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Chapter 7 Microbial Genetics 7/6/11 MDufilho 1 The Structure and Replication of Genomes • Genetics – Study of inheritance and inheritable traits as expressed in an organism’s genetic material • Genome – The entire genetic complement of an organism – Includes its genes and nucleotide sequences 7/6/11 MDufilho 2 Figure 7.1 The structure of nucleic acids-overview 7/6/11 MDufilho 3 The Structure and Replication of Genomes • The Structure of Prokaryotic Genomes – Prokaryotic chromosomes – Main portion of DNA, along with associated proteins and RNA – Prokaryotic cells are haploid (single chromosome copy) – Typical chromosome is circular molecule of DNA in nucleoid 7/6/11 MDufilho 4 Figure 7.2 Bacterial genome-overview 7/6/11 MDufilho 5 The Structure and Replication of Genomes • The Structure of Prokaryotic Genomes – Plasmids – Small molecules of DNA that replicate independently – Not essential for normal metabolism, growth, or reproduction – Can confer survival advantages – Many types of plasmids – Fertility factors – Resistance factors – Bacteriocin factors – Virulence plasmids 7/6/11 © 2012 Pearson Education Inc. MDufilho 6 The Structure and Replication of Genomes • The Structure of Eukaryotic Genomes – Nuclear chromosomes – Typically have more than one chromosome per cell – Chromosomes are linear and sequestered within nucleus – Eukaryotic cells are often diploid (two chromosome copies) 7/6/11 © 2012 Pearson Education Inc. MDufilho 7 The Structure and Replication of Genomes • The Structure of Eukaryotic Genomes – Extranuclear DNA of eukaryotes – DNA molecules of mitochondria and chloroplasts – Resemble chromosomes of prokaryotes – Only code for about 5% of RNA and proteins – Some fungi and protozoa carry plasmids 7/6/11 © 2012 Pearson Education Inc. MDufilho 8 The Structure and Replication of Genomes • DNA Replication – 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 DNA composed of one original and one daughter strand 7/6/11 © 2012 Pearson Education Inc. MDufilho 9 The Structure and Replication of Genomes ANIMATION DNA Replication: Overview 7/6/11 © 2012 Pearson Education Inc. MDufilho 10 The Structure and Replication of Genomes ANIMATION DNA Replication: Synthesis 7/6/11 © 2012 Pearson Education Inc. MDufilho 11 The Structure and Replication of Genomes • DNA Replication – Other characteristics of bacterial DNA replication – Bidirectional – DNA is methylated – Control of genetic expression – Initiation of DNA replication – Protection against viral infection – Repair of DNA 7/6/11 © 2012 Pearson Education Inc. MDufilho 12 Gene Function • The Relationship Between Genotype and Phenotype – Genotype – Set of genes in the genome – Phenotype – Physical features and functional traits of the organism 7/6/11 © 2012 Pearson Education Inc. MDufilho 13 Gene Function • The Transfer of Genetic Information – Transcription – Information in DNA is copied as RNA – Translation – Polypeptides synthesized from RNA – Central dogma of genetics – DNA transcribed to RNA – RNA translated to form polypeptides 7/6/11 © 2012 Pearson Education Inc. MDufilho 14 Figure 7.8 The central dogma of genetics 5´ 3´ DNA (genotype) 5´ 3´ Transcription 5´ 3´ mRNA Translation by ribosomes NH2 Methionine 7/6/11 Arginine Tyrosine MDufilho Phenotype Leucine Polypeptide 15 Gene Function ANIMATION Transcription: The Process 7/6/11 © 2012 Pearson Education Inc. MDufilho 16 Gene Function • Translation – Process where ribosomes use genetic information of nucleotide sequences to synthesize polypeptides 7/6/11 © 2012 Pearson Education Inc. MDufilho 17 Figure 7.12 The genetic code 7/6/11 MDufilho 18 Figure 7.13 Prokaryotic mRNA Promoter Gene 1 Gene 2 Gene 3 Terminator 5´ Template DNA strand 3´ Transcription 5´ Start codon AUG UAA Ribosome binding site (RBS) Start codon AUG Stop RBS codon UAG Start codon AUG Stop RBS codon UAA 3´ mRNA Stop codon Untranslated mRNA Translation Polypeptide 1 7/6/11 Polypeptide 2 MDufilho Polypeptide 3 19 Figure 7.14 Transfer RNA-overview 7/6/11 MDufilho 20 Figure 7.16 Transfer RNA binding sites in a ribosome Large subunit Large subunit Nucleotide bases mRNA 5´ Small subunit P A site site site 3´ mRNA Prokaryotic ribosome (angled view) attached to mRNA 7/6/11 E tRNAbinding sites Small subunit Prokaryotic ribosome (schematic view) showing tRNA-binding sites MDufilho 21 Figure 7.17 The initiation of translation in prokaryotes Initiator tRNA Large ribosomal subunit tRNAfMet Anticodon mRNA E Start codon 5´ 3´ P A P Small ribosomal subunit 7/6/11 A P A Initiation complex MDufilho 22 Figure 7.18 The elongation stage of translation-overview E Peptide bond E 5´ 3´ P 5´ A 3´ P A Movement of ribosome one codon toward 3´ end 5´ 3´ E P A P A P A E 5´ 3´ E 5´ 3´ Two more cycles Growing polypeptide E 7/6/11 5´ MDufilho 3´ P A 23 Gene Function • Regulation of Genetic Expression – 75% of genes are expressed at all times – Other genes transcribed and translated when cells need them – Allows cell to conserve energy – Regulation of protein synthesis – Typically halts transcription – Can stop translation directly 7/6/11 © 2012 Pearson Education Inc. MDufilho 24 Mutations of Genes • Mutation – Change in the nucleotide base sequence of a genome – Rare event – Almost always deleterious – Rarely leads to a protein that improves ability of organism to survive 7/6/11 © 2012 Pearson Education Inc. MDufilho 25 Mutations of Genes • Mutagens – Radiation – Ionizing radiation – Nonionizing radiation – Chemical mutagens – Nucleotide analogs – Disrupt DNA and RNA replication – Nucleotide-altering chemicals – Result in base-pair substitutions and missense mutations – Frameshift mutagens 7/6/11 – Result in nonsense mutations MDufilho 26 Mutations of Genes • Frequency of Mutation – Mutations are rare events – Otherwise organisms could not effectively reproduce – Mutagens increase the mutation rate by a factor of 10 to 1000 times 7/6/11 MDufilho 27 Genetic Transfer • Horizontal Gene Transfer Among Prokaryotes – Horizontal gene transfer – Donor cell contributes part of genome to recipient cell – Three types – Transformation – Transduction – Bacterial conjugation 7/6/11 © 2012 Pearson Education Inc. MDufilho 28 Figure 7.33 Transformation in Streptococcus pneumoniae-overview 7/6/11 MDufilho 29 Figure 7.34 Transduction-overview Bacteriophage Host bacterial cell (donor cell) Bacterial chromosome Phage injects its DNA. Phage enzymes degrade host DNA. Phage DNA Phage with donor DNA (transducing phage) Cell synthesizes new phages that incorporate phage DNA and, mistakenly, some host DNA. Transducing phage Recipient host cell Transducing phage injects donor DNA. Transduced cell Inserted DNA 7/6/11 MDufilho Donor DNA is incorporated into recipient’s chromosome by recombination. 30 Figure 7.35 Bacterial conjugation-overview F plasmid Origin of Conjugation pilus transfer Chromosome Donor cell attaches to a recipient cell with its pilus. F+ cell F– cell Pilus may draw cells together. One strand of F plasmid DNA transfers to the recipient. Pilus The recipient synthesizes a complementary strand to become an F+ cell with a pilus; the donor synthesizes a complementary strand, restoring its complete plasmid. 7/6/11 F+ cell MDufilho F+ cell 31 Figure 7.36 Conjugation involving an Hfr cell-overview Donor chromosome Pilus F+ cell F plasmid integrates into chromosome by recombination. Hfr cell Pilus F+ cell (Hfr) F plasmid Cells join via a conjugation pilus. F– recipient Donor DNA Part of F plasmid Portion of F plasmid partially moves into recipient cell trailing a strand of donor’s DNA. Incomplete F plasmid; cell remains F– Conjugation ends with pieces of F plasmid and donor DNA in recipient cell; cells synthesize complementary DNA strands. Donor DNA and recipient DNA recombine, making a recombinant F– cell. 7/6/11 MDufilho Recombinant cell (still F–) 32