mRNA encodes the amino acid sequence of the polypeptide specified by a gene. The function of mRNA is the same in all cells, but there are some differences in the synthesis and structure of prokaryotic and eukaryotic mRNA.
1) In bacteria, mRNA is transcribed and translated in the single cellular compartment, i.e, there is no spatial separation of the two processes. These two process in bacteria are closely linked and occur simultaneously. Translation starts, and ribosomes attach to 5′ end of bacterial mRNA, even before the transcription is complete. Bacterial mRNA is usually unstable, and translated into proteins for only a few minutes.
2) In a eukaryotic cell, however, mRNA transcription and translation occur in nucleus and cytoplasm, respectively. Thus the two process are spatially separated. The synthesis and maturation of mRNA occur exclusively in the nucleus. After these events, the mRNA is exported to the cytoplasm, where it is translated. Eukaryotic mRNA is highly stable and continues to be translated for several hours. However, many unstable mRNAs are known in eukaryotes.
The rate of transcription and translation in bacteria are similar. At 37°C, mRNA transcription occurs at a rate of approximately 40 nucleotides/second, which is very close to the rate of protein synthesis, roughly 15 amino acids/second. Thus it takes around 2 minutes to transcribe and translate an mRNA of 5000 bp, corresponding to an 80 kDa protein.