micro
Messenger RNA strands(mRNA)
microRNA(abbreviated miRNA)about 22 nucleotides long
Blocked ribosome
Argonaute proteins
microRNA
microRNA duplex
Base mismatch
Hairpin pri-microRNA(primary micro RNA)
nucleus
Exportin-5
1
2
An enzyme called dicer(not shown) trims thepri-microRNA and removes the hairpin loop,leaving a double stranded microRNA duplexmolecule.
A protein called exportin-5 transportsa hairpin primary microRNA (pri-miRNA)out of the nucleus.
microRNA-protein complex
2½
Meanwhile, one of the strands joins agroup of proteins, forming anmicroRNA-protein complex. The otherstrand, known as a passenger strandis usually discarded.How this all happens is still notvery well understood.
Endonucleolyticcleavage
3' untranslated region(3' UTR)
3' Poly-A tail
3' UTR
3' Poly-A tail
Coding region
4
In animal cells, the microRNA nucleotidestypically don't pair up with the mRNAnucleotides as well. Their base pairing oftenfollows a pattern though.
Seed Region (Nucleotides 2–8)Perfect base pairing
Nucleotide 1Has an A across from it
Nucleotide 9Has an A or U across from it
5
The microRNA-protein complex's presenceblocks translation as well as speeding updeadenylation(breakdown of the Poly-A tail),which causes the mRNA to be degraded soonerand translated less.
Deadenylation
Nucleotides 13–16Good base pairing
Base mismatches
5'
3'
3'
3'
5'
Protein
Passenger strand
RNA
s
formation
and
function
the
of
3
In plant cells, the microRNA isusually perfectly complementaryto its target mRNA molecule.The microRNA will bond with it,and cause the mRNA tobreak down.