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.