Chapter 7 Microbial Genetics 7/6/111MDufilho

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 7/6/112MDufilho

Figure 7.1 The structure of nucleic acids-overview 7/6/113MDufilho

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 7/6/114MDufilho

Figure 7.2 Bacterial genome-overview 7/6/115MDufilho

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 © 2012 Pearson Education Inc. 7/6/116MDufilho

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) © 2012 Pearson Education Inc. 7/6/117MDufilho

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 © 2012 Pearson Education Inc. 7/6/118MDufilho

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 © 2012 Pearson Education Inc. 7/6/119MDufilho

The Structure and Replication of Genomes © 2012 Pearson Education Inc. ANIMATION DNA Replication: Overview 7/6/1110MDufilho

The Structure and Replication of Genomes © 2012 Pearson Education Inc. ANIMATION DNA Replication: Synthesis 7/6/1111MDufilho

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 © 2012 Pearson Education Inc. 7/6/1112MDufilho

Gene Function The Relationship Between Genotype and Phenotype –Genotype –Set of genes in the genome –Phenotype –Physical features and functional traits of the organism © 2012 Pearson Education Inc. 7/6/1113MDufilho

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 © 2012 Pearson Education Inc. 7/6/1114MDufilho

Figure 7.8 The central dogma of genetics Transcription Translation by ribosomes 5´ 3´ DNA (genotype) mRNA Polypeptide Phenotype NH 2 MethionineArginineTyrosine Leucine 7/6/1115MDufilho

Gene Function © 2012 Pearson Education Inc. ANIMATION Transcription: The Process 7/6/1116MDufilho

Gene Function Translation –Process where ribosomes use genetic information of nucleotide sequences to synthesize polypeptides © 2012 Pearson Education Inc. 7/6/1117MDufilho

Figure 7.12 The genetic code 7/6/1118MDufilho

Figure 7.13 Prokaryotic mRNA PromoterTerminatorGene 1Gene 2Gene 3 Transcription 5´ Template DNA strand 3´ 5´3´ mRNA Start codon AUG UAAUAGUAA Ribosome binding site (RBS) RBS Stop codon Untranslated mRNA Translation Polypeptide 1Polypeptide 2Polypeptide 3 3´ mRNA 7/6/1119MDufilho

Figure 7.14 Transfer RNA-overview 7/6/1120MDufilho

Figure 7.16 Transfer RNA binding sites in a ribosome Large subunit Small subunit mRNA 5´ 3´ Prokaryotic ribosome (angled view) attached to mRNA Large subunit mRNA Prokaryotic ribosome (schematic view) showing tRNA-binding sites Small subunit Nucleotide bases tRNA- binding sites E site A site P site 7/6/1121MDufilho

Figure 7.17 The initiation of translation in prokaryotes Initiator tRNA tRNA fMet mRNA 5´3´ Small ribosomal subunit PA Anticodon Start codon PA Large ribosomal subunit PA E Initiation complex 7/6/1122MDufilho

Figure 7.18 The elongation stage of translation-overview Peptide bond Movement of ribosome one codon toward 3´ end 5´3´5´3´ 5´3´ 5´3´ 5´3´ 5´3´ E PA E PA E PA EPA E PA E PA Two more cycles Growing polypeptide 7/6/1123MDufilho

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 © 2012 Pearson Education Inc. 7/6/1124MDufilho

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 © 2012 Pearson Education Inc. 7/6/1125MDufilho

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 –Result in nonsense mutations 7/6/1126MDufilho

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 7/6/1127MDufilho

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 © 2012 Pearson Education Inc. 7/6/1128MDufilho

Figure 7.33 Transformation in Streptococcus pneumoniae-overview 7/6/1129MDufilho

Figure 7.34 Transduction-overview Bacteriophage Phage injects its DNA. Phage enzymes degrade host DNA. Phage DNA Host bacterial cell (donor cell) Bacterial chromosome Phage with donor DNA (transducing phage) Cell synthesizes new phages that incorporate phage DNA and, mistakenly, some host DNA. Transducing phage Transducing phage injects donor DNA. Recipient host cell Donor DNA is incorporated into recipient’s chromosome by recombination. Transduced cell Inserted DNA 7/6/1130MDufilho

Figure 7.35 Bacterial conjugation-overview F plasmid Origin of transfer Conjugation pilusChromosome F + cell F – cell Donor cell attaches to a recipient cell with its pilus. Pilus may draw cells together. One strand of F plasmid DNA transfers to the recipient. F + cell 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. 7/6/1131MDufilho

Figure 7.36 Conjugation involving an Hfr cell-overview Donor chromosome Pilus F + cell Hfr cell F + cell (Hfr) F plasmid F – recipient Part of F plasmid Donor DNA F plasmid integrates into chromosome by recombination. Cells join via a conjugation pilus. Portion of F plasmid partially moves into recipient cell trailing a strand of donor’s DNA. 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. Incomplete F plasmid; cell remains F – Recombinant cell (still F – ) 7/6/1132MDufilho