Bacterial Genetics Review • Genome: genetic blueprint • Gene: • Most organisms-DNA • Viruses – DNA or RNA.
Download ReportTranscript Bacterial Genetics Review • Genome: genetic blueprint • Gene: • Most organisms-DNA • Viruses – DNA or RNA.
Bacterial Genetics Review • Genome: genetic blueprint • Gene: • Most organisms-DNA • Viruses – DNA or RNA Nucleic acids • Nucleotides – Sugar – Phosphate – Nitrogenous base • DNA – Guanine-G; cytosine-C; adenine-A; thymine-T • RNA – Uracil-U replaces thymine Review • DNA –ds helix • Strands held together by H bonds • Complimentary base pairing – A pairs with T; in RNA, A pairs with U – C with G • Linear sequence of bases contains info • 3 nucleotides code for 1 amino acid Duplication of DNA • • Replication of chromosome(s) Precise duplication of DNA • DNA polymerase • Denaturation: separate strands Terms • Genotype: set of genes carry instructions • Phenotype: expression of those genes • Genome of prokaryotes includes chromosomal DNA and plasmids Flow of Information • DNA to RNA to polypeptides • Transcription-synthesis of complimentary strand of RNA from DNA-mRNA – Separates strands of DNA – Copies only DNA needed for protein synthesis-mRNA Flow of Information • Translation: protein synthesis – Decodes sequence of nucleotides into amino acids (20) – Genetic Code: codons, group of 3 nucleotides-AAU,CGC • AUG-start codon Degenerate Code • Each codon specifies a particular amino acid • Several codons code same aa Flow of Information • Translation – Ribosomes-site of protein synthesis – tRNA recognizes the specific codon – tRNA forms complimentary base pairing with the codon – Has anticodon & carries the required aa Question • AUG is first codon on mRNA. • What is the anticodon on the tRNA? Regulation of Gene Expression • Prokaryotes: Where is mRNA transcribed in cell? – Translation can begin before completion of mRNA molecule Regulation of Gene Expression • Eukaryotes: Where? – mRNA contains exons and introns – Exons are expressed, encode for aa – Introns do not – Processing by ribozymes to remove introns & splice exons Regulation of Gene Expression • Conserve energy • • • Turn on & off transcription of genes Constitutive genes not regulated Operon Operons • Regulation of metabolic genes • Uses repressors (regulatory genes) – Block RNA polymerase from attaching Induction • • • Turn on the transcription of gene Inducer- induces transcription Inducible enzymes – Synthesized only when substrate is present • Glycolysis genes constitutive Lac Operon • Inducible operon: enzymes to metabolize lactose • Default is “off” • Regulatory sites – Promoter– – Operator- Lac Operon • i genes code for repressor-regulatory protein – Always turned on – Binds to operator • Structural genes – Lac operon-3 genes Lactose in Medium • • • • Binds repressor changing shape Repressor can’t bind RNA polymerase can bind Enzymes for lactose metabolism produced • To turn on operon: Repressible Operon • • Tryptophan operon Usually occurs in anabolism • Repressor is inactive so tyrptophan is synthesized Default in “on” • Tryptophan Operon • Amino acid in media – Binds to repressor activating it • Genes to synthesize amino acid produced Mutation • Change (heritable) in base sequence of DNA – – – Called mutant Genotype differs from parent Phenotype may be altered Types of Mutations • Spontaneous mutations – DNA replication errors – Occur at low frequencies • Induced mutations-mutagens – Alter structure of bases – Errors in base paring Point Mutations • Change in 1 base-pair – Single base is replaced with another – Change in genotype – May be change in phenotype Types of Point Mutations • Silent mutation- no phenotypic change • Degenerate code Missense Mutations • Change in amino acid • Can result in significant changes in polypeptide Examples of Missense • Sickle cell anemia- hemoglobin – Change from glutamic acid (hydrophilic) to valine (hydrophobic) • Change in shape of protein Nonsense Mutation • • Base-pair substitution Create stop codon in middle of mRNA – Premature termination of translation Frame Shift Mutation • Bases deleted or inserted • Shifts translational reading frame • Large insertions are transposons Frame Shift • THE FAT CAT ATE THE BAD RAT • Remove a C • THE FAT ATA TET HEB ADR AT Induced Mutations • • • Mutagens Increase mutation rate Chemical mutagens – Nitrous acid – Occurs at random sites Radiation • UV light -nonionizing • Covalent bonds between certain bases • Adjacent thymines(Ts) can cross link – Pyrimidine dimers • Some enzymes that repair UV damage Nucleoside Analog • Structurally similar to normal bases • Have altered base pairing • 2 aminopurine replaces A & may pair with C • 5-bromouracil replaces T but may pair with G Genetic Recombination • Physical exchange of genes between 2 homologous DNA molecules • Contributes to population’s genetic diversity Horizontal Transfer • Microbes of same generation • Involves a donor cell - gives DNA to recipient cell • Part of DNA incorporated into recipient’s DNA Transformation • Free (naked) DNA in solution – Cells after death, release DNA • Cells may take up DNA – Only in certain stage of cell cycle Competence • Cells able to take up DNA & be transformed • Release competence factor that helps in uptake Transformation • Enzymes cut DNA into small pieces • Recombination between donor & recipient • Few competent bacteria First Evidence of Transformation • Griffith in 1920s • Streptococcus pneumoniae in mice Conjugation • Mediated by one kind of plasmid – F plasmid or F factor • Genes to control conjugation • Donor cells must have F plasmid Differs from Transformation • Cells must be of opposite mating types – Donor is F plus – Recipient is F minus • Requires direct cell contact • Transfers larger quantities of DNA Conjugation • Gram negative cells • Gram positive cells produce sticky surface molecules – Keeps cells together Conjugation • Plasmid is replicated – A copy of plasmid transferred to recipient • F minus cell becomes F plus • Receptors on new F plus change F Plasmid • Plasmid integrates into the chromosome converts cell to Hfr cell( high frequency of recombination) • F factor DNA can separate and become plasmid Conjugation • Hfr and F- cell • Replication of Hfr begins in middle of integrated F factor • Small piece leads the chromosome into Fcell • Donor DNA can recombine (DNA not integrated is degraded) Transduction • Bacterial DNA is transferred via a virus -Bacteriophage • Virulent phages –lytic cycle • Generalized transduction – Any gene on donor chromosome transferred Generalized Transduction • All genes are equally likely to be packaged inside phage – Virus cannot replicate in new bacteria – Defective virus • Specialized transduction-only certain bacterial genes transferred Plasmids • • Extra chromosomal material F factor or plasmid is a conjugative plasmid – Carries genes for sex pili and for transfer Dissimilation Plasmids • Enzymes break down unusual sugars and hydrocarbons – Pseudomonas use toluene and petroleum as carbon and energy sources – Used to clean up oil spills – Allows bug to grow in adverse environments Virulence Plasmids • E. coli carries plasmids that code for toxins –diarrhea • Bacteriocins- toxic proteins kills other bacteria – E. coli produces colicins Resistance Factors • R factors- resistance to antibiotics, heavy metals or cellular toxins • Wide spread use of antibiotics- led to selection of bacteria with R factors with resistant genes Transposition • Segment of DNA moves from one place in chromosome to another – Rare event – Transposable elements – Insert within a gene & inactivate it Transposons • Contain information for own transposition • Insertion sequences (IS) contain only gene for transposase • Complex transposons carry genes for enterotoxins or antibiotic resistance