1. The Genetics of Viruses and Bacteria Ch 18

2. 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)

3. 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)

4. 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)

5. 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

6. 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.

7. 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

8. 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))

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

10. 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

11. 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

12. Bacteria

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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.

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

20. 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

21. Hyperlink for more information
HIV Statistics