The Genetics of Viruses and Bacteria Gene Expression Chapter 17 and Chapter 35

1. The Genetics of Viruses and Bacteria Gene Expression Chapter 17 and Chapter 35

2. The Genetics of Viruses • Discovery – researchers discovered viruses by studying the TMV (tobacco mosaic virus) • Infected sap was sprayed 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 • ViralGenomes – 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 • HostRange– 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) • LyticCycle– 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. 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

8. The Genetics of Viruses (Continued) • LysogenicCycle– 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 • Prophagegenes are copied along with bacteria genome. As cell divides the prophage is passed to each new daughter cell • LysogenicCell– host cell carrying a prophage in its chromosome • Excision (exiting) of the prophage from the bacteria chromosome may begin the lytic cell

10. 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) • EmergingViruses – make a sudden impact; most likely an existing virus that has expanded its host range

11. 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) • BinaryFission – reproduction preceded by DNA replication

12. GeneticRecombination – 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

13. 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

14. 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

15. 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

16. Control of Gene Expression (Continued) • TRPOperon – 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

17. Control of Gene Expression (Continued) • LACOperon – 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 • Allolactoseis an isomer of lactose that enters the cell and induces the binding of the active repressor to the operator