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.

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.

Ribosome structure indicating small subunit (A) and large subunit (B). Side and front view.

1 Head
2 Platform
3 Base
4 Ridge
5 Central protuberance
6 Back
7 Stalk

A tiny organelle that is the site of protein synthesis (protein translation) in the living cell. Ribosomes are complex, bead-like structures composed of about 40% protein and 60% ribosomal RNA (rRNA). In eukaryotes, ribosomes are made of four strands of RNA and are often attached to the membranes of the endoplasmic reticulum to form rough ER. In prokaryotes, they are made of three strands of RNA and occur free in the cytoplasm.

Eukaryote ribosomes are produced and assembled in the nucleolus. Three of the four strands are produced there, but one is produced outside the nucleolus and transported inside to complete the ribosome assembly. Ribosomal proteins enter the nucleolus and combine with the four strands to create the two subunits that will make up the completed ribosome. The ribosome units leave the nucleus through the nuclear pores and unite once in the cytoplasm. Some ribosomes will remain free-floating in the cytoplasm, creating proteins for the cell's use. Others will attach to the endoplasmic reticulum and produce the proteins that will be "exported" from the cell.

Protein synthesis requires the assistance of two other RNA molecules. Messenger RNA (mRNA) provides instructions from the cellular DNA for building a specific protein. Transfer RNA (tRNA) brings the protein building blocks, amino acids, to the ribosome. Once the protein backbone amino acids are polymerized, the ribosome releases the protein and it is transported to the Golgi apparatus. There, the proteins are completed and released inside or outside the cell. For more details of the role played by ribosomes in protein synthesis, see protein translation.