Bio1151 Chapter 18 Regulation of Gene Expression
  1. Regulation of protein production can occur by           inhibition or by                control, as illustrated in the E. coli      operon.

      An operon is a group of functionally related genes under the control by a single on-off switch called an operator which is usually located within the promoter. Enzyme activity can be controlled by
    • feedback inhibition when the product of the metabolic pathway depresses activity of the first enzyme in the pathway.
    • gene regulation when the genes coding for the enzyme are not transcribed.
     
     
     
     
  2. The trp operon is a              system that is turned     unless repressed by the repressor.

      Tryptophan absent, repressor inactive, operon on. In the repressible trp operon, 5 genes encoding the enzymes to synthesize tryptophan (an amino acid) are regulated by a promoter and an operator. When tryptophan is absent, the repressor (controlled by its own promoter) is inactive and the operon is on. RNA polymerase attaches to the DNA at the promoter and transcribes the operons genes.


      Tryptophan present, repressor active, operon off. The presence of tryptophan (a corepressor) activates the repressor which binds to the operator to turn the operon off by inhibiting the transcription of these genes by RNA polymerase.
     
     
     
     
    • Repressible enzymes usually function in           pathways.


      Catabolic reactions are exergonic: they release energy from the break down of complex molecules into simpler compounds. Anabolic reactions are endergonic: they consume energy to build large molecules from simpler ones.
     
     
     
     
  3. The E. coli lac operon is an            system that is turned      unless induced by the inducer.

      Lactose absent, repressor active, operon off. E. coli uses 3 enzymes to take up and metabolize the sugar lactose. The genes for these 3 enzymes are clustered in the lac operon. In the absence of lactose, a repressor switches off the operon by binding to the operator.


      Lactose present, repressor inactive, operon on. Allolactose, an isomer of lactose, serves as an inducer and derepresses the operon by inactivating the repressor, turning on transcription and translation of enzymes for lactose digestion.
     
     
     
     
    • Inducible enzymes usually function in            pathways.
       
       
       
       
    • Expression of the lac operon is also subject to positive control: the operon is             by the catabolite-activating protein (CAP), together with Cyclic adenosine monophosphate ( cAMP ) when          is absent.


      Lactose present, glucose scarce (cAMP level high): lac transcription stimulated. RNA polymerase has high affinity for the lac promoter only when the activator, catabolite-activating protein (CAP), is bound next to the promoter. CAP is active only when associated with cyclic AMP (cAMP), whose concentration in the cell rises when the concentration of the preferred glucose falls.


      Lactose present, glucose abundant (cAMP level low): lac transcription reduced. When glucose levels increase, cAMP levels drop, and the catabolite-activating protein (CAP) is inactive. The cell preferentially catabolizes glucose and does not make the lactoseutilizing enzymes.


      The enzyme adenyl cyclase catalyzes the formation of cAMP from ATP. The activity of adenyl cyclase is inhibited by glucose. Thus cAMP levels fall when glucose levels rise.
     
     
     
     
  4. In eukaryotes, gene regulation controls cell specialization and                  .
     
     
     
     
  5. Eukaryotic gene expression can be regulated at many stages, from the          to the            .

      Regulation of eukaryotic gene expression may take place at different stages. In the nucleus, regulation can occur by
    • Chromatin modification.
    • Transcription control.
    • RNA processing. Cytoplasm


      In the cytoplasm, regulation can occur by
    • mRNA degradation.
    • Protein processing.
    • Protein degradation.
     
     
     
     
    • Chromatin modification: acetylation of          "tails" loosens the configuration of chromatin and enhances                by making DNA more accessible to enzymes.

        Chromatin modification: histone acetylation. Enzymes can add negative acetyl groups (COCH[3]) to positively charged lysines in histone tails. This process loosens chromatin structure, making the DNA accessible to transcription. Such chromatin modifications may be passed to future generations of cells in a process called epigenetic inheritance.


        DNA and positively-charged histone molecules form "beads on a string," the 10-nm fiber comprising the interphase chromatin. A nucleosome has 8 histone molecules with the amino end (tail) of each projecting outward. A different histone, H1, acts as a spacer between nucleosomes.
       
       
       
       
    • Transcription control: many genes contain control elements such as            and             that can stimulate transcription.

        Transcription control. Distal control elements (far from the promoter, where RNA polymerase binds,) can be grouped together as enhancers and may interact with activators or repressors to control initiation of transcription.


        Activator proteins bind to distal control elements grouped as an enhancer in the DNA. A DNA-bending protein brings the bound activators closer to the promoter. The activators bind to transcription factors and mediator proteins, forming a transcription initiation complex on the promoter with RNA polymerase. Other transcription factors function as repressors.


        The control elements can activate transcription only when the appropriate activator proteins are present. The particular combination of control elements and activator proteins enables a liver cell to express the albumin gene, while a lens cell expresses the crystallin gene.
       
       
       
       
    • RNA processing: different       molecules can be produced from the same primary transcript, in a process called alternative RNA           .

        RNA processing: alternative RNA splicing. The primary transcripts of some genes can be spliced in more than one way, generating different mRNA molecules. In this example one mRNA molecule has ended up with the green exon and the other with the purple exon. With alternative splicing, an organism can produce more than one type of polypeptide from a single gene.


        Alternative RNA Splicing
      • In the early stage of Drosophila embryogenesis, the sxl protein is expressed in female embryo, but not in the male embryo.
      • In the late female embryo, the sxl protein produced in the early stage may mask the splicing signal for the second intron, resulting in a different protein than in the male embryo. Adapted from Web Book Publications
       
       
       
       
    • mRNA degradation: single-stranded            (miRNAs) can lead to degradation of an mRNA, thus limiting its life span.

        mRNA degradation.
      • Single-stranded microRNAs (miRNAs) are noncoding RNAs that can form hairpin loops by hydrogen bonding.
      • These are cut by the Dicer and can degrade complementary mRNA or block its translation.
       
       
       
       
    • Protein processing and degradation: protein complexes called              degrade proteins by binding to proteins tagged by            and digesting them.


      Protein degradation. Unneeded proteins can be tagged with the protein ubiquitin. The tagged protein is then chopped up by a proteasome.
     
     
     
     
  6. Multicellular organisms develop from a single-celled         to cells of many different types through cell           , cell                  , and morphogenesis.

      The cell cycle. The mitotic (M) phase alternates with interphase, which is composed of 2 growth phases (G[1] and G[2]) and a S (DNA synthesis) phase. In the M phase, mitosis divides the nucleus, and cytokinesis divides the cytoplasm, producing 2 daughter cells.


      Differences between cells is mainly due to differential gene expression, even though different cells share genomic equivalence.


      Morphogenesis encompasses the processes that give shape to the organism and its parts. It takes just one week for cell division, differentiation, and morphogenesis to transform a fertilized frog egg into a hatching tadpole.
     
     
     
     
    • Differential distribution of              determinants in the egg can lead to subsequent            of embryonic cells.

        Cytoplasmic determinants. Uneven distribution of determinants, such as RNA, proteins, and organelles, in the cytoplasm of the unfertilized egg affect expression of genes and ultimately the cells developmental fate.


        Induction by nearby cells. The cells at the bottom of this early embryo release chemicals that signal nearby cells to change their gene expression (transcription).


        Cytoplasmic determinants such as mRNA establish the axes of the body in Drosophila. Asymmetrically distribution of these molecules in the unfertilized egg eventually lead to differentiation of specialized segments of the adult. One important egg-polarity gene that encodes for such mRNA is the bicoid gene.


        Gradients of bicoid mRNA and bicoid protein in egg and early embryo leads to normal development of the head. Continue
       
       
       
       
    • Expression of genes for tissue-specific proteins result in cell                , and leads to observable                  .


      Cell determination. MyoD is a master regulatory gene that produce proteins that commit the cell to becoming skeletal muscle. If a myoblast cell produces the MyoD protein (a transcription factor), it binds to enhancers of many target genes. The cell is now determined to be a skeletal muscle cell.


      Transcription control. Distal control elements (far from the promoter, where RNA polymerase binds,) can be grouped together as enhancers and may interact with activators or repressors to control initiation of transcription.
     
     
     
     
  7. Cancer results from genetic changes that affect cell        control.
     
     
     
     
    • Mutations in        - oncogenes can turn them into cancer-causing            that lead to abnormal cell growth.

        Proto-oncogenes are normal genes involved in cell growth and division. Mutations to a proto-oncogene can lead it to become a cancer-causing oncogene by producing growth-stimulating proteins that are in excess levels or that are hyperactive or degradation- resistant .


        Protein degradation. Unneeded proteins can be tagged with the protein ubiquitin. The tagged protein is then chopped up by a proteasome.
       
       
       
       
    • Tumor-             genes regulate cell growth; mutations in these genes can lead to uncontrolled cell growth and cancer.


      The p53 tumor suppressor gene encodes a transcription factor that regulates transcription of more than 50 different genes involved in the cell cycle. Cells with mutant p53 are unable to arrest at cell cycle checkpoints and become cancerous.


      The cell cycle. The mitotic (M) phase alternates with interphase, which is composed of 2 growth phases (G[1] and G[2]) and a S (DNA synthesis) phase. In the M phase, mitosis divides the nucleus, and cytokinesis divides the cytoplasm, producing 2 daughter cells.
     
     
     
     
  8.           mutations are generally needed for full-fledged cancer; at least one active           and accumulation of several mutant tumor-             genes characterize most cancers.

      A multistep model for the development of colorectal cancer. Affecting the colon and/or rectum, this type of cancer is one of the best understood. Next


      Mutations affecting tumorsuppressor genes (such as p53) may lead to benign growth in the colon lining called a polyp.


      Mutations in other tumorsuppressor genes and development of oncogenes can cause enlargement of the benign growth into an adenoma. Next


      Continured accumulation of mutations can culminate in the development of full-fledged cancer. This malignant tumor is called a carcinoma.