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repressor:see nucleic acidnucleic acid,
any of a group of organic substances found in the chromosomes of living cells and viruses that play a central role in the storage and replication of hereditary information and in the expression of this information through protein synthesis.
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a special regulating protein formed in bacterial cells that halts transcription, which is the synthesis of messenger ribonucleic acid (m-RNA) from a specific operon (a group of genes that carry out the synthesis of functionally related enzymes). The number of different repressors corresponds to the number of operons.
Unlike other proteins, a repressor present in a cell consists of ten to 20 molecules. The synthesis of m-RNA ceases when a repressor combines with an operator, which is the regulating part of an operon. An effector, for example, lactose in a lactose operon, interacts with a repressor to form a complex that inactivates and produces a reversible spatial change in a repressor molecule. This type of repressor can no longer combine with an operator and, as a result, m-RNA synthesis resumes.