Chapter 18~ Microbial Models: The Genetics of Viruses and Bacteria

Viral structure

Virus: “poison” (Latin); infectious particles consisting of a nucleic acid in a protein coat

1) Capsid; 2) DNA or RNA; 3) (viral envelopes)

Bacteriophages (phages)

Viral Classes

Viral reproduction: Lytic Cycle

Host range: infection of a limited range of host cells (receptor molecules on the surface of cells)

The lytic cycle:

1- attachment

2- injection

3- hydrolyzation

4- assembly

5- release

Results in death of host cell

Virulent virus (phage reproduction only by the lytic cycle)

Lytic Cycle (T4 phage)

Viral reproduction: Lysogenic Cycle

Genome replicated w/o destroying the host cell

Genetic material of virus becomes incorporated into the host cell DNA (prophage DNA)

Temperate virus (phages capable of using the lytic and lysogenic cycles)

May give rise to lytic cycle

Lambda Virus Replication

Lysogenic Cycle

RNA viruses

Retroviruses: transcribe DNA from an RNA template (RNA--->DNA)

Reverse transcriptase (catalyzing enzyme)

HIV--->AIDS

Retrovirus Replication Retrovirus (HIV)

Viroids and prions

Viroids : tiny, naked circular RNA that infect plants; do not code for proteins, but use cellular enzymes to reproduce; stunt plant growth

Prions: “infectious proteins”; “mad cow disease”; trigger chain reaction conversions; a transmissible protein

Bacterial genetics

Nucleoid: region in bacterium densely packed with DNA (no membrane)

Plasmids: small circles of DNA

Reproduction: binary fission (asexual)

Bacterial DNA Replication

Bacterial DNA-transfer processes

Transformation: genotype alteration by the uptake of naked, foreign DNA from the environment ( Griffith expt.)

Transduction: phages that carry bacterial genes from 1 host cell to another •generalized~ random transfer of host cell chromosome •specialized~ incorporation of prophage DNA into host chromosome

Conjugation : direct transfer of genetic material; cytoplasmic bridges; pili; sexual

Bacterial Plasmids

Small, circular, self-replicating DNA separate from the bacterial chromosome

F (fertility) Plasmid: codes for the production of sex pili (F+ or F-)

R (resistance) Plasmid: codes for antibiotic drug resistance

Transposons: transposable genetic element; piece of DNA that can move from location to another in a cell’s genome (chromosome to plasmid, plasmid to plasmid, etc.); “jumping genes”

Transposons: Jumping Genes Metabolic Pathway

Operons, I

Repressible (trp operon):

tryptophan (a.a.) synthesis

promoter : RNA polymerase binding site; begins transcription

operator : controls access of RNA polymerase to genes (tryptophan not present)

repressor : protein that binds to operator and prevents attachment of RNA polymerase ~ coded from a regulatory gene (tryptophan present ~ acts as a corepressor)

transcription is repressed when tryptophan binds to a regulatory protein

Repressible Operon

Operons, II

Inducible (lac operon):

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

Inducible Operon