Genes are usually portrayed as coding for proteins, and a simple schematic of this relationship is given by Figure 4-10. This shows the modular nature of the coding logic, but it is not entirely accurate. Genes code for polypeptides—strings of amino acids. Polypeptides are not mature proteins. Functional proteins often comprise several polypeptides—from the same gene (homomultimers) or different genes (heteromultimers). Its shape gives a protein its ability to interact with some other specific compound, but a polypeptide does not fold into this shape without help, and other enzymes (that each must itself be coded by a gene) fold, modify, repair, protect, transport the polypeptide, and/or complex it with other polypeptides, to form the active protein. Many active proteins have signal peptides or other domains that must be enzymatically cleaved for the protein to be activated (or secreted from the cell, and so on, as needed in each case).
This processing machinery largely consists of other proteins, which means that, like the TFs that regulate the expression of a protein, these processing enzymes are also the product of genes. In general, although much of the information for a protein's function is in a sense contained in its amino acid code, a gene does not really code for a protein.
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