Bio

• Anableps anableps


  Also known as "cuatro ojos" ("four eyes"), glides through lakes and ponds in South America, the upper half of each eye protruding from the water
• Regulation of gene expression


  How two cells with the same set of genes can function differently
• Natural selection has favored bacteria that express only the genes that encode products needed by the cell
• Feedback inhibition


  The end product of a metabolic pathway shuts down further synthesis of the product by inhibiting enzyme activity
• Gene regulation


  Cells can adjust the production level of certain enzymes by regulating expression of the genes encoding the enzymes

  
• Operon


  A cluster of functionally related genes that can be coordinately controlled by a single "on-off switch"
• Operator


  A segment of DNA that acts as the "on-off switch" for an operon, positioned within the promoter or between the promoter and enzyme-coding genes
• Repressor


  A protein that prevents gene transcription by binding to the operator and blocking RNA polymerase
• Corepressor


  A molecule that cooperates with a repressor protein to switch an operon off
• Trp operon


  A repressible operon in E. coli that is turned off (repressed) if tryptophan levels are high
• Lac operon


  An inducible operon in E. coli that contains genes for enzymes used in the hydrolysis and metabolism of lactose
• Inducer


  A molecule that inactivates the repressor and turns on transcription of an inducible operon
• Allolactose


  The inducer for the lac operon, an isomer of lactose that binds the lac repressor protein and alters its shape so it can no longer bind to the operator sequence
• Inducible enzymes


  Usually function in catabolic pathways; their synthesis is induced by a chemical signal

  
• Repressible enzymes


  Usually function in anabolic pathways; their synthesis is repressed by high levels of the end product

  
• Cyclic AMP receptor protein (CRP)


  An activator of transcription that binds to the promoter of the lac operon and increases the affinity of RNA polymerase, thus accelerating transcription
• Differential gene expression is the expression of different genes by cells with the same genome, resulting in differences between cell types
• Heterochromatin


  Highly packed chromatin where genes are usually not expressed
• Euchromatin

  Less packed chromatin where gene transcription is affected by the location of nucleosomes and sites where DNA attaches to the protein scaffolding
• Histone acetylation


  The attachment of acetyl groups to histones, which appears to open up the chromatin structure and promote the initiation of transcription
• DNA methylation


  The addition of methyl groups to certain bases in DNA, associated with reduced transcription and long-term inactivation of genes in cellular differentiation
• Epigenetic inheritance


  The inheritance of traits transmitted by mechanisms not directly involving the nucleotide sequence
• Control elements


  Segments of noncoding DNA that serve as binding sites for transcription factors that help regulate transcription

  
• General transcription factors


  Essential for the transcription of all protein-coding genes
• Specific transcription factors


  Bind to control elements and are required for high levels of transcription of some genes
• Proximal control elements


  Located close to the promoter
• Distal control elements


  Also called enhancers, may be far away from a gene or even located in an intron, and are generally associated with only one gene
• General transcription factors


  • A few bind to the TATA box within the promoter
  • Many bind to proteins, including other transcription factors and RNA polymerase
• Transcription initiation


  1. Only when the complete initiation complex has assembled can the RNA polymerase begin to move along the template strand of the DNA

  2. It produces a complementary strand of RNA
• Specific transcription factors


  For genes that are not expressed all the time, high levels of transcription depend on the presence of another set of factors
• Enhancers


  • Proximal control elements are located close to the promoter
  • Distal control elements, groupings of which are called enhancers, may be far away from a gene or even located in an intron
  • Each enhancer is generally associated with only one gene and no other
• Transcriptional activator


  • A protein that binds to an enhancer and stimulates transcription of a gene
  • Activators have two domains, one that binds DNA and a second that activates transcription
  • Bound activators facilitate a sequence of protein-protein interactions that result in enhanced transcription of a given gene
• Enhancer-mediated transcription activation


  1. Protein-mediated bending of the DNA brings the bound activators into contact with a group of mediator proteins

  2. The mediator proteins interact with general transcription factors at the promoter

  3. This helps assemble and position the preinitiation complex
• The nucleus is the control center of the cell, containing genetic material (DNA) that determines an organism's characteristics.
• A gene can be thought of as a set of instructions for building a protein.
• Ribosomes are responsible for protein synthesis.
• Chromosomes are structures within the nucleus made up of DNA and proteins.
• Cell membrane: A thin layer surrounding the cytoplasm that regulates what enters and exits the cell.
• The nucleus is the control center of the cell, containing genetic material (DNA) that determines an organism's characteristics.
• Transcriptional repressors


  • Some transcription factors function as repressors, inhibiting expression of a particular gene in several different ways
  • Some repressors bind directly to control elements, and block activator binding
  • Others interfere with activators, so they cannot bind the DNA
  • Some activators and repressors may indirectly affect transcription by altering chromatin structure