RNA Processing

A freshly transcribed eukaryotic RNA, called pre-mRNA, is not ready for translation. It's a mix of exons, the stretches that actually code for protein, and introns, noncoding stretches sitting in between them. A massive RNA-protein machine called the spliceosome recognizes short marker sequences at each intron's boundaries, cuts the intron out, and stitches the flanking exons together. The looped-out intron is degraded, and what's left is a continuous coding message with no gaps.

Before that mRNA leaves the nucleus, it gets two protective modifications. A 5' cap, a modified guanine nucleotide, is added to the front end almost as soon as transcription starts; it protects the mRNA from being chewed up by enzymes and later helps the ribosome find the start of the message. At the other end, a poly-A tail, a long stretch of adenine nucleotides, gets added after the transcript is cleaved; it also resists degradation and helps the mRNA stay stable long enough to be translated many times. Only a capped, tailed, fully spliced mRNA is allowed to exit through a nuclear pore.

Splicing isn't always done the same way twice. Alternative splicing lets a cell keep some exons and drop others from the same pre-mRNA transcript, so one gene can produce several different mature mRNAs and therefore several different proteins, depending on which exons end up in the final message. This is a big reason why a genome with a limited number of genes can still build a far larger number of proteins, and it links straight into the next idea: regulation decides which version gets made in which cell.