corrects it if it is not the right one. There are enzymes that can replace the wrong nucleotide with the right one. Dimers of thymine can be excised and repaired as well. The DNA is sealed up again with DNA ligase.
OPERONS AND GENE REGULATION Because all cells contain every possible gene in an organism, there needs to be a mechanism for some genes to be expressed and others to be repressed, depending on conditions in the environment or on the cell with multicellular organisms. There are regulatory genes that specifically turn on or turn off the different structural genes. For microorganisms, this regulation allows the organism to survive under different circumstances. There are differences and similarities in how prokaryotes and eukaryotes undergo gene regulation. Prokaryotes but not eukaryotes have a block of genes with related functions called operons. They are usually transcribed together. This allows the entire operon to be regulated at the same time. The best studied operon is the lac operon, which has a single lac promotor region. Operons are not seen in eukaryotes but some of the same principles exist in eukaryotic genes. There is a regulatory region that includes a promotor region, which binds to certain transcription factors. These transcription factors can influence the ability of RNA polymerase to bind to the promotor region. Transcription factors can be activators, repressors, or inducers. Activator and repressors will increase or decrease the transcription of a gene. An inducer can interact with an activator or repressor. It is a small molecule that can increase or decrease transcription along with the activator or repressor. The repressor will bind to the operator, blocking attachment of RNA polymerase. Activators bind to the promotor site to increase transcription. Figure 48 describes an operon and how it works:
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