Chapter 17 Gene Expression Bacteria How do bacteria regulate which genes are expressed in what amounts and when Review Translation is the conversion of into Translation begins with the sequence and terminates with a codon The amino acid of tRNA is attached with a bond to the growing amino acid chain making up our protein Translation terminates when a codon is reached in the site recruiting a factor Initiating Translation 1 mRNA binds to small ribosomal subunit 2 Initiator aminoacyl tRNA binds to codon carrying f Met The first codon is always AUG 3 Large subunit binds completing the complex Elongation Extending the Polypeptide When both the P site and A site are occupied by tRNAs they are in the enzyme s site peptide bond formation occurs Elongation Extending the Polypeptide 1 aminoacyl tRNA arrives an binds in A site 2 bond formation 3 repeated down the length of mRNA Terminating Translation When translocation opens the A site and exposes one of the stop codons a protein called release factor fills the A site This breaks the bond linking the tRNA in the P site to the polypeptide chain Central Dogma Does this happen the same way for every gene Does this happen the same way for every organism Are all genes transcribed at the same rate Percentage of identical DNA Sequence of Sequence of Sequence of 99 5 96 99 29 code for the same amino acid sequences Part of the reason is gene expression and regulation http www uchospitals edu news 2006 20060309 chimp htm http genome wellcome ac uk doc WTD020730 html Gene Expression Regulation No two genes are transcribed translated or post translational processed the same Every organism regulates gene expression In multi cellular organisms cells regulation can vary greatly depending on the location of the cell In fact it is gene expression that differentiates us from other organisms Mechanisms of Regulation Think Pair Share Split into your groups and discuss the following question Why do cells regulate gene expression What are the advantages and disadvantages to each type of control and why Energy cost to the cell Every time the cell a genes an mRNA and an mRNA into protein requires energy Competition for resources is what drives some organism to survive while others die If an organism misuses energy it requires that much more energy and makes it more difficult to compete with its neighbor Different Levels of Control Transcriptional control is efficient and saves the cell the most Translational control allows a cell to make rapid changes in the relative a of different proteins Post translational control provides the most rapid response of the the three mechanisms Studying Gene Expression Regulation Model organism are often used to research transcription translation and gene expression A model organism is a species that is used for research because it is practical and because conclusions drawn from studying it turns out to apply to many other as well Examples include E coli S cerevisiae and Fruit flies Lactose Metabolism Model System Glucose is E coli s preferred carbon source but when glucose levels are l it will use lactose This requires the enzyme galactosidase to break lactose down into and To conserve energy does E coli tightly regulates the lactose processing pathway so that it is only used when glucose is low This is a great model for scientists to study regulation Lactose Metabolism Model System Hypothesis E coli does not produce galactosidase when glucose is present even when in the presence of lactose Conclusion prevents the expression of galactosidase galactosidase is only expressed in the presence of lactose and absence of glucose How are these genes for lactose metabolism regulated Identify Genes Under Control 1 Generate a large number of mutants using mutagens 2 the mutants to find individuals with defects Lactose Metabolism Mutants Constitutive mutants Expression is on all the time Lactose Metabolism Mutants Mechanism of Control Negative control occurs when a regulatory protein binds to DNA and shuts down transcription Positive control occurs when a regulatory protein binds to DNA and triggers transcription The Hypothesis of Negative Control
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