1. Microbial Models The Genetics of Viruses and Bacteria
Chapter 18

2. The Genetics of Viruses
Discovery – researchers discovered viruses by studying the TMV (tobacco mosaic virus)
Infected sap was [rayed on other plants; Beijerinck concluded that the pathogen was reproducing because the concentration of it was undiluted in each new generation of infected plants,
Stanley crystallized the pathogen known as TMV

3. The Genetics of Viruses (Continued)
Viral structure
Viral Genomes – include double strand DNA, single stranded DNA, double strand RNA, or single stranded RNA
Linear or circular
May have four genes or several hundred
Capsids and Envelopes
Capsid – protein coat that encloses the viral genome
Rod-shaped, polyhedral, or complex
Composed of small protein spherical subunits call capsomeres
Envelope – membrane that coats viral capsids
Helps virus infect host
Bacteriophages – most complex virus with a icosohedral head; named T1 – T7

4. The Genetics of Viruses (Continued)
Viral Reproduction
Host Range – virus contain viral proteins that fit into specific cell surface receptor sites of the host
Some ranges may be broad while others are small or even one host ranges
Mechanisms – viruses commandeer their host cell machinery and produce copies of themselves
Patterns of viral genome replication (viral protein production)
DNA to DNA = virus will use hot cells DNA polymerase to copy its DNA genome
RNA to RNA = most host cells do not have an enzyme to copy RNA, the virus cell will carry RNA replicase, an enzyme that uses viral RNA as a template produce complementary RNA
RNA to DNA to RNA – viral cell carries reverse transcriptase, an enzyme that transcribes DNA from an RNA template

5. The Genetics of Viruses (Continued)
Lytic Cycle – virulent phages reproduce only by this mechanisms; virulent phages lyse the host cells resulting in host cell death
Phage attaches to cell surface
Viral surface proteins recognize receptor sites of the host cell
Phage contract sheath and inject viral genome
ATP stored in phage tailpiece powers contraction
Capsid ghost is left behind; capsid with no genetic material present
Hydrolytic enzymes destroy host cell’s DNA
Host cell transcribes then translates viral proteins

6. Cell lyses and releases phage particles
Phage genome directs the host cell to produce phage components: DNA and capsid proteins
Using nucleotides from its degraded genome, the host cell makes copies of the phage genome
Host assembles tail fibers, phage tails, and polyhedral heads
Phage components assemble spontaneously through weak hydrogen bond interactions
Cell lyses and releases phage particles
Lysozymes specified by the viral genome digest bacteria cell wall
Lytic cycle takes 20–30 minutes and may increase the population a hundredfold

7. The Genetics of Viruses (Continued)
Lysogenic Cycle – coexistence of host and phage
Viral replication where the viral genome becomes incorporated into the bacterial genome
Phage  binds to bacteria surface
Phage  injects DNA into host
 DNA forms a circle and either begins lytic or lysogenic cycle
 DNA crosses over into the bacterial DNA and becomes a prophage
Prophage genes are copied along with bacteria genome. As cell divides the prophage is passed to each new daughter cell
Lysogenic Cell – host cell carrying a prophage in its chromosome
Excision (exiting) of the prophage from the bacteria chromosome may begin the lytic cell

8. The Genetics of Viruses (Continued)
Animal Viruses
Provirus – viral DNA that inserts into a host cell chromosome (animals chromosome)
Retrovirus – RNA virus that uses reverse transcriptase to transcribe DNA from the viral RNA (HIV)
Emerging Viruses – make a sudden impact; most likely an existing virus that has expanded its host range

9. The Genetics of Bacteria
Bacterial chromosome is a circular, double stranded DNA found in the nucleoid region
Plasmids – extra chromosomal units found in most bacteria that contain extra genes (double stranded rings)
Binary Fission – reproduction preceded by DNA replication

10. Genetic Recombination – gene transfer between bacteria and other sources of DNA
Transformation – gene transfer during which a bacterial cell assimilates foreign (naked) DNA from its surroundings (Avery’s experiment, chapter 16)
Transduction – gene transfer from one bacteria to another by a bacteriophages
Host cell DNA is packaged along with phage DNA during the lytic cycle and that phage infects another a host

11. Conjugation – direct transfer of genes between two cells that are temporarily joined via a sex pilli (ability to form sex pilli is found in the genes of the F plasmid)
Characteristics of Plasmids
Contain a few genes, not necessary for survival
Replicate independently or in synchrony of the bacterial chromosome
No extracellular stage, unlike viruses
F plasmid – “fertility” plasmid consisting of 25 genes which are involved in production of sex pilli
Replicates in synchrony of the bacterial chromosome
Each daughter cell will be F+
R plasmid – “resistance” plasmid carrying up to ten genes fro antibiotic resistance
Increased antibiotic use has produced many pathogenic resistant strains of bacteria

12. Transposons – pieces of DNA that move from different locations on the chromosome (McClintock, 1940’s)
Causes mutations by interrupting the transcription of mRNA, and therefore, disrupting the translation of that protein
Increase/decrease protein production by inserting within a regulatory gene sequence that controls transcription rates

13. Control of Gene Expression
Operon – a system that allows for the turning on or the turning off of metabolic activites
Operator – the DNA switch found within the promoter region
Repressor – protein that may shut the operon down by binding to the operator so the RNA polymerase can not bind to the promoter to transcribe RNA
Product of a regulator gene

14. Control of Gene Expression (Continued)
TRP Operon – regulates the production of repressible enzymes by producing the amino acid tryptophan
Five genes encode the polypeptides that make the enzymes
When “on”, RNA polymerase binds to the DNA and transcribes the gene’s; inactive repressor; no tryptophan present
When “off”, tryptophan (corepressor) binds to the inactive repressor making it active allowing it to bind to the operator; tryptophan present
When tryptophan is present, it inhibits its own production by activating the repressor

15. Control of Gene Expression (Continued)
LAC Operon – production of enzymes to take up and metabolize lactose
Three genes code for the enzymes necessary to metabolize lactose
When “off”, lactose is absent, repressor will bind to operator stopping the transcription by RNA polymerase to produce the enzymes
When “on”, lactose present, repressor in inactive because of an inducer (allolactose) that will bind to the repressor rendering it inactive
Allolactose is an isomer of lactose that enters the cell and induces the binding of the active repressor to the operator