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Ribosome with large ribosomal subunit (red) and small ribosomal subunit (blue). The rRNAs in the large subunit (red) and the ribosomal proteins (pink) are shown. rRNA in the small subunit (dark blue) and ribosomal proteins (light blue) are also shown.

Credit: Vossman


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 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 by the nucleolus. 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.

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 or polyribosomes during protein synthesis.

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.

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. Ribosomal RNA helps to link amino acids together to produce the polypeptide chain. The polypeptide chain undergoes several modifications before becoming 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:
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