Understanding:
• Translation can occur immediately after transcription in prokaryotes due to the absence of a nuclear membrane
In eukaryotes, the ribosomes are separated from the genetic material (DNA and RNA) by the nucleus
- After transcription, the mRNA must be transported from the nucleus (via nuclear pores) prior to translation by the ribosome
- This transport requires modification to the RNA construct (e.g. 5’-methyl capping and 3’-polyadenylation)
Prokaryotes lack compartmentalised structures (like the nucleus) and so transcription and translation need not be separated
- Ribosomes may begin translating the mRNA molecule while it is still being transcribed from the DNA template
- This is possible because both transcription and translation occur in a 5’ → 3’ direction
Skill:
• Identification of polysomes in electron micrographs of prokaryotes and eukaryotes
A polysome (or a polyribosome) is a group of two or more ribosomes translating an mRNA sequence simultaneously
- The polysomes will appear as beads on a string (each 'bead' represents a ribosome ; the ‘string’ is the mRNA strand)
- In prokaryotes, the polysomes may form while the mRNA is still being transcribed from the DNA template
- Ribosomes located at the 3’-end of the polysome cluster will have longer polypeptide chains that those at the 5’-end
Polysomes
