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Lytic
Infect or kill immediately
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Lysogenic
Can enter and lay dormant and can go into the lytic cycle
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Phoenix cell
- Incomplete retrovirus
- Gag and pol construct viral structure
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Arbor viruse need?
A go between.. insects � west Nile
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Bacteria � asexual reproduction
Parasexual - sexual relationships with another organism � one is sharing information - no go between
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Prototrophs � have everything they need to carry out processes if optimal conditions
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Transformation � uptake of free DNA
Calcium chloride method � breaks down cell membrane from plasmids to enter � cold
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Electroporation � hook up to give electric shock and ipens channels in cell wall and DNA go in
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Conjugation
Direct transfer from a bacteria to a host
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Transduction
Transfer of bacterial DNA by bacteriophages
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Capsule
LPS lipopolysaccharides � immune response � shuts this off
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Competence
Cells are capable of taking up DNA and using it
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Hfr cells
High frequency recombination � recombine with main chromosomes
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Cre-Recombinase � rely on a food additive to induce recombination and insert or eject pieces of DNA - can turn on and off different things
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Episomes � genetic element
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Transformation � is cell contact required? Sensitive to DNase?
NO, YES
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Conjugation � is cell contact required? Sensitive to DNase?
Yes, no
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Transduction � is cell contact required? Sensitive to DNase?
No, no.
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Bacteria and Viruses
- � Small size
- � Rapid reproduction
- � Selective media (e.g., antibiotics)
- � Simple structures and physiology
- � Genetic variability
- � Complete genome sequences
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Their small size, short generation time, and simple structures have made
bacteria and viruses valuable model systems for genetic studies.
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Many basic concepts of genetics were first deduced from studies of
bacteria and viruses.
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The Genetics of Viruses
Viruses can only reproduce by infecting living host cells.
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Bacteriophages are viruses that
infect bacteria.
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Several important genetic concepts have been discovered through studies of bacteriophages.
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Bacteriophage T4
- � Double-stranded DNA genome
- � Protein head
- � Genome contains 168,800 base pairs and 150 characterized genes
- � Lytic phage
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Bacteriophage? Lambda
- � Double-stranded DNA genome
- � Genome contains, 48,502 base pairs and about 50 genes
- � May be lytic or lysogenic
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Mapping Genes in Bacteriophage
- � Genes may be mapped based on recombination frequencies.
- � Host bacteria are infected with two types of phage; progeny phage are screened for recombination.
- � Map distances are calculated as the average number of crossovers between genetic markers.
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Phage phenotypes
- � Temperature-sensitive (ts) mutations
- � Plaque morphology (rapid lysis; lysis inhibition)
- � Host range
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Map of Bacteriophage T4
- � Linear chromosome
- � Circular map
- Explanation: The T4 chromosome is terminally redundant and circularly permutated.
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Viruses are obligate parasites that can reproduce only by
infecting living host cells.
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Bacteriophages are viruses that
infect bacteria.
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Bacteriophage T4 is a lytic phage that infects E. coli, reproduces, and lyses the host cell.
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Bacteriophage lambda can enter a lytic pathway, like T4, or it can enter a lysogenic pathway, during which its chromosome is inserted into the chromosome of the bacterium.
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In its integrated state, the lambda chromosome is called a
prophage, and it�s lytic genes are kept turned off.
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Bacteria contain genes that mutate to produce altered phenotypes. Gene transfer in bacteria is unidirectional�
from donor cells to recipient cells.
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Bacteria
- � One main chromosome with a few thousand genes.
- � Variable number of plasmids and episomes.
- � Asexual reproduction by simple fission.
- � Parasexual processes occur.
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Phenotypes in Bacteria
- � Colony color and morphology
- � Nutritional mutants for energy sources
- � Prototrophs and auxotrophs
- � Dur and antibiotic resistance
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Bacteria usually contain one main
chromosome.
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Wild-type bacteria are prototrophs;
they can synthesize everything they need to grow and reproduce given an energy source and some inorganic molecules.
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Auxotrophic mutant bacteria require
additional metabolites for growth.
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Gene transfer in bacteria is unidirectional;
genes from a donor cell are transferred to a recipient cell, with no transfer from recipient to donor.
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Mechanisms of Genetic Exchange in Bacteria
Bacteria exchange genetic material through three different parasexual processes.
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In transduction,
a bacteriophage transfers DNA from a donor cell to a recipient cell.
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In generalized transduction,
a random fragment of bacterial DNA is packaged in the phage head in place of the phage DNA.
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In specialized transduction,
recombination between the phage chromosome and the host chromosome produces a phage chromosome containing a piece of bacterial DNA.
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A plasmid is a
genetic element that can replicate independently of the main chromosome in an extrachromosomal state.
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Most plasmids are not required for the
survival of the host cell.
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Plasmids in E. coli
- �F Factor (Fertility Factor)
- �R Plasmids (Resistance Plasmids)
- �Col Plasmids (synthesize compounds that kill sensitive cells)
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Episomes
- An episome is a genetic element that is not essential to the host and that can either replicate autonomously or be integrated into the bacterial chromosome.
- Integration depends on the presence of IS elements.
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Three parasexual processes occur in bacteria.
�transformation, conjugation, and transduction�
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These processes can be distinguished by two criteria:
whether the gene transfer is inhibited by deoxyribonuclease and whether it requires cell contact.
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Transformation involves the uptake of
free DNA by bacteria.
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Conjugation occurs when a
donor cell makes contact with a recipient cell and then transfers DNA to the recipient cell.
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Transduction occurs when a
virus carries bacterial genes from a donor cell to a recipient cell.
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Plasmids are
self-replicating extrachromosomal genetic elements.
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Episomes can replicate
autonomously or as integrated components of bacterial chromosomes.
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F factors that contain chromosomal genes (F� factors) are trasnferred to F-cells by
sexduction.
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Closely linked genes can be mapped in bacteria by
three-factor crosses.
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Genetic Exchange in Bacteria
- � Mutation is the source of new genetic variation.
- � Recombination produces new combinations of allele.
- � Transformation, conjugation, and transduction generate new combinations of genes in bacteria to allow bacteria to adapt to new environments.
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Parasexual recombination mechanisms produce
new combinations of genes in bacteria.
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Parasexual mechanisms enhance the ability of
bacteria to adapt to changes in the environment.
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