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Bio103-05%20genetics-XD

Bio103-05%20genetics-XD - STUDY GUIDE 5 Bio 103-General...

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1 STUDY GUIDE 5 Bio 103-General Microbiology Microbial Molecular Genetics-I: Gene Regulation Central Dogma of Biology transcription translation DNA RNA Protein replication reverse transcription Regulation of bacterial chromosome replication The genome of E. coli is replicated bidirectionally from a single origin ( ori C). The origin initiates a replication cycle, controls the frequency of initiation events and segregate replicated chromosome to daughter cells. Bacteria have two links between replication and cell growth: (a) The frequency of initiation of cycles of replication is adjusted to fit the rate at which the cell is growing. (b) The completion of a replication cycle is connected with division of the cell. Regulation of gene expression by transcription Differences in transcription in prokaryotes and eukaryotes Transcription regulation in prokaryotes: (1) regulation by the sigma factor A wholesale reorganization of transcription in bacterial in response to environmental stress (heat shock, nitrogen starvation, sporulation) is carried out by changing the sigma factor of the RNA polymerase. Sigma factor is involved only in transcription initiation. It is released from the core enzyme when RNA synthesis has been initiated. Sigma factor introduces a major change in the affinity of RNA polymerase for DNA and confers the ability to recognize specific binding sites (promoters). bacterial promoter consensus sequence for general use ( σ 70 ): -35 TTGACA (hexamer), -10 TATAAT (Pribnow box). (2) regulation by the two-component system: Conserved domains were found in bacterial regulatory proteins that respond to environmental stimuli. One component of each system acts as an environmental sensor that transmits a signal to the second component which is the transcription regulator of the corresponding genes. The N-terminal domain of the sensor perceives an environmental stimulus, by binding to the ligand, and transmits a signal to the conserved C-terminal cytoplasmic domain through an allosteric alteration. The activated C-terminal domain of the sensor modifies ( phosphorylates ) the conserved N-terminal domain of the transcription regulator causing a switch in the
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2 conformation of the regulator. The C-terminal domain of the regulator functions by interacting with a specific sigma factor of RNA polymerase and activates the corresponding genes. (3) ligand-regulated induction and repression of transcription To avoid synthesizing the enzymes of a pathway in the absence of the substrate (ligand) or the in abundance of the product (ligand), several different mechanisms for controlling enzyme synthesis are known in bacteria that are regulated by the presence or absence of these specific small molecules. Examples: lactose operon: (i) Three structural genes: lacZ: beta-galactosidase; lacY: galactoside permease; lacA: thiogalactoside transacetylase.
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