This module looks at the mechanics involved in translating the information encoded in an mRNA molecule into a polypeptide. The components of the translation (protein synthesis) machinery will be introduced, and the process of protein synthesis will be outlined.
Translation of the genetic code involves using the genetic information to produce a polypeptide. Therefore, translation is synonymous with protein synthesis. Proteins, or to be more precise, polypeptides, are linear chains of amino acids. (You don't need to know all of the amino acids, but you should work to become more familiar with them.) As outlined in the module on the genetic code, genetic information encoded in DNA, and transferred to mRNA, is used to determine the sequence of amino acids in a polypeptide. The actual synthesis of polypeptides is carried out by ribosomes.
Protein synthesis is one of the most important processes in a cell, since most cellular functions are mediated by proteins. Therefore, cells contain many ribosomes to ensure that they are able to synthesize enough protein. Bacteria contain approximately 10,000 ribosomes, and eukaryotic cells contain over 50,000 ribosomes. Amphibian eggs, which are highly specialized single cells, contain more than a million ribosomes (!) because a tremendous amount of protein synthesis occurs after fertilization.
Ribosomes are composed of two subunits, one large and one small. The intact ribosome, with both subunits resent, is called a monosome. Each of the subunits is a complex of RNA and protein. The specific type of RNA used in ribosomes is called ribosomal RNA, or rRNA. There are four specific types of rRNA in eukaryotes, and these are designated by their size: 28S, 18S, 5.8S, and 5S. ('S' refers to the Svedberg unit, which is a unit of relative size of a molecule, based on sedimentation as a result of centrifugation. Svedberg units are not additive; they are merely relative measurements of size.) Of the ribosomal RNAs, the 28S, 5.8S and 5S are found in the large subunit, and the 18S is found in the small subunit. Subunit sizes and general makeup of the eukaryotic ribosomal subunits are shown in the following figure:
Prokaryotic ribosomes are constructed in a similar fashion, although the sizes are somewhat different. The prokaryotic monosome is 70S, and is made of a 50S large subunit and a 30S small subunit. The 50S subunit is composed of 23S rRNA (analogous to the eukaryotic 28S), 5S rRNA (analogous to eukaryotic 5.8S), and 31 ribosomal proteins. The 30S subunit is composed of 16S rRNA (analogous to eukaryotic 18S) and 21 ribosomal proteins. In all, the differences between eukaryotic and prokaryotic ribosome constituents (rRNA sizes, number of proteins) simply reflects an increase in complexity of the eukaryotic ribosome over its prokaryotic counterpart.
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