M03.04.05 Polysomes: Structure and Function

Polysomes, or polyribosomes, are clusters of ribosomes that simultaneously translate a single messenger RNA (mRNA) molecule. This mechanism enhances the efficiency of protein synthesis, allowing for the rapid production of proteins in cells.

Structure of Polysomes

Messenger RNA molecules are relatively long compared to ribosomes, which enables multiple ribosomes to translate the same mRNA strand simultaneously.

Key Features:

• Translation Direction: Ribosomes translate mRNA in the 5′ to 3′ direction.
• Peptide Length: The ribosome closest to the 3′ end of the mRNA will have the longest nascent peptide chain, as it is the last to begin translation.

Location of Polysomes

Polysomes can be found in two primary locations within the cell:

1. Free in the Cytoplasm: Polysomes are present in the cytosol, synthesizing proteins that function within the cytoplasm.
2. Attached to the Rough Endoplasmic Reticulum (RER): When polysomes are bound to the RER, they typically translate proteins destined for secretion or for membrane insertion.

Points to Remember

• Polysomes increase the efficiency of protein synthesis by allowing multiple ribosomes to translate the same mRNA molecule simultaneously.
• The ribosome closest to the 3′ end of the mRNA has the longest peptide chain.
• The location of polysomes (free vs. attached to RER) indicates the type of protein being synthesized.

Bibliography

1. Alberts, B., et al. (2015). Molecular Biology of the Cell. 6th ed. Garland Science.
2. Lodish, H., et al. (2016). Molecular Cell Biology. 8th ed. W.H. Freeman and Company.
3. Campbell, N. A., & Reece, J. B. (2017). Biology. 11th ed. Pearson Education.
4. Karp, G. (2013). Cell and Molecular Biology: Concepts and Experiments. 7th ed. Wiley.