Chapter18 Microbial Models

The genetics of Virus and Bacteria

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

Figure 18.1 Comparing the size of a virus, a bacterium, and a eukaryotic cell

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

Figure 18.9x Tobacco mosaic virus

Rod shape

> 1000 molecules

Figure 18.02x1 Adenovirus

Infect respiratory

tract

252 identical

protein

polyhedral

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 18.2 Viral structure

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

Figure 18.4 The lytic cycle of phage T4

Figure 18.5 The lysogenic and lytic reproductive cycles of phage , a temperate phage

Figure 18.02x2 Phages

Table 18.1 Classes of Animal Viruses, Grouped by Type of Nucleic Acid

Figure 18.6 The reproductive cycle of an enveloped virus

Figure 18.7 HIV, a retrovirus

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

Figure 18.7x1 HIV infection

Figure 18.7x2 Couple at AIDS quilt

Figure 18.x1 Smallpox

Figure 18.x2 Measles

Figure 18.x3 Polio

Figure 18.x4 Hepatitis

Figure 18.x5 Influenza epidemic

Figure 18.8 Emerging viruses

Ebola virus

Hemorrhagic fever

Figure 18.8x Deer Mouse

Hanta virus

Figure 18.x6 Herpes

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

Figure 18.9 Viral infection of plants

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

Bacterial genome

d.s circular DNA

DNA localized in the nucleoid region

Divide by binary fission

Figure 18.11 Replication of the bacterial chromosome

Figure 18.x7 E. coli

Figure 18.x8 E. coli dividing

Figure 18.x9 Bacterium releasing DNA with plasmids

Figure 18.x10 Plasmids

Plasmids

Small circular, self replicating DNA

Figure 18.12 Detecting genetic recombination in bacteria

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

Figure 18.13 Transduction (Layer 1)

Figure 18.13 Transduction (Layer 2)

Figure 18.13 Transduction (Layer 3)

Figure 18.13 Transduction (Layer 4)

3. Conjugation

Direct transfre of genetic materials between

two bacterial

donar: male receiver: female

Figure 18.14 Bacterial mating

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

Figure 18.15 Conjugation and recombination in E. coli

Figure 18.15 Conjugation and recombination in E. coli

Figure 18.15 Conjugation and recombination in E. coli

Figure 18.15 Conjugation and recombination in E. coli

Transposon( jumping gene)

A transposable genetic element

Movement occur only when recombination of transposon and target site occur

Figure 18.16 Insertion sequences, the simplest transposons

Figure 18.17 Insertion of a transposon and creation of direct repeats

Figure 18.18 Anatomy of a composite transposon

Include extra genes beside insertion

sequence

Helps bacterial adapt to the new environment

Figure 18.19 Regulation of a metabolic pathway

Figure 18.20a The trp operon: regulated synthesis of repressible enzymes

Figure 18.20b The trp operon: regulated synthesis of repressible enzymes (Layer 1)

Figure 18.21a The lac operon: regulated synthesis of inducible enzymes

Figure 18.21b The lac operon: regulated synthesis of inducible enzymes

Figure 18.22a Positive control: cAMP receptor protein

Figure 18.22b Positive control: cAMP receptor protein

Figure 18-22x cAMP

Bacterial and viral growth curves

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