Lect24_regulation - Gene regulation Genes must be turned on...

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1 Regulation of Gene Expression (Suppl reading – Appleman) Gene regulation Genes must be turned on an off to allow an organism to acclimate to changes in its environment Which is an example of a gene? A. Any DNA sequence B. A discrete DNA sequence that codes for a particular polypeptide C. A particular location on a chromosome Control of gene expression Expression of genes is regulated in order to conserve ATP and protect cell from harmful products. – Cells that make unnecessary proteins waste energy that could be directed towards growth – It costs about 7 ATP for each amino acid placed in a protein! – Cells that make unnecessary membrane proteins or enzymes may also be damaged by accumulation of un-needed end products or intermediates Control of gene expression Two classes of proteins exist Constitutive proteins: – Always needed! – Constantly produced at a fixed rate (can increase rate, if needed) – 60-80% of genes in E. coli . – Examples: Enzymes for glycolysis, DNA proofreading Inducible/repressible genes / proteins – Only made when needed. – Example: enzymes for alternative energy sources
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2 What ultimately controls where transcription begins on the DNA? A. The start codon B. RNA polymerase C. A section of DNA called the promoter D. The information in the gene Promoter: RNA polymerase binds to DNA region to start transcription Prokaryotic Promoters are ~70bp in length. Consensus sequence = most typical base sequence for a strong promoter – A strong promoter regularly binds RNA polymerase If the base sequence varies a lot from consensus sequence this causes weak binding to RNA polymerase ( weak promoter ) & reduced transcription of gene(s), thus less of the protein. RNA pol A weak promoter rarely binds RNA polymerase Base sequence determines promoter strength A strong promoter frequently binds RNA polymerase Gene Regulation in Bacteria In bacteria, related genes may be located together in a series (an operon) – A single promoter causes RNA polymerase to transcribe all these genes – Start and stop codons in the RNA differentiate the proteins Organization into operons (top) is especially efficient when the genes encode a pathway (independent transcription of separate genes has a higher probability of error) Gene A Gene B Gene C Gene A Gene B Gene C GeneD One promoter and terminator Sometimes multiple genes are transcribed together as an operon. Gene Regulation in Bacteria In bacteria, related genes may be located together in a series (an operon) – An protein binding site, or operator , exists within the promoter region – A regulatory protein called a repressor can bind to the operator – The presence / absence of the repressor determines whether RNA polymerase can bind or not, to transcribe the genes – That is, the operator can act as an “on / off” switch
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