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Download Topic 12 (Ch9/7) – Microbial Genetics Genetics Chromosome
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Topic 12 (Ch9/7) – Microbial Genetics Topics - Genetics - Flow of Genetics - Regulation - Mutation - Recombination 1 Genetics • Genome (The sum total of genetic material of a cell is referred to as the genome.) – Chromosome – Gene – Protein • Genotype • Phenotype 2 Chromosome • Prokaryotic – Histone like proteins condense DNA • Eukaryotic – Histone proteins condense DNA • Subdivided into basic informational packets called genes 3 1 Genes • Three categories – Structural – Regulatory – Encode for RNA • Genotype – sum of all gene types • Phenotype – Expression of the genotypes 4 Flow of Genetics • DNA =>RNA=>Protein – Replication – Transcription – Translation 5 Genetic Information Flow 6 2 DNA • Structure • Replication 7 Nucleic Acids 8 Prokaryotic Genome – 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 9 3 DNA is long and occupies a small part of the cell by coiling up into a smaller package. An Escherichia coli cell disrupted to release its DNA molecule. 10 Another Look 11 Eukaryotic Genome – 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) 12 4 Eukaryotic Nuclear Packaging 13 Structure • Nucleotide – Phosphate – Deoxyribose sugar – Nitrogenous bases (purines- adenine, guanine; pyramidines – thymine, cytosine) • Double stranded helix – Antiparallel arrangement 14 Purines and pyrimidines pair (A-T or G-C), backbone sugars linked by phosphate. 15 5 Replication • • • • Semiconservative Enzymes Leading strand Lagging strand – Okazaki fragments 16 Replication is Semiconservative • Means each new molecule is made from one new and one original • New strands are always synthesized in 5’ to 3’ direction 17 Semiconservative replication of DNA synthesizes a new strand of DNA from a template strand. 18 6 Enzymes • • • • • • Helicase DNA polymerase III Primase DNA polymerase I Ligase Gyrase 19 Function of some enzymes involved in DNA replication 20 Leading strand • RNA primer initiates the 5’ to 3’ synthesis of DNA in continuous manner 21 7 Lagging strand • Multiple Okazaki fragments are synthesized • Okazaki fragments are ligated together to form one continuous strand 22 DNA replication process steps. 23 Prokaryotic Bi-direction! 24 8 What is the Process? – 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 25 The Dogma! 26 RNA • Transcription – Message RNA (mRNA) – Transfer RNA (tRNA) – Ribosomal RNA (rRNA) – Others… (see website info) • Codon 27 9 Transcription • A single strand of RNA is transcribed from a template strand of DNA • RNA polymerase catalyzes the reaction • Synthesis in 5’ to 3’ direction 28 mRNA • Copy of a structural gene or genes of DNA – Can encode for multiple proteins on one message • Thymidine is replaced by uracil • The message contains a codon (three bases) 29 mRNA synthesis from DNA 30 10 tRNA • Copy of specific regions of DNA • Complimentary sequences form hairpin loops – Amino acid attachment site – Anticodon • Participates in translation (protein synthesis) 31 Structural Characteristics for tRNA and mRNA 32 rRNA • Consist of two subunits (70S) • A subunit is composed of rRNA and protein • Participates in translation 33 11 Ribosomes bind to the mRNA. This enables tRNAs to bind, followed by protein synthesis. 34 Codons • Triplet code that specifies a given amino acid • Multiple codes for one amino acid • 20 amino acids • Start codon • Stop codons 35 mRNA Codons Specify Amino Acids 36 12 How DNA codes lead to amino acids 37 Protein • Translation – Protein synthesis have the following participants • mRNA • tRNA with attached amino acid • Ribosome 38 Participants in the translation process 39 13 Ribosome Structures 40 Assembled Ribosome 41 3 Stages of Translation • Initiation Ribosomes bind mRNA near the start codon (ex. AUG) tRNA anticodon with attached amino acid binds to the start codon • Elongation Ribosomes move to the next codon, allowing a new tRNA to bind and add another amino acid Series of amino acids form peptide bonds • Termination Stop codon terminates translation 42 14 The process of translation 43 For prokaryotes, translation can occur at multiple sites on the mRNA while the message is still being transcribed = speedy! 44 Transcription and translation in eukaryotes • Similar to prokaryotes except – AUG encodes for a different form of methionine – mRNA code for one protein – Transcription and translation are not simultaneous – Pre-mRNA • Introns • Exons 45 15 Processing pre-mRNA into mRNA requires removal of introns 46 Regulation • Lactose operon – sugar • Repressible operon – Amino acids, nucleotides • Antimicrobials 47 About Gene Regulation... – 75% of genes are expressed at all times – Other genes transcribed and translated when cells need them • (energy conservation) – Regulation of protein synthesis • Typically halts transcription • Can stop translation directly 48 16 Control of Translation • Genetic expression can be regulated at level of translation – Riboswitch • mRNA molecule that blocks translation of the polypeptide it encodes – Short interference RNA (siRNA) • RNA molecule complementary to a portion of mRNA, tRNA, or a gene that binds and renders the target inactive 49 What an Operon Looks Like 50 Operon Overview 51 17 Lac operon example for regulation of sugar metabolism Repression in the absence of lactose Induction in the presence of lactose 52 Example for regulation of amino acids, like arginine No repression when product is being used Repression when product accumulates 53 Antimicrobials • Antibiotics and drugs can inhibit the enzymes involved in transcription and translation 54 18 Mutations • Changes made to the DNA – – – – Spontaneous – random change Induced – chemical, radiation. Point – change a single base Nonsense – change a normal codon into a stop codon – Back-mutation – mutation is reversed – Frameshift – reading frame of the mRNA changes 55 Point Mutation Effects 56 57 19 Mutation repairs -- enzymes that recognize, remove, replace bases 58 Excision repair The Ames test screens environmental and dietary chemicals for mutagenicity and carcinogenicity. 59 Effects of mutations • Positive effects for the cell – Allow cells to adapt • Negative effects for the cell – Loss of function – Cells cannot survive 60 20 Recombination • Sharing or recombining parts of their genome – Conjugation – Transformation – Transduction 61 Conjugation • Transfer of plasmid DNA from a F+ (F factor) cell to a F- cell • An F+ bacterium possesses a pilus • Pilus attaches to the recipient cell and creates pore for the transfer DNA • High frequency recombination (Hfr) donors contain the F factor in the chromosome 62 Conjugation (genetic transmission through direct contact between cells) 63 21 Transformation • Nonspecific acceptance of free DNA by the cell (ex. DNA fragments, plasmids) • DNA can be inserted into the chromosome • Competent cells readily accept DNA 64 Griffith’s classic experiment in transformation DNA released from a killed cell can be accepted by a live competent cell, that expresses a new phenotype. 65 Transduction • Bacteriophage infect host cells • Serve as the carrier of DNA from a donor cell to a recipient cell – Generalized – Specialized 66 22 Genetic transfer (generalized transduction) 67 Genetic transfer (specialized transduction) 68 Transposon • “Jumping genes” • Exist in plasmids and chromosomes • Contains genes that encode for enzymes that remove and reintegrate the transposon • Small transposons are called insertion elements 69 23 Movement of transposons can occur in plasmids and chromosomes 70 24