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Selasa, 27 Januari 2009

Ribosomes

In Journey into the Cell, we looked at the structure of the two major types of cells: prokaryotic and eukaryotic cells. Now we turn our attention to the protein assemblers of a eukaryotic cell, the ribosomes.

Ribosomes are organelles that consist of RNA an proteins. They are responsible for assembling the proteins of the cell. Depending on the protein production level of a particular cell, ribosomes may number in the millions.

Distinguishing Characteristics:

Ribosomes are typically composed of two subunits: a large subunit and a small subunit. Ribosomal subunits are synthesized by the nucleolus. These two units join together when the ribosome attaches to messenger RNA to produce a protein in the cytoplasm.


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Location in the Cell:

There are two places that ribosomes usually exist in the cell: suspended in the cytosol and bound to the endoplasmic reticulum. These ribosomes are called free ribosomes and bound ribosomes respectively. In both cases, the ribosomes usually form aggregates called polysomes.

Free ribosomes usually make proteins that will function in the cytosol, while bound ribosomes usually make proteins that are exported or included in the cell's membranes. Interestingly enough, free ribosomes and bound ribosomes are interchangeable and the cell can change their numbers according to metabolic needs.

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