the 5' 7-methylguanosine cap

Think of a sealed wax stamp on the front of an important letter. It tells the recipient three things at once: this is genuine, it is meant for you, and it is not damaged. The 5' cap is that stamp placed on the front end of a eukaryotic mRNA — a small chemical badge that marks the message as real and ready to be read.

Chemically, the cap is a single modified guanine nucleotide stuck onto the very first base of the RNA, but joined in an unusual backwards way. Normally nucleotides link 5'-to-3'; here the capping guanine is attached 5'-to-5' through a bridge of three phosphates, and that guanine carries an extra methyl (CH3) group on position 7 of its ring — hence 7-methylguanosine, often written m7G. This odd 5'-to-5' linkage is the key: it leaves no ordinary free 5' end for an enzyme to grab. The cap is added very early, after only about 25 to 30 bases have been transcribed, while RNA polymerase is still working.

The cap earns its keep in several ways. It protects the mRNA's front from exonucleases — enzymes that chew RNA from a free 5' end — so a capped message lives far longer. It is the docking signal that the ribosome's small subunit uses to find and load the mRNA before translation begins. And it helps the mature mRNA get exported from the nucleus and helps the spliceosome remove the first intron. An uncapped mRNA in a eukaryotic cytoplasm is treated as garbage and quickly destroyed.