Chapter 7: Microbial Genetics By Dr. St. Laurent
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The Structure and Replication of Genomes Genome – the entire genetic complement of an organism; includes its genes and nucleotide sequences Chapter 7: Microbial Genetics By Dr. St. Laurent The Structure of Nucleic Acids The Structure of Nucleic Acids Figure 7.1a–c The Structure of Prokaryotic Genomes Contained in two structures Chromosomes Plasmids Figure 7.1d Prokaryotic Chromosomes Main portion of DNA, along with associated proteins and RNA, are packaged in 1-2 distinct chromosomes Prokaryotic cells have a single copy of each chromosome (haploid) Typical chromosome – circular molecule of DNA in nucleoid 1 Plasmids Plasmids Small molecules of DNA that replicate independently Carry information required for their own replication, and often for one or more cellular traits Not essential for normal bacterial metabolism, growth, or reproduction Can confer survival advantages The Structure of Eukaryotic Genomes Many types of plasmids Fertility factors Resistance factors Bacteriocin factors Virulence plasmids Cryptic plasmids Eukaryotic Chromosomes Contained in two structures Typically have more than one chromosome per cell Chromosomes are linear and sequestered within membrane-bound nucleus Eukaryotic cells often have two copies of each chromosome (diploid) Nuclear DNA Extranuclear DNA Eukaryotic Chromosomal Packaging Extranuclear DNA of Eukaryotes DNA molecules of mitochondria and chloroplasts are circular and resemble chromosomes of prokaryotes Only codes for about 5% of RNA and proteins Nuclear DNA codes for 95% of RNA and proteins Some fungi and protozoa carry plasmids Figure 7.3 2 DNA Replication Initial Processes in DNA Replication An anabolic polymerization process that requires monomers and energy Triphosphate deoxyribonucleotides serve both functions Key to replication is complementary structure of the two strands Replication is semiconservative – new strands composed of one original strand and one daughter strand PLAY Animation: DNA Replication Figure 7.5a Initial Processes in DNA Replication Synthesis of the Leading Strand DNA polymerase binds to each strand and adds nucleotides to hydroxyl group at 3′ end of nucleic acid Replicates DNA only 5′ to 3′ Because strands are antiparallel, new strands synthesized differently Leading strand synthesized continuously Lagging strand synthesized discontinuously Figure 7.5b Synthesis of the Lagging Strand Other Characteristics of DNA Bacterial Replication Bidirectional Topoisomerases remove supercoils in DNA molecule DNA is methylated; methylation plays role in variety of processes Control of genetic expression Initiation of DNA replication Protection against viral infection Repair of DNA Figure 7.5c 3 Replication of Eukaryotic DNA Gene Function Similar to bacterial replication Some differences Use four DNA polymerases Thousands of replication origins Shorter Okazaki fragments Plant and animal cells methylate only cytosine bases Transfer of Genetic Information Genotype – set of genes in the genome Phenotype – physical features and functional traits of organism Events in Transcription Transcription – information in DNA is copied as RNA nucleotide sequences Translation – polypeptides synthesized from RNA nucleotide sequences Central dogma of genetics DNA transcribed to RNA RNA translated to form polypeptides Four types of RNA transcribed from DNA RNA primers mRNA rRNA tRNA Occurs in nucleoid of prokaryotes Three steps Initiation Elongation Termination Initiation of Transcription PLAY Elongation of the RNA Transcript Animation: Transcription Figure 7.8a Figure 7.8b 4 Concurrent RNA Transcription RNA Polymerase Versus DNA Polymerase RNA polymerase does not require helicase RNA polymerase slower than DNA polymerase Uracil incorporated instead of thymine RNA polymerase proofreading function is less efficient than DNA polymerase (more errors) Figure 7.9 Termination of Transcript Transcription in Eukaryotes RNA transcription occurs in the nucleus Transcription also occurs in mitochondria and chloroplasts Three types of RNA polymerases Numerous transcription factors mRNA processed before translation Capping Polyadenylation Splicing Figure 7.8c Genetic Code Prokaryotic mRNA Figure 7.11 Figure 7.12 5 Eukaryotic mRNA tRNA Figure 7.10 Ribosomes and rRNA Figure 7.13a-b Ribosomes and rRNA Figure 7.14a Ribosomes and rRNA Figure 7.14b Stages of Translation Three stages Initiation Elongation Termination All stages require additional protein factors Initiation and elongation require energy (GTP) PLAY Animation: Translation Figure 7.14c 6 Initiation Elongation Figure 7.15 Elongation Figure 7.16.1 Elongation Figure 7.16.2 Elongation Figure 7.16.3 Elongation Figure 7.16.4 Figure 7.16.5 7 Elongation Polyribosome Figure 7.16.6 Termination Release factors somehow recognize stop codons and modify ribosome to activate ribozymes which sever polypeptide from final tRNA Ribosome dissociates into subunits Polypeptides released at termination may function alone or together Regulation of Genetic Expression 75% of genes are expressed at all times Other genes are regulated so they are only transcribed and translated when cell needs them Allows cell to conserve energy Regulation of protein synthesis Typically halt transcription Can stop translation directly Figure 7.17a Translation in Eukaryotes Initiation occurs when ribosomal subunit binds to 5′ guanine cap First amino acid is methionine Control of Translation Regulation of genetic expression can be at the level of translation Riboswitch mRNA molecule that blocks ribosomes and translation of the polypeptide they encode Short interference RNA (siRNA) RNA molecule complementary to a portion of mRNA, tRNA, or a gene that binds and renders the target inactive 8 The Operon Operons Inducible operons – must be activated by inducers Lactose Operon Repressible operons – transcribed continually until deactivated by repressors Tryptophan Operon PLAY Animation: Operons Figure 7.18 The Lactose Operon The Lactose Operon Figure 7.19a The Tryptophan Operon Figure 7.19b The Tryptophan Operon Figure 7.20a Figure 7.20b 9
