Ribosome: 3D Model
This is a 3D computer graphic model of a ribosome. Ribosomes are composed of protein and RNA. They consist of subunits that fit together and work as one to translate mRNA (messenger RNA) into a polypeptide chain during protein synthesis (translation). Credit: Callista Images/Cultura/Getty Images

There are two major types of cells: prokaryotic and eukaryotic cells. Ribosomes are cell organelles that consist of RNA and 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 in the nucleolus and cross over the nuclear membrane to the cytoplasm through nuclear pores. These two subunits join together when the ribosome attaches to messenger RNA (mRNA) during protein synthesis. Ribosomes along with another RNA molecule, transfer RNA (tRNA), help to translate the protein-coding genes in mRNA into proteins. Ribosomes link amino acids together to form polypeptide chains, which are further modified before becoming functional proteins.

Location in the Cell:

There are two places that ribosomes usually exist within a eukaryotic 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 or polyribosomes during protein synthesis. Polyribosomes are clusters of ribosomes that attach to a mRNA molecule during protein synthesis. This allows for multiple copies of a protein to be synthesized at once from a single mRNA molecule.

Free ribosomes usually make proteins that will function in the cytosol (fluid component of the cytoplasm), while bound ribosomes usually make proteins that are exported from the cell 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.

Organelles such as mitochondria and chloroplasts in eukaryotic organisms have their own ribosomes. Ribosomes in these organelles are more like ribosomes found in bacteria with regard to size. The subunits comprising ribosomes in mitochondria and chloroplasts are smaller (30S to 50S) than the subunits of ribosomes found throughout the rest of the cell (40S to 60S).

Ribosomes and Protein Assembly

Protein synthesis occurs by the processes of transcription and translation. In transcription, the genetic code contained within DNA is transcribed into an RNA version of the code known as messenger RNA (mRNA). In translation, a growing amino acid chain, also called a polypeptide chain, is produced. Ribosomes help to translate mRNA and link amino acids together to produce a polypeptide chain. The polypeptide chain eventually becomes a fully functioning protein. Proteins are very important biological polymers in our cells as they are involved in virtually all cell functions.

Eukaryotic Cell Structures

Ribosomes are only one type of cell organelle. The following cell structures can also be found in a typical animal eukaryotic cell: