Identifying Polysomes
- Translation can occur simultaneously at multiple positions along mRNA
- Polysomes (or polyribosomes) are groups of two or more ribosomes translating the same mRNA transcript
- Multiple copies of the same polypeptide chain can be made simultaneously from a single mRNA transcript
- Polysomes effectively increase the amount of polypeptide produced
- In electron micrographs, polysomes look like ‘beads on a string’ with each bead representing a ribosome
- There are visible differences between eukaryotes and prokaryotes
- In prokaryotes, the lack of a nucleus means transcription and translation are coupled
- Translation starts before the mRNA has finished being transcribed from the DNA
- On an electron micrograph, multiple polysomes can appear on growing mRNA strands along the DNA molecule
- In eukaryotes, mRNA is transported out of the nucleus prior to translation
- On an electron micrograph, polysomes are seen on the mRNA with no involvement of DNA
- As ribosomes move in the same 5’ to 3’ direction along the mRNA, ribosomes towards the 3’ end have longer polypeptide chains being synthesised
Electron micrograph of prokaryotic polysomes, the image shows simultaneous transcription and translation of a bacterial gene
Electron micrograph of eukaryotic polysomes, polypeptide chains can be seen emerging from the ribosomes