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Polyribosomes

Role of the Ribosome

Polyribosomes

Clusters of ribosomes may sit on a mRNA and make proteins, each making a strand of polypeptides.  These clusters are called polyribosomes. When they are free in the cytoplasm, they are called free polyribosomes (linked by the mRNA). Or, they may bind to rough endoplasmic reticulum.


Ribosomes are visualized as small (20 X 30 nm) ribonucleoprotein particles. They are formed from two subunits. As you learned in the lecture on the nucleolus , the subunits are produced in the nucleolus in organizing centers on certain chromosomes. The two ribosomal subunits leave the nucleus separately through the nuclear pores . The pores are structured to allow transit of only the subunits. Whole ribosomes are formed outside in the cytoplasm. This prevents protein synthesis from occurring in the nucleus. Why might this be important?

The above photograph shows a group of ribosomes in action. They are connected by a strand of messenger RNA which runs between the large and small subunits. They read the 3 nucleotide code for an amino acid and the appropriate transfer RNA brings the amino acid to the growing polypeptide chain. In this photograph, we see the growing peptide chain radiating at right angles to the mRNA. It extends from the base of the large ribosomal subunit.


#Introduction to the Ribosome-Endoplasmic Reticulum Unit



The left hand view of this cartoon shows the free polyribosomes connected by the mRNA. They are arranged in rosettes and these can be seen in the cytoplasm in conventional electron micrographs. The right hand view shows the arrangement of polyribosomes on the rough endoplasmic reticulum. Note that the growing polypeptide chain (which projects down from the large subunit) is inserted through the membrane and into the cisterna of the rough endoplasmic reticulum. 


This cartoon shows the binding site on the rough endoplasmic reticulum. The membrane of the rough endoplasmic has a receptor that binds the larger subunit of the ribosome. Next to the receptor is a pore that allows newly synthesized proteins to enter and be stored initially in the rough endoplasmic reticulum cisterna or lumen. Note that the ribosomes are still connected to one another outside the rough endoplasmic reticulum by the mRNA which runs between the large and small subunits.


This electron micrograph shows a high magnification of a longitudinal section through the rough endoplasmic reticulum. The electron dense ribosomes are on its outside surface. Inside the sac (cisterna) is flocculent material, the newly synthesized proteins. The details of ribosomal structure cannot be appreciated in this micrograph. They look like small irregular balls on the outside of the membrane. Note that the sacs of rough endoplasmic reticulum are bridged by a junction. This is shown diagrammatically in the following figure.



The cartoon in this figure shows the rough endoplasmic reticulum with a bridge adjoining two sacs. In this way, the sacs communicate and proteins fill the spaces all over the cell. They even communicate with the inside of the nuclear envelope. Recall that the outside membrane of the nuclear envelope is studded with ribosomes and is part of the rough endoplasmic reticulum. An immunocytochemical labeling protocol, such as that found in the above figure, will delineate the reticulum filled with the newly synthesized proteins.


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