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Gene Regulation

Gene Regulation. certain genes are transcribed all the time – constitutive genes synthesis of some proteins is regulated and are produced only when needed under special conditions. Gene Regulation in Prokaryotes. The Jacob-Monad Model. The Lac Operon (Inducible Operon):

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Gene Regulation

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  1. Gene Regulation • certain genes are transcribed all the time – constitutive genes • synthesis of some proteins is regulated and are produced only when needed under special conditions

  2. Gene Regulation in Prokaryotes

  3. The Jacob-Monad Model The Lac Operon (Inducible Operon): Jacob and Monad demonstrated how genes that code for enzymes that metabolize lactose are regulated

  4. An operon consists of three elements: • the genes that it controls • a promotor region where RNA polymerase first binds • an operator region between the promotor and the first gene which acts as an “on-off switch”.

  5. Intestinal bacteria (E. coli) are able to absorb the disaccharide, lactose, and break and break it down to glucose and galactose (E. coli will only make these enzymes when grown in the presence of lactose) • Requires the production of 3 enzymes: •  - galactosidase – breaks down the lactose to glucose and galactose • galactose permease – needed to transport lactose efficiently across bacterial cell membrane • galactoside transacetylase – function is not clear

  6. Production of these enzymes is controlled by three structural genes and some closely linked DNA sequences responsible for controlling the structural genes – entire gene complex is called the operon • Structural genes of the lactose operon: • lacZ – codes for  - galactosidase • lacY – codes for galactose permease • lacA – codes for galactoside transacetylase

  7. Next to the structural genes are 2 overlapping regulatory regions: • promotor – region to which RNA polymerase binds to initiate transcription • operator – region of DNA that acts as the switch that controls mRNA synthesis; sequence of bases that overlaps part of the promotor region

  8. when lactose is absent, a repressor protein (in this case the lactose repressor) binds to the operator region – repressor protein is large enough to cover part of the promotor sequence, too, and blocks RNA polymerase from attaching to promotor – transcription is blocked • when lactose is present, it acts as an inducer and “turns on” the transcription of the lactose operon • lactose binds to repressor protein, inactivates it, and unblocks the promotor region allowing RNA polymerase to attach and begin transcription

  9. Gene Regulation in Eukaryotes • most cells in a multicellular organism contain the same DNA but they don’t all use the DNA all the time • individual cells express only a small fraction of their genes – those genes that are appropriate to the function of that particular cell type • transcription of a cell’s DNA must be regulated • factors such pregnancy may affect gene expression (genes for milk production are not used all the time) • the environment may affect which genes are transcribed (length of day may increase a change in size of sex organs affecting the production of sex hormones in birds)

  10. Gene expression may be regulated by: 1. the rate of transcription of genes • the cell may regulate the transcription of individual genes through regulatory molecules (ex. steroids may stimulate the production of certain proteins) • certain parts of eukaryotic chromosomes are in a highly condensed, compact state making it inaccessible to RNA polymerase • some of these areas are structural and don’t contain genes • other of these regions are functional genes that are not currently being transcribed • entire chromosomes may be inactivated • ex – Female mammals have two X chromosomes in each cell but only one is available for transcription – the other chromosome is condensed into a tight mass called a Barr body

  11. 2. mRNAs may be translated at different rates • mRNAs vary in stability (how long they last before they are degraded) and in the rate at which they are translated into protein 3. Proteins may require modification before they can carry out their functions in a cell 4. The rate of enzyme activity may be regulated (previously discussed in organic chemistry)

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