Transcript Chapter 8b
Comparison of Genetic Material and Replication for Eukaryotes and Prokaryotes Bacteria Archaea Eukaryotes Genome haploid; circular haploid; circular diploid; linear Histones Absent Present; nucleosome Faster Present; nucleosome Slower Rate Faster Point of origin Single Multiple Multiple Telomeres Absent Absent Present ~5 ~5 # DNA Polymerase ~15 Comparison of Transcription for Eukaryotes and Prokaryotes Bacteria Archaea 1 1 – similar to # RNA eukaryotic Polymerase polycistronic polycistronic # genes on transcript None Introns Posttranscription modification No Yes Transcription factors Sigma Factor Promoter Unique Similar Eukaryotes 3 monocistronic Introns, cap and tail Yes Similar Regulation of Gene Expression Enzymes are common feature of biochemical pathways Constitutive enzymes (60-80%) Inducible enzymes Default position off Repressible enzymes Default position on Operon model of gene expression Regulatory gene, operator, promoter and series of structural genes divided into three regions: Regulatory gene – codes for regulatory protein Control region - operator and promoter Structural genes - genes being transcribed Operon structure Control region Regulatory gene Operator Gene 1 Gene 2 Gene 3 Promoter Regulatory gene – DNA sequence for repressor protein Promoter – Binding site for RNA polymerase Operator – binding site for the repressor protein Structural Genes – DNA sequence for proteins of interest Operon controlled by regulatory region Protein acts as “on/off” switch Can act as repressor or inducer Operon model based on studies of induction of the enzymes of lactose catabolism on E. coli Inducible enzyme Default position is off Enzymes not made until needed Catabolite Repression glucose represses enzymes for lactose degradation Low glucose levels corresponds to high cAMP cAMP binds to catabolite activating protein (CAP) alarmone CAP binds to promoter and induces RNA polymerase to bind E.coli grows on either substrate 2-step diauxic growth caused by catabolite repression Repressible enzyme Default position is on Enzymes made until no longer needed Operons rare in eukaryotes Function differently Eukaryotes utilize transcription factors or alternate splicing of exons Expression may be regulated at translation level Unsure of regulation of expression in archaea May be more similar to eukaryotes than bacteria Many microbes adapt to changing environments by altering level of gene expression Global Regulatory Systems Signal transduction Transmits information from external environment to inside cell Allows cell to respond to environmental changes Two-component regulatory systems Sensors recognize change in environment Kinase protein in membrane Response regulators activate or repress gene expression DNA binding protein Quorum sensing Based on density of cell population Activation of genes beneficial only when produced by multiple cells Vibrio fisheri Biofilm formation Natural selection Antigenic variation Alteration in characteristics of certain surface proteins Ex. Neisseria gonorrhoeae varies pilin gene at expression locus Regulation may occur at the translation level Riboswitches Antisense RNA Bacterial Genetics and Genetic Transfers Genetic Diversity Eukaryotes - sexual reproduction Gametes have various genetic combinations Prokaryotes - asexual reproduction All offspring are clones of parent cell No genetic variation Diversity in Bacteria Bacterial mechanisms for genetic diversity Mutation Gene transfer Mutations Change in genotype May or may not cause phenotypic changes Wild type vs. mutant silent, beneficial, or harmful Passed vertically to all offspring Selective pressure can lead to evolution through natural selection Types of Mutations Point Mutation (base substitution) •Change • Missense in one base Results in change of amino acid Nonsense • Results in a stop codon Frame-shift mutation •Insertion or deletion of one or more bases Mutagen Agent that induces mutations Physical or chemical agents Spontaneous mutations Occur in the absence of a mutagen May be due to error or transposons Transposable Elements (Transposons) May disrupt proper gene function Contain insertion sequences (transposase) Complex (composite) transposons carry other genes •Nucleotide excision repair •Endonuclease, DNA ligase & DNA Polymerase •Light repair •Direct repair •Photoactivation of enzymes (photolyase) Induced Mutations Mutations are essential for understanding genetics Intentionally produced (induced) to demonstrate function of particular gene or set of genes Mutations can be induced via Chemical mutagens Transposition Radiation •Ames Test •Mutational reversion assay •Tests mutagenicity of compounds •Utilizes a histidine auxotroph Mutations followed by selection may produce microbes with desirable traits Positive (direct) selection detects mutant cells because they grow or appear different Ex. Penicillin resistant mutants growing on penicillin containing agar – non mutants will not grow Eliminates wild type Negative (indirect) selection detects mutant cells because they do not grow Replica plating to isolate mutants requiring a specific growth factor – auxotroph Selects for wild type Replica Plating Figure 8.21