CappingA cap is added to the 5′ end. It goes on as the new RNA comes out of RNA polymerase.
SplicingThe introns are cut out and the exons joined.
PolyadenylationA tail of adenine nucleotides is added to the 3′ end. The transcript is cut there first.
On the pre-mRNA 7
5′ capA modified guanine on the 5′ end. It shields that end from enzymes that break down RNA.
Poly-A tailA stretch of adenine nucleotides on the 3′ end.
ExonA stretch that stays in the finished mRNA. The exons are joined when the introns come out.
IntronA stretch that is cut out of the pre-mRNA. It is not in the finished mRNA.
Pre-mRNAThe RNA as first transcribed, introns and all. It is also called the primary transcript.
SpliceosomeThe machine that cuts out introns and joins exons. It is built from small RNAs and many proteins.
Cut-out intron (lariat)The intron leaves as a loop with a tail. The loop is called a lariat.
The finished mRNA 5
Mature mRNAThe finished mRNA, made only of exons. It has its cap and tail and is ready to leave the nucleus.
Coding sequenceThe stretch that is translated into protein. It runs from the start codon to a stop codon.
5′ untranslated regionThe stretch between the cap and the start codon. It is not translated into protein.
3′ untranslated regionThe stretch between the stop codon and the tail. It is not translated into protein.
Nuclear poreAn opening through the nuclear envelope. The finished mRNA leaves the nucleus through it.
How this list was compiled: Every name checked on 2026-10-01 in at least two places, for facts only and with nothing copied: Kimball's Biology Pages (Gene Expression: Transcription; Gene Translation; The Nucleus); Wikipedia, text only (Post-transcriptional modification, Five-prime cap, Polyadenylation, RNA splicing, Mature messenger RNA, Untranslated region, Messenger RNA); Nature Education, Scitable (RNA Splicing: Introns, Exons and Spliceosome; Translation: DNA to mRNA to Protein); and the US National Human Genome Research Institute glossary (Exon, Intron, Messenger RNA). Licence: own work
The drawing: Drawn for this site on 2026-10-01 from plain geometry; nothing traced. Licence: own work
How it works
Label the steps in eukaryotic mRNA processing on one chart: capping, splicing and polyadenylation turn a pre-mRNA into mature mRNA inside the nucleus. Read the names first, then press Quiz me.
Read the full guide
Read the chart from top to bottom
The top strand is the pre-mRNA as it was transcribed: three exons joined by two introns, with its 5′ and 3′ ends marked. A cap goes on the 5′ end and a poly-A tail on the 3′ end. A spliceosome holds each intron in a loop, cuts it free as a lariat and joins the exons on either side.
What leaves the nucleus
The finished mRNA is a coding sequence with an untranslated region on each side, between the cap and the tail. It leaves the nucleus through a nuclear pore.
The steps overlap in the cell: the cap goes on first, and splicing can begin while the gene is still being transcribed.
Questions and answers
What are the three steps of mRNA processing?
Capping, splicing and polyadenylation. A cap is added to the 5′ end, the introns are cut out and the exons joined, and a poly-A tail is added to the 3′ end.
What is the difference between pre-mRNA and mature mRNA?
Pre-mRNA is the RNA as first transcribed, with its introns still in it. Mature mRNA has a 5′ cap and a poly-A tail, and its introns are gone, so it is made only of exons.
Are introns or exons removed during splicing?
Introns are removed. The exons on either side are joined and stay in the mature mRNA.
Where does mRNA processing take place?
In the nucleus of a eukaryotic cell. The finished mRNA then leaves through a nuclear pore to the cytoplasm, where it is translated into protein.
What does the spliceosome do?
It cuts the introns out of a pre-mRNA and joins the exons. It is a large complex of small RNAs and proteins.