MMG301 1st EditionExam # 3 Study Guide Lectures: 22-32Lecture 22-virus: a DNA or RNA genetic element surrounded by a protein coat capable of replicating in cells as a parasite and exists in a free state -virion: virus particle-bacteriophage: bacterial virus (aka phage)-characteristics of viruses-obligate parasites: only replicate in host cells-contain a genome surrounded by a protective protein coat that allows this genome to be transmitted from one host organism to another; host can be eukaryotic or procaryotic(mostly bacteria, a few archaea)-exists in two states:-extracellular- nucleic acid surrounded by proteins (and sometimes a membrane):termed a virion or virus particle-intracellular- where the virus genome is introduced into the host cell (called infection) and replicates itself-viruses can contain either DNA or RNA in a circular or linear form, or go through DNA and/or RNA intermediates during their life cycle-inner core of nucleic acid: can be single or double stranded, single or multiple segments, circular or linear, DNA or RNA-capsid: protein coat surrounding nucleic acid-proteins assemble in subassemblies called capsomers-capsomers aggregate into protein coat-the entire capsid + nucleic acid = nucleocapsid-complex viruses: composed of separate subassemblies which then assemble into virus-example: T4 bacteriophage-lysis: host cell destruction, does not happen for all viruses-titer: virus concentration-plaque: clear zone on agar plate-bacteriophage lambda: two pathways of existence – lytic and lysogenic-lysogeny: incorporation of phage genome into the host E. coli genome to form a latent state called a prophage-a lysogen (cell containing prophage) is immune to infection by the same virus-induction: excision of viral genome from the host genome – cell then follows a lytic pathway-retrovirus life cycle1. entry by fusion of envelope with cell membrane2. uncoating of virion to release ssRNA3. reverse transcription4. travel to nucleus; integration into host genome5. integration of DNA into host genome6. transcription of viral DNA to make RNA genome copies and mRNA7. encapsidation8. budding and releaseLecture 23-mutations in bacteria-point mutations: substitutions involving one DNA base pair-silent mutation: there is a change in the DNA sequence, but the new codon codes for the same amino acid (DNA sequence changes, protein sequence does not)-nonsense mutation: results in a stop codon-missense mutation: results in a different amino acid-frameshift mutation: results from an insertion or deletion that changes the triplet reading frame-most mutations are caused by chemicals or radiation that cause chemical changes to DNA bases-radiation-ultraviolet: formation of pyrimidine dimers in DNA (DNA can’t replicate properly)-ionizing radiation: breaks in DNA, chemical damage to DNA bases, damage to other molecules-prokaryotes: have ability to repair many types of DNA damage, have…-enzymes that replace damaged DNA bases-polymerases that can replicate through damaged regions-E. coli, like most bacteria, have DNA repair systems-SOS system: a regulon of about 40 genes involved in repair of DNA damage and damage tolerance-types of enzyme-catalyzed DNA repair mechanisms-photo-repair (light repair): enzyme uses light energy to replace thymine dimers-excision repair: enzymes remove a single-strand piece of damaged DNA; DNA polymerase fills in the resulting gap-mismatch repair: enzymes remove damaged DNA base; resulting single base gap is filledin by DNA polymerase-SOS regulon control enzymes-RecA: is activated by DNA damage; multifunctional enzyme-LexA: a repressor of many SOS genes; repressor prevents transcription of genes of operons-activated RecA becomes a LexA-specific protease and degrades the LexA repressor protein; DNA repair genes are therefore switched “on”-reversions: point mutations that give restoration of a wild-type phenotype; can occur two ways-same site reversion: base pair mutates back to original genotype-second site reversion: mutation at a different site restores the wild-type phenotype…frameshift-example: a base pair insertion that corrects an altered reading frame caused by a nearby deletion-types of mutations involving larger segments of DNA-insertions: new stretches of DNA are added (often longer than a few base pairs)-translocations: rearrangements of regions of DNA; large segments of DNA move to another location on the genome-inversions: orientation of a segment of DNA is reversed; sometimes, large portions of genomes involved-auxotroph: a mutant with a nutritional requirement for growth-wild-type: a strain of microbe as isolated from nature-induced mutations: result from deliberate manipulation of cells: chemical or UV irradiation-spontaneous mutations: occur naturally without human manipulation-mutagens: substances that cause mutations; often chemical compounds that react with or bindto DNALecture 24-conjugation: direct plasmid transfer or integrated plasmid-mediated chromosome transfer-in E. coli, the F (fertility) plasmid is involved. -cells having the F plasmid are designated F+-F plasmid encodes the pilus + other genes-F+ cells donate DNA through the sex pilus -the F plasmid can exist either of two states:-as a separate plasmid-incorporated into the cell genome-an F plasmid that is incorrectly excised so that it carries some chromosomal genes is termed an F’ (F-prime) plasmid-plasmid-mediated transfer of genes from one cell to another is also termed mobilization: genes adjacent to the integration site are moved from the donor cell to the recipient cell-a method called Interrupted Mating used Hfr genome transfer to map the relative location of genes on a genome-merodiploid: two copies of a gene within a single cell-a second copy of a gene is carried on a transducing phage-a second copy of a gene is carried on a F plasmid -gene: segment of DNA that codes for a polypeptide, tRNA, or rRNA-operon: contiguous set of genes transcribed as a single mRNA under the control of a single regulatory region-regulon: spatially separated genes regulated by a common regulatory molecule-sigma factor: protein that recognizes specific DNA promoter site; aids binding of RNA polymerase; is released as transcription begins-repressor: dimeric protein binds to symetric inverted repeat sequences on DNALecture 25-regulation of transcription by repression-example: arginine-add arginine to medium of growing cells, genes for arginine synthesis shut down-arginine binds to
View Full Document
Unlocking...