tail, eukaryotic transcripts rely on these structures to regulate nuclear export, translational initiation, and RNA longevity. The added modifications create a robust framework that ensures only properly processed mRNAs reach the ribosome.
Viruses that infect eukaryotic cells often hijack the host’s translation machinery. For instance, polioviruses encode proteases that cleave the host eIF4G protein, a component essential for ribosome recruitment to capped mRNAs. Consequently, cellular mRNAs are silenced, allowing the virus’s uncapped RNA to dominate protein synthesis—a clever exploitation of the host’s own regulatory system.
