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The ribosome consists of a small subunit and a large subunit, which is about three times as big as the small one. The large subunit sits on top of the small one. A chain of m R N A threads between the large and small subunits. A protein chain extends from the top of the large subunit.
A large subunit (top) and a small subunit (bottom) comprise ribosomes. During protein synthesis, ribosomes assemble amino acids into proteins.

Source: OpenStax Biology 2e

OpenStax Biology 2e

Ribosomes are the cellular structures responsible for protein synthesis. When we view them through an electron microscope, ribosomes appear either as clusters (polyribosomes) or single, tiny dots that float freely in the cytoplasm. They may be attached to the plasma membrane’s cytoplasmic side or the endoplasmic reticulum’s cytoplasmic side and the nuclear envelope’s outer membrane. Electron microscopy shows us that ribosomes, which are large protein and RNA complexes, consist of two subunits, large and small. Ribosomes receive their “orders” for protein synthesis from the nucleus where the DNA transcribes into messenger RNA (mRNA). The mRNA travels to the ribosomes, which translate the code provided by the sequence of the nitrogenous bases in the mRNA into a specific order of amino acids in a protein. Amino acids are the building blocks of proteins.

– What is a protein complex that is active in mitochondria and functions as a riboprotein for translating mitochondrial mRNAs encoded in mtDNA?

Because protein synthesis is an essential function of all cells (including enzymes, hormones, antibodies, pigments, structural components, and surface receptors), there are ribosomes in practically every cell. Ribosomes are particularly abundant in cells that synthesize large amounts of protein. For example, the pancreas is responsible for creating several digestive enzymes and the cells that produce these enzymes contain many ribosomes. Thus, we see another example of form following function.

– What is a molecular component that produces quasi-mechanical movements (output) in response to specific stimuli (input)?

By taking a deep dive into the molecular underpinnings of Diamond-Blackfan anemia, scientists from Harvard Medical School and Boston Children’s Hospital have made a new discovery about what drives the development of mature red blood cells from the earliest form of blood cells, called hematopoietic (blood-forming) stem cells. For the first time, cellular machines called ribosomes, which create proteins in every cell of the body, have been linked to blood stem cell differentiation.


Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/details/books/biology-2e