(linked amino acids)
messenger RNA (mRNA) are used. These are RNA copies of genetic information stored by the DNA and are made by a process called transcription. The messenger RNA molecules carry information from the genome to the cytoplasm, where the information is used by the ribosomes to synthesize proteins. In eukaryotes, mRNA is not made directly. Instead, transcription yields precursor RNA molecules (pre-mRNA) that must be processed, to produce the actual mRNA as detailed in Ch. 12.
The DNA that carries the primary copy of the genes is present as gigantic molecules, each carrying hundreds or thousands of genes. In contrast, any individual messenger RNA molecule carries only one or a few genes' worth of information. Thus, in practice, multiple short segments of DNA are transcribed simultaneously to give many different messenger RNA molecules. In eukaryotes, each mRNA normally carries only a single gene, whereas in prokaryotes, anywhere from one to a dozen genes may be transcribed as a block to give an mRNA molecule carrying several genes, usually with related functions (Fig. 3.18).
Translation is the synthesis of proteins using genetic information carried by messenger RNA. Proteins consist of one or more polymer chains known as polypeptides. These are made from subunits called amino acids. Translation thus involves transfer of information from nucleic acids to an entirely different type of macromolecule. This decoding process is carried out by ribosomes. These submicroscopic machines read the messenger RNA and use the information to make a polypeptide chain. Proteins, which make up about two-thirds of the organic matter in a typical cell, are directly responsible for most of the processes of metabolism. Proteins perform most of the enzyme reactions and transport functions of the cell. They also provide many structural components and some act as regulatory molecules, as described below.
Proteins are made by a subcellular machine, the ribosome, that uses the genetic code to read information encoded by nucleic acids.
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