(Science: molecular biology) single stranded rNA molecule that specifies the amino acid sequence of one or more polypeptide chains. This information is translated during protein synthesis when ribosomes bind to the mRNA.
In prokaryotes, mRNA is normally formed by splicing a large primary transcript from a dNA sequence and protein synthesis starts while the mRNA is still being synthesised. Prokaryote mRNAs are usually very short lived (average t 1/2 is 5mins.).
In contrast, in eukaryotes the primary transcripts (hnRNA) are synthesised in the nucleus and they are extensively processed to give the mRNA that is exported to the cytoplasm where protein synthesis takes place.
This processing includes the addition of a 5' 5' linked 7 methyl guanylate cap at the 5' end and a sequence of adenylate groups at the 3' end, the poly A tail, as well as the removal of any introns and the splicing together of exons, only 10% of hnRNA leaves the nucleus.