Translation is the first step in the system by which cells build proteins. It is part of the overall procedure that allows the expression of genes contained in the DNA and is critical for the continuation of cellular life. The entire process is made possible by an extremely small cellular organelle known as the ribosome.
All living cells need to make proteins. Some proteins provide the amino acids necessary for the growth and repair of tissue, while some function as enzymes or provide support for other cellular functions. To make proteins, a cell needs ribosomes. Ribosomes are tiny organelles that function to synthesize or build proteins. They connect one amino acid to another, forming long strings of protein known as polypeptide chains.
Where they're found
Ribosomes are generally thought of as organelles. But unlike other organelles, they are not bound by a membrane and are much smaller than average, requiring the use of an electron microscope to see them. They are found many places in the cell. Some ribosomes may be found floating in the cellular cytoplasm; some are bound to the nuclear envelope or the endoplasmic reticulum. It all depends on whether the cell is animal, plant or bacteria. Endoplasmic reticulum with ribosomes attached takes on a bumpy appearance and is called rough endoplasmic reticulum. You can find an abundance of rough endopoasmic reticulum in cells that are very active in protein synthesis, such as those in the brain and the pancreas. Those attached ribosomes are the ones that make the proteins to be used inside the cell, as well as the proteins that will be transferred out of the cell.
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Ribosomes are not one solid piece; they are made of two pieces or subunits. In eukaryotic cells (those with membrane bound organelles), the larger unit of the ribosome is called the 60-S and the smaller is called the 40-S. The pieces are slightly smaller for prokaryotic cells. When protein synthesis is not underway, the two sub-units of the ribosome remain apart. When the cell needs to make a protein, it creates a strand of messenger RNA (mRNA) in the nucleus. The mRNA is then sent out of the nucleus and into the cell in search of the ribosomes. To make the protein, the two sub-units of the ribosome join together and combine with the mRNA. The completed ribosome locks onto the mRNA and begins the process of protein synthesis.
The procedure for making a protein is fairly straightforward. Messenger RNA is created in the nucleus and sent out into the cell. The mRNA provides the template for building a specific protein. While the mRNA is passing through the ribosome offering instructions (or being translated), transfer RNA (tRNA) brings the necessary building blocks--the amino acids. There are three different places on the ribosome for the tRNA to attach, which allows the ribosome to pull off the necessary amino acid and add it to the chain. One amino acid at a time, the ribosome creates a growing chain that will become part of a finished protein.
Once the protein backbone of amino acids is complete, the ribosome will reach what is known as the "stop codon." This tells the ribosome to cease translation. The ribosome will then release the protein, and it will either be transported out of the cytoplasm or taken to the Golgi apparatus for packaging. Once completed, the proteins created by the ribosome can be used in a number of ways, such as damage repair or building new structures.