Ribosomes are large molecules that are responsible for the production of all the protein that your cells need to perform their specific function. Ribosomes are unique within the cell because they are made up of both protein and a special type of RNA, ribosomal RNA (rRNA). Together, they form the two subunits of a ribosome, called the large and small subunits. They work by accepting messages the DNA, called mRNA, on the small subunit and translating the code into a specific protein by linking together different amino acids on the large subunit. The amino acids, which are the building blocks of proteins, are brought to the ribosome by another type of RNA called transfer RNA (tRNA) that acts as a delivery driver for the amino acids. The amino acids are added to an ever growing chain one by one as shown below:
Ribosomes can be found freely floating within the cell or attached to another organelle called the rough endoplasmic reticulum (RER). Proteins that are made by free floating ribosomes are usually ones that perform their function inside of the cell where as proteins that are made by ribosomes attached to the RER are usually ones that perform their function on the surface of the cell or outside the cell.
Prokaryotes, or bacteria, have a different type of ribosomes then eukaryotes. This comes in handy when trying to develop drugs to target bacteria without damaging other cells. Many antibiotics exploit this difference to kill bacteria by stopping them from making proteins.
In a previous post we talked about how mitochondria have their own DNA, well they also have their own ribosomes that are similar to those seen in bacteria. This is another reason why scientists think the mitochondria in eukaryotes were originally free-living bacteria.