The Genetics of Viruses and Bacteria Ch 18

Discovery of the virus (Mayer) 1883: tobacco plants stunted or mottled leaves. Contagious disease through sap contact Pathogen not seen through microscope Ivanowsky filtered sap to remove bacteria, still sap is contagious. 1897: Beijerinck discovered agent could reproduce 1935: Wendell Stanley crystallized agent now known as TMV (Tobacco Mosiaic Virus)

Viral Structure Virus only 20nm diameter, smaller than ribosome. Genetic Material Either DNA or RNA Single strand or double strand Single linear or circular genetic material 4-100 genes Capsid Protein shell surrounding genetic material Various shapes Envelope (Influenza virus) Membranes surrounding the capsid May be host or virus derived Proteins and glycoproteins (some have enzymes w/i)

Reproduction Overview Viruses needs a host cell to reproduce Obligate intracellular parasites Limited host range (receptor recognition of host) and can be tissue specific Zoonotic: jumps/crosses species (HIV) Viral DNA reprograms host cell to make viral proteins and DNA/RNA (hostile takeover)

Two Reproductive Cycles Lytic Cycle Last stage of infection Host cell death Virulent Virus/phage Virus or phage that only goes through the lytic cycle T4 phage infect bacteria using lytic cycle

Some of prophage DNA may be expressed w/i host cell Lysogenic Cycle Replicates virus DNA w/I host’s Temperate viruses Insert DNA into host forming a Prophage Each time host cell undergoes mitosis, viral DNA is also replicated. Eventually enters lytic cycle Prophage leaves host DNA causing lytic cycle to occur Some of prophage DNA may be expressed w/i host cell Diptheria, botilism, scarlet fever would be harmless to humans if prophage genes did not make toxins in host bacteria.

Viral Envelopes Lipid bylayer w/ glycoproteins Glycoproteins act as receptors for binding to host cell Fuses with host cell Herpes Virus (viral envelope) Remains latent in host cell’s nuclei (provirus)(lysogenic) Stress (emotional or physical) causes to go lytic Blister are active form of virus

Retroviruses (RNA) Types Reverse Transcriptase ssRNA serves as mRNA ssRNA template for mRNA ssRNA template for DNA (retroviruses) Reverse Transcriptase Enzyme transcribes DNA from RNA Inserted into host’s DNA Lysogenic Cycle (eventually lytic (AIDS))

Emerging Viruses HIV (1980, California) Ebola (1976) Central Africa, hemorrhagic fever Smallpox (eradicated by WHO in 1977) Worlwide vaccination Measles Polio

Virus and Cancer 1911: Rous discovered that some viruses cause cancer Tumor Viruses Hepatitis B causes liver cancer Oncogenes in viruses affect mitosis of host cell

Viroids and Prions Prions: Viroids Protein molecules replication unknown yet is transmissible Mad Cows Disease/Creutzfeldt Jakob Converts normal proteins to the prion version Normal Cow Cow w/ Creutzfeldt Jakob Viroids Naked circular RNA Infect plants Several hundred nucleotides No proteins Replicates in host cells Disrupts metabolism of plants

Bacteria

Genetics ds DNA, circular Forms dense region in cell called Nucleoid No membrane Binary Fission preceded by duplication of DNA Asexual forming daughter cells identical to parent

Recombinations Recombination generates diversity (natural selection) Transformation Changing of genotypes by uptake or foreign naked DNA Bacteria recognizes specific fragments of DNA that MAY contain alleles needed. Transduction Phages carry bacterial genes from one host to another Generalized Specialized

Conjugation Direct transfer of genetic material b/w two bacteria “F-Factor”; DNA segment that codes for the formation of a sex pilus (male) Can exist as either w/i bacterial chromosome or As a plasmid Episome: genetic section that can exist as either a plasmid or w/i chromosome

F Plasmid 25 genes that control production of sex pilus F+ has F plasmid and is inheritable F- has no F plasmid F+ condition is “contagious” Hrf cell has incorporated the F plasmid into the bacterial genome

R plasmids Plasmids that contain genes that confer resistance to antibiotics Has genes that encode for sex pilus Can carry multiple genes for many antibiotics

Transposons “Jumping Genes” Genes that can move locations w/i chromosome/plasmids Allows for multiple genes in the R plasmid Has inverted sequence on either side of gene Needs transposase enzyme to transpose gene.

Composite Transposons Inverted repeats surround “hitchhiker genes” Antibiotic resistance genes are often these hitchhikers

Gene Therapy Uses transposons to “cure” genetic disorders Insert working gene to replace damaged ones Controversial Space Doctor. Test your Gene Therapy Skills Try this at home Treatment of SCID in humans

Hyperlink for more information HIV Statistics