9. Genetics Of Bacteria And Viruses

Genetics of Bacteria & Viruses CYTOGENETICS AY 2008-2009

Relevance of Bacterial & Viral Genetics • Microorganisms are the most important component of environmental health • Microorganisms cause diseases • Microorganisms can help heal as well as prevent disease • Microorganisms have numerous commercial/industrial applications • Mitochondria and chloroplasts are microorganisms • Microorganisms serve as model system • Microorganisms are extremely abundant

VIRUSES (Characteristics) • Smaller than bacteria (typically, at least) • Obligate intracellular parasites (some bacteria are also) • structurally simpler than cellular organisms • possess a relative dearth of metabolic machinery • Many possess unusual genomes • Relative dearth of antivirals • Go through an acellular stage

VIRUSES (Parts) – The genomes of viruses are typically much smaller than the genomes of cellular organisms – Virus genomes are also not always composed dsDNA

(i) dsDNA (ii) ssDNA (iii) dsRNA (iv) ssRNA                      

                   

                  

                 

VIRUSES (Parts) – Virus genomes can also take on a variety of configurations, depending on the virus including • (i)          Linear • (ii)         Circular • (iii)        Segmented (more than one DNA molecule, each holding a different gene or genes) • (iv)        Diploid (most viruses are haploid, though)

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                    

                   

                   

Polyhedral Viruses

Figure 13.2a, b

Helical Viruses

Figure 13.4a, b

Complex Viruses

Figure 13.5a

VIRUSES (Host Range) – Many viruses are limited to only a single host species (e.g. bacteriphage)

– Other viruses have broader host ranges, being capable of successfully infecting more than one host species – Many viruses are additionally limited in the cell types they are able to infect within a host (i.e primary & secondary target) – One determinant of the host range of a virus is the "lock-and-key" fit between the virus capsid or envelope proteins and virus receptors, the latter of which typically

Viruses (Life Cycle) The simplified virus life cycle consists of (i) Adsorption to a host cell (ii) Uptake of the virus genome into the cell (iii) Transcription of virus genes (iv) Translation of the resulting virus mRNAs (v) Replication of the virus genome (vi) Packaging of the new virus genomes into capsids (vii) Progeny-virus release from the host cell                      

                   

                  

                 

                   

                 

                

Bacteria l cell

Bacterial chromoso

Capsi

DN Capsi

Sheat

1 Attachment: Phage attaches to

Tail fiber Base Pin Cell wall

Tail

Plasma

2 Penetration:

Phage pnetrates host cell and injects

Sheath

Tail core

3 Merozoites

released into bloodsteam from liver may infect new red blood Figure 13.10.1

Tail

DNA

4 Maturation:

Viral components are assembled into virions.

Capsi

5 Release:

Host cell lyses and new virions are released.

Tail fibers

Figure 13.10.2

Viruses (Life Cycle) – A lytic life cycle requires the destruction of the host cell before progeny release may occur – This host-cell destruction is called lysis

Viruses (Life Cycle) • Lysogenic life cycle (prophage, provirus, temperate virus) – In a lysogenic life cycle virus progeny are neither produced nor released • Temperate virus = a virus capable of going through a lysogenic cycle (e.g., phage lambda, a.k.a., )

• Prophage = a bacteriophage whose genome has integrated into its host's genome during lysogenic growth • Provirus = equivalent to prophage but more generally applicable (e.g., to animal viruses)

Beneficial Uses of Viruses in Biotechnology • Gene therapy • Vaccines and vaccine carrier / delivery vehicles • Antibacterial agents • Insecticides

Bacteria • • • •

Circular chromosome Extrachromosomal: plasmids Constitutive and repressible genes Transposons

Bacteria • Sex: – Transformation – Transduction – Conjugation