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Cells need to make proteins. Proteins are necessary for many things like fighting viruses, structural support for cells, and movement among other things. When you need to make proteins, you look for ribosomes. Ribosomes are the protein builders or the protein synthesizers of the cell. They are like construction guys who connect one amino acid at a time and build long chains called polymers.

Ribosome's occur both as free particles in prokaryotic and eukaryotic cells and as particles attached to the membranes of the endoplasmic reticulum in eukaryotic cells. Ribosome's can vary in size, but the average ribosome consists of about 40 percent protein and 60 percent RNA. Ribosomes are usually made up of three or four RNA molecules and anywhere from 40 to 80 different proteins.

A ribosome is not just one piece. There are two pieces or subunits. Scientists named them 60-S (large) and 40-S (small). When the cell needs to make protein, mRNA is created in the nucleus. The mRNA is then sent into the cell and the ribosomes. When it is time to make the protein, the two subunits come together and combine with the mRNA. The pieces lock onto the mRNA and start the protein synthesis.

The 60-S/ 40-S model works fine for eukaryotic cells. Prokaryotic cells have ribosome's made of 50-S and 30-S subunits. It's a small difference, but one of many you will find in the two different types of cells. Scientists have used this difference in ribosome size to develop drugs that can kill prokaryotic microorganisms that cause disease.

The process of making proteins is quite simple. We just explained that mRNA is made in the nucleus and sent into the cell. The mRNA then combines with the ribosome subunits. Another nucleic acid lives in the cell - tRNA, which stands for transfer RNA. tRNA is bonded to the amino acids floating around the cell. With the mRNA offering instructions, the ribosome connects to a tRNA and pulls off one amino acid. Slowly the ribosome makes a long amino acid chain that will be part of a larger protein.

Ribosomes are the best because ribosome's contain proteins which are important for cell growth and reproduction. If a cell had no ribosome's, it would produce no protein and would die very quickly, as the cell needs proteins for all functions, including metabolism and making more DNA.

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