The 5' cap is a modified guanine nucleotide (7-methylguanylate) added to the 5' end of a eukaryotic mRNA during RNA processing. It protects the mRNA from breakdown, helps export it from the nucleus, and lets the ribosome recognize and translate it.
The 5' cap is one of the modifications eukaryotic cells make to a freshly transcribed mRNA before it can be used. Specifically, it's a 7-methylguanylate (7mG) nucleotide attached to the very front (the 5' end) of the transcript through an unusual 5'-to-5' triphosphate bridge. That backwards linkage is part of what makes the cap special: it doesn't look like the rest of the RNA chain, so enzymes that chew up loose RNA ends can't grab it.
The cap gets added co-transcriptionally, meaning it's slapped on while RNA polymerase II is still building the rest of the transcript. So a pre-mRNA gets capped early, then goes on to be spliced and given a poly-A tail before it leaves the nucleus. Once the mRNA reaches the cytoplasm, the cap is the part that translation initiation proteins latch onto to load up the ribosome.
In Honors Biology, the 5' cap shows up in Unit 8, Topic 8.1: Transcription and RNA Processing, where you learn how eukaryotic cells turn a raw transcript into a finished, working mRNA. It's one of the three big mRNA processing steps you're expected to know, alongside splicing and adding the poly-A tail. Knowing the cap helps you explain why eukaryotic gene expression has extra steps that prokaryotes skip, and it connects transcription to translation by showing how the ribosome finds the start of the message.
Keep studying Honors Biology Unit 8
Visual cheatsheet
view gallery3' Poly-A Tail (Unit 8)
The cap protects the front of the mRNA and the poly-A tail protects the back. Together they shield both ends from degrading enzymes and mark the transcript as a complete, ready-to-use message.
Translation Initiation (Unit 8)
The cap is the docking site for the proteins that recruit the ribosome. Without a cap, the ribosome can't find the start of the mRNA, so translation never gets going.
pre-mRNA (Unit 8)
Capping is one of the first things that happens to a pre-mRNA. It's the start of turning a raw transcript into mature mRNA, before splicing removes introns.
RNA Polymerase (Unit 8)
Because the cap is added co-transcriptionally, it goes on while RNA polymerase II is still building the strand. The cap and the growing transcript are made at the same time.
On Honors Biology tests, the 5' cap usually appears in questions about mRNA processing. You might get a multiple-choice question asking which modifications happen to a eukaryotic transcript, or a diagram of an mRNA where you label the cap, the poly-A tail, and the coding region. Short-answer and free-response prompts often ask you to explain a function of the cap (protection from breakdown, nuclear export, or starting translation) or to compare prokaryotic and eukaryotic gene expression. Be ready to say what the cap is made of (a modified guanine nucleotide) and at least one job it does.
Both are added during RNA processing and both protect the mRNA, but the cap goes on the 5' (front) end and is a single modified guanine nucleotide, while the poly-A tail goes on the 3' (back) end and is a long stretch of adenine nucleotides.
The 5' cap is a modified guanine nucleotide (7-methylguanylate) added to the front end of a eukaryotic mRNA during RNA processing.
The cap protects the mRNA from exonucleases that would otherwise break down an unprotected end.
It's added co-transcriptionally, meaning it goes on while RNA polymerase II is still making the transcript.
The cap helps the mRNA get exported from the nucleus and is recognized by proteins that start translation.
Capping, splicing, and adding the poly-A tail are the three main RNA processing steps in eukaryotes.
It's a modified guanine nucleotide (7-methylguanylate) attached to the 5' end of a eukaryotic mRNA during RNA processing. It protects the mRNA, helps it leave the nucleus, and lets the ribosome recognize it for translation.
No. Capping is a eukaryotic feature. Prokaryotes don't process their mRNA this way, which is one reason they can begin translation before transcription even finishes.
No. The cap is a single modified guanine nucleotide on the front (5') end, while the poly-A tail is a long string of adenine nucleotides on the back (3') end. Both protect the mRNA, but they're at opposite ends.
The cap uses an unusual 5'-to-5' triphosphate bridge that doesn't look like normal RNA. Exonucleases that chew up loose RNA ends can't grab the capped end, so the message lasts longer.
Right away, co-transcriptionally. The cap goes on the 5' end while RNA polymerase II is still building the rest of the transcript, before splicing and the poly-A tail.