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GENE EXPRESSION and the LAC OPERON We have about 42 000 genes inside our DNA that codes for proteins. Obviously not all the proteins are needed at the same time. Cells have developed methods to control the transcription and translation of genes, depending on their needs. HOUSEKEEPING GENES are always needed and are therefore constantly being transcribed and translated. TRANSCRIPTION FACTORS turn genes on when required. Gene regulation is important for organism survival and it can occur at four levels: TRANSCRIPTIONAL : regulates which genes are transcribed (DNA to mRNA) or regulates the rate of transcription POSTTRANSCRIPTIONAL: removing the introns of the mRNA before it goes off to translation. TRANSLATIONAL: controls how often and how fast mRNA transcripts are converted into proteins. POSTTRANSLATIONAL: controlling the amount of time before a protein becomes functional. THE lac OPERON Lactose is milk sugar and is broken down by E. coli in the intestines of mammals. BETA GALACTOSIDASE is the enzyme responsible for the breaking down of lactose. The bacteria have a NEGATIVE CONTROL SYSTEM that blocks the production of beta- galactosidase if lactose is NOT present. The system works by inhibiting TRANSCRIPTION of the gene that codes for the beta-galactosidase protein. The beta-galactosidase gene is part of an OPERON. An operon is on a prokaryotic DNA molecule. It contains a PROMOTER, OPERATOR and some structural genes. The promoter and operator do not code for protein but are important in the REGULATION of the genes' transcription. The lac operon consists of a promoter, operator and three genes (lac Z, lac Y, and lac A) which code for the proteins and enzymes that break down lactose. A REPRESSOR protein, called lacI, binds to the operator and prevents the genes for beta- galactosidase from being transcribed (blocks RNA polymerase). The promoter region and the operator region overlap (share nucleotides) so when lacI binds to the operator it is essentially blocking the promoter site where RNA POLYMERASE wants to bind. In the lac operon system lactose is known as a signal molecule or an INDUCER. When it is present it binds to the lacI protein which pulls it away from the operator site. This frees up the promoter site for RNA polymerase and allows for the proper TRANSCRIPTION of the genes responsible for lactose degredation. THE trp OPERON Tryptophan is an amino acid used by E. Coli to make protein. The trp operon is another example of co-ordinated regulation. It contains 5 genes that make the three enzymes responsible for synthesizing tryptophan. While the lac operon has transcription induced when lactose is present (in order to make enzymes to break it down), the trp operon is repressed when high levels of trp are present (no need to make any more). The EFFECTOR, the cause of the gene being repressed, is trp. Trp is called a CO-REPRESSOR. When trp levels are high, trp binds to an inactive repressor protein making it ACTIVE. This repressor-trp COMPLEX then binds to the operator site and prevents the gene from being transcribed. When levels of trp drop it RELEASES from the repressor protein and the repressor protein disengages from the operator so that the trp genes can be transcribed.
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