Post on 21-Dec-2015
The Genetics of Virus
Researchers discovered virus by studying a plant
disease
A virus is a genome enclosed in a protective coat
Phage reproduces using lytic or lysogenic cycle
Animal virus are diverse in their modes of infection
and reproduction
Plant virus are serious agriculture pests
Viroid and prion are infectious agent even simpler
than virus
Viruses may have evolved from other mobile genetics elements
1883 Adolph Mayer
Tobacco Mosaic Virus-- contagious
1890 Dimitri Ivanowsky
Bacteria makes filterable toxins
1897 Martinus Beijerinck
Infectious agent in the filtered sap could reproduce and cannot inactivate by alcohol
1933 Wendall Stanley
Crystallized the TMV particle
Capsid
Protein shell that encloses the viral genome
Capsomere
Capsid build from a large number of protein subunit
Viral envelope
Membrane cloaking the capsid, derived from host cell
Figure 18.3 A simplified viral reproductive cycle
Limited host range
Identify host by lock-and-key
Virus of eukaryotic are
tissue specific
Uses host DNA polymerase to
synthesize genome
Lytic cycle
A phage reproductive cycle that culminate in death of host cell, bacteria lyse, phages release
Three process for emergence of viral disease:
1. Mutation of existing virus
i.e.. High mutation of RNA virus
flu virus
2. Spreading existing virus from one host to another
i.e.. SARS, Hanta virus
3. Dissemination of viral disease from a small isolated population
I.e. AIDS
Plant virus
mostly are RNA virus
Two major route to spread virus:
1. Horizintal transmission
a plant infect from external source of the virus
I.e wind, chilling, injury, insects bite………
2. Vertical transmission
inherit the viral infection from a parent
Viroid
Naked circular RNA
Replicate by using host enzyme
Cause error in regulatory system and control plant growth
Figure 18.10 A hypothesis to explain how prions propagate 1997 Stanley Prusiner
PrionInfectious proteinMad cow disease; degenerative in brain
Virus may have evolved from mobile genetic elements
1. Plasmids
Circular DNA separate from genome
2. Transposon
DNA fragments that move from one location to another
The Genetics of Bacteria
The short generation span of bacteria
helps them adapt to changing environments
Genetic recombination produces new
bacterial strain
The control of gene expression enables
individual bacterial to adjust their metabolism
to environmental change
Different process bring bacterial DNA from different individuals:
1. Transformation
uptake of naked, foreign DNA from surrounding
i.e. uptake of pathogenic pneumonia DNA from
broken bacteria pieces
2. Transduction
DNA transfer process by bacterial phage
3. Conjugation
Direct transfre of genetic materials between
two bacterial
donar: male receiver: female
Plasmids
Small circular, self replicating DNA
Incorporate reversible into bacterial genome
Episome exist as plasmids or in bacteria genome
F plasmid
Required for sex pili
Hfr cells( high frequency of recombination)
F factor integrate into bacterial chromosome
R plasmid
Plasmids carrying antibiotic resistance gene
Transposon( jumping gene)
A transposable genetic element
Movement occur only when recombination of transposon and target site occur
Figure 18.18 Anatomy of a composite transposon
Include extra genes beside insertion
sequence
Helps bacterial adapt to the new environment