Lecture 3

The Ribosome, rRNA and mRNA

Major players in protein synthesis: mRNA, tRNA and the ribosome

mRNA	Messenger RNA, a copy of DNA blueprint of the *gene* to be expressed.	Information
tRNA	Aminoacyl transfer RNA, also called *anticodon* or *adaptor* molecule. One or more tRNAs for each amino acid.	Supply
Ribosome	A very large complex of several rRNAs (ribosomal RNA) and many protein molecules. Total molecular weight almost 3 million dalton.	Factory
Protein	Polypeptide chain with sequence dictated by the mRNA sequence. Also called the *gene product*.	Product

The Ribosome

Polysome

Ribosomes can be found either free in the cytosol (cytoplasm) or attached to intracellular membranes.

Free ribosomes

Found in the cytosol.
May occur as a single ribosome or in groups known as polyribosomes or polysomes.
Occur in greater number than bound ribosomes in cells that retain most of their manufactured protein.
Responsible for proteins that go into solution in the cytoplasm or form important cytoplasmic structural or motile elements.

Bound ribosomes

Found bound to the exterior of the endoplasmic reticulum (ER) constituting the rough ER.
Occur in greater number than free ribosomes in cells that secrete their manufactured proteins (e.g., pancreatic cells, producers of digestive enzymes).
Responsible for proteins that insert into membranes or are packaged into vesicles for storage in the cytoplasm or export to the cell exterior.

Ribosomes attached to rough ER

Electronmicrograph of ribosomes (black dots) attached to the rough endoplasmic reticulum.

About 15,000 ribosomes in a single E. coli cell, comprising ~25% of the dry cell mass.
All ribosomes within one cell are identical.
All ribosomes are composed of two subunits (called small and large).
A substantial fraction of ribosomes are dissociated into free subunits in the cell.
Prokaryotic and eukaryotic ribosomes differ in composition.
Mitochondrial ribosomes resemble prokaryotic ribosomes.

Next: The Composition of Ribosomes

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