Microbial Models The Genetics of Viruses and Bacteria Chapter 18

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

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

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

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

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

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

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

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

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

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

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

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

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

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