9. genetics of bacteria and viruses

8/14/2019 9. Genetics of Bacteria and Viruses

1/23

Genetics of Bacteria &

Viruses

CYTOGENETICS

AY 2008-2009


2/23

Relevance of Bacterial & Viraln ti

Microorganisms are the most importantcomponent of environmental health

Microorganisms cause diseases

Microorganisms can help heal as well asprevent disease

Microorganisms have numerouscommercial/industrial applications

Mitochondria and chloroplasts aremicroorganisms

Microorganisms serve as model system

Microorganisms are extremely abundant


3/23


4/23

VIRUSES (Characteristics)

Smaller than bacteria (typically, at least)

Obligate intracellular parasites (somebacteria are also)

structurally simpler than cellularorganisms

possess a relative dearth of metabolicmachinery

Many possess unusual genomes

Relative dearth of antivirals

Go through an acellular stage


5/23

VIRUSES (Parts)

The genomes of viruses are typicallymuch smaller than the genomes ofcellular organisms

Virus genomes are also not alwayscomposed dsDNA

(i) dsDNA

(ii)

ssDNA(iii) dsRNA

(iv)

ssRNA


6/23

VIRUSES (Parts)

Virus genomes can also take on avariety of configurations, depending onthe virus including

(i) Linear (ii) Circular

(iii) Segmented (more than one DNAmolecule, each holding a different gene

or genes) (iv) Diploid (most viruses are

haploid, though)


7/23

VIRUSES (Parts)

Capsids and envelopes(a) Defining characteristic of viruses

is their protected extracellular state

(b) Protection is achieved via a capsi(c) In addition, an envelope may be

present, surrounding the capsid


8/23

Polyhedral Viruses

Figure 13.2a, b


9/23

Helical Viruses

Figure 13.4a, b


10/23

Complex Viruses

Figure 13.5a


11/23

VIRUSES (Host Range)

Many viruses are limited to only a singlehost species

(e.g. bacteriphage)

Other viruses have broader host ranges,being capable of successfully infectingmore than one host species

Many viruses are additionally limited in thecell types they are able to infect within ahost (i.e primary & secondary target)

One determinant of the host range of avirus is the "lock-and-key" fit between thevirus capsid or envelope proteins and virus


12/23

Viruses (Life Cycle)

The simplified virus life cycle consists of(i) Adsorption to a host cell

(ii) Uptake of the virus genome into thecell

(iii) Transcription of virus genes

(iv) Translation of the resulting virusmRNAs

(v) Replication of the virus genome

(vi) Packaging of the new virus genomesinto capsids

(vii) Progeny-virus release from the hostcell


13/23


14/23

Figure 13.10.1

Attachment:Phageattaches to

Penetration:Phagepnetrates hostcell and injects

Merozoitesreleased intobloodsteam fromliver may infect

new red blood

1

2

3

Bacterial cell

Bacterialchromoso

Capsi DN

Capsi

Sheat

Tail fiber

Base

Pin

Cell wall

Tail

Plasma

Sheath

Tail core


15/23

Figure 13.10.2

Maturation:Viral componentsare assembledinto virions.

Tail

5 Release:Host cell lysesand new virionsare released.

DNA

Capsi

Tail fibers


16/23


A lytic life cycle requires the destructionof the host cell before progeny releasemay occur

This host-cell destruction is called lysis


17/23


Lysogenic life cycle (prophage,provirus, temperate virus) In a lysogenic life cycle virus progeny

are neither produced nor releasedTemperate virus = a virus capable of going

through a lysogenic cycle (e.g., phage lambda,a.k.a., )

Prophage = a bacteriophage whose genome hasintegrated into its host's genome duringlysogenic growth

Provirus = equivalent to prophage but more

generally applicable (e.g., to animal viruses)


18/23

Beneficial Uses of Viruses inBeneficial Uses of Viruses inBiotechnologyBiotechnology

Gene therapy

Vaccines and vaccine carrier / deliveryvehicles

Antibacterial agents

Insecticides


19/23

Bacteria

Circular chromosome

Extrachromosomal: plasmids

Constitutive and repressible genes Transposons


20/23

Bacteria

Sex:

Transformation

Transduction

Conjugation


21/23


22/23


23/23

9. genetics of bacteria and viruses

Documents