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Understanding Operons in Prokaryotes: The Lac Operon Mechanism

Operons are groups of genes in prokaryotes that are transcribed as a single unit, enabling efficient regulation of gene expression. The lac operon, a well-studied example, is controlled by a promoter and an operator. When lactose is absent, a repressor protein binds to the operator, preventing transcription and protein production. In contrast, presence of lactose binds to the repressor, altering its shape and allowing transcription to occur. This mechanism facilitates the breakdown of lactose into galactose and glucose, providing a survival advantage for the cell.

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Understanding Operons in Prokaryotes: The Lac Operon Mechanism

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  1. Operons in Prokaryotes

  2. Operons • Group of genes that are transcribed together • They are turned ON or OFF as a unit to produce mRNA • Promoter • RNA polymerase binding site; begins transcription • Operator • Controls access of RNA polymerase to genes • Genes • Genes that need to be transcribed for protein(s)

  3. lac operon • Transcription is started repressor protein is removed from the operator • Lactose breakdown • Lactose  galactose + glucose • WHY would a cell want to do this?

  4. How does lac operon work? • If lactoseis NOT present in cell • operon is “off” • the repressor protein is attached to the operator, which means • No transcriptionof genes • No translation of mRNA • No breakdown of lactose

  5. How does lac operon work? • If lactoseis PRESENT in cell: • Lactose binds to repressor, which changes its shape, making the protein inactive • Remember: Allosteric inhibitor???? • Operon is “on” because the repressor protein is removed from the operator, which means • Transcriptionof genes • Translationof mRNA • Breakdownof lactose

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