Chapter 18. Tobacco mosaic virus (TMV) – 1 st identified Structure ◦ Smallest are only 20nm in...
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Transcript of Chapter 18. Tobacco mosaic virus (TMV) – 1 st identified Structure ◦ Smallest are only 20nm in...
Genetics of Viruses and Bacteria
Chapter 18
Tobacco mosaic virus (TMV) – 1st identified
Structure◦ Smallest are only 20nm
in diameter◦ Genome may be double or
single stranded DNA or double or single stranded RNA
◦ Enclosed by a protein shell – capsid Protein subunits – capsomeres Shape differs with virus
◦ May have a membranous envelope (viral envelope) to help with infecting host Ex. Influenza virus
Viruses
Bacteriophages (phages) – viruses that infect bacteria◦ Have most complex capsids◦ Ex. T4 phage – infects E. coli.
May only reproduce within host cells.◦ Host range – range of cells a virus may infect
Determined by receptor molecules on surface of cells May be very wide or very narrow Ex. West Nile – wide; polio – narrow
Lytic cycle – virulent virus (phage) only goes through this cycle◦ Ex. Cold, flu, rabies◦ Results in death of host cell◦ Steps
Attachment to host cell Injection of viral DNA Hydrolyzation – break down
of host DNA Assembly of new viruses Release – cell lyses (breaks
open)
Viral Reproductive Cycles
Lysogenic cycle◦ Ex. HIV, Herpes◦ Does not destroy host cell◦ Temperate phages – may
use either type of reproduction
◦ Steps Attachment to host cell Injection of viral DNA Insertion of viral DNA into
host DNA – called prophage
Reproduction of cell Eventually switches to
lytic cyle
Reproductive cycle cont.
Single stranded RNA viruses may act as mRNA in the host cell – automatically producing viral proteins
Retrovirus – RNA viruses that use reverse transcriptase to transcribe DNA from and RNA template◦ New viral DNA permanently
integrates in the hosts DNA◦ Host cell the produces viral
proteins based on the new viral DNA
◦ Ex. HIV
RNA exceptions
Vaccines – harmless variants of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen
Emerging viruses◦ Mutation of existing viruses◦ Spread of existing viruses across species◦ Spreading of rare viruses as a population becomes
less isolated Plant viruses may spread in two ways
◦ Horizontal transmission – external source◦ Vertical transmission – from parent
Viroids - tiny, naked circular RNA that infect plants◦ Do not code for proteins, but use
cellular enzymes to reproduce◦ Stunt plant growth
Prions – misfolded, infectious proteins that cause degenerative brain diseases◦ Ex. mad cow disease◦ Long incubation periods◦ Trigger chain reaction
conversions of normal proteins
Viroids and Prions
Structure◦ Double-stranded circular
DNA◦ Nucleoid – region in
bacterium densely packed with DNA (no membrane)
◦ Plasmids – small circles of DNA – in addition to the chromosome
BACTERIA
◦ Cell wall~ maintains shape and provides protection – composed of peptidoglycan (sugars & proteins) Gram stain – used to help id Gram +: w/peptidoglycan penicillin action
breaks down walls Gram -: little peptidoglycan, have outer
membranes w/ lipopolysaccharides; most pathogens; impede drug action
◦ Capsule: adherence; protection◦ Fimbriae and Pili: adherence
and conjugation◦ Endospore: resistant cells for harsh conditions (250 million years!)
Structure, II
Domain: Bacteria Domain: Archaea Size – 1-5 μm Shape
•cocci (sphere) •bacilli (rod) •helical (spiral)
Combined biomass is @ least 10X all eukaryotes
Classification
1- Flagella – thinner than eukaryotes 2- Helical shape (spirochetes) – rotate
internal flagella-like filaments 3- Slime 4-Taxis (movement away or toward a
stimulus)
Motility – about 50 μm/s
Asexual – binary fission ◦ Most 1-3 hours, some only
20 min Gene transfer and
recombination◦ Transduction – phages carry
bacterial genes from 1 host cell to another Generalized – random
transfer of host cell chromosome
Specialized – incorporation of prophage DNA into host chromosome
Bacterial reproduction, I
◦ Transformation – genotype alteration by the uptake of naked, foreign DNA from the environment
◦ Conjugation – direct transfer of genetic material cytoplasmic bridges and pili connect two
bacterial cells sexual reproduction
Rapid reproduction allow beneficial mutations to
spread through population quickly
Reproduction, II
Plasmids◦ Small, circular, self-replicating DNA separate from the
bacterial chromosome◦ F (fertility) Plasmid – codes for the production of sex pili
F+ (donor) or F- (recipient) Hfr cell – F factor is built into chromosome
◦ R (resistance) Plasmid – codes for antibiotic drug resistance◦ Transposons – transposable genetic element
Piece of DNA that can move from 1 location to another in a cell’s genome (chromosome to plasmid, plasmid to plasmid, etc.)
“jumping genes”
Bacterial plasmids
Bacteria respond to environmental change by regulating gene expression.
Operons – Unit of genetic function consisting of coordinately related clusters of genes with related functions (transcription unit) – includes the operator, the promoter, and the genes they control
Operons
Repressible (trp operon) – operon is usually on but can be inhibited ◦ tryptophan synthesis ◦ promoter – RNA polymerase
binding site; begins transcription
◦ operator – controls access of RNA polymerase to genes (if tryptophan not present)
◦ repressor – protein that binds to operator and prevents attachment of RNA polymerase – turns operon off coded from a regulatory gene
(tryptophan present – acts as a corepressor)
◦ transcription is repressed when tryptophan binds to a regulatory protein
Operons
Inducible (lac operon) – operon is usually off but can be turned on (stimulated) ◦ lactose metabolism ◦ lactose not present –
repressor active, operon off no transcription for lactose
enzymes ◦ lactose present – repressor
inactive, operon on inducer molecule
inactivates protein repressor (allolactose)
◦ transcription is stimulated when inducer binds to a regulatory protein
Operons cont.