Viruses Lecture 16 Fall 2008

Viruses What is a virus? Are viruses alive? Read Discovery of Viruses pgs and Fig

Virus Structure Capsid - protein shell –Capsomere – protein subunits Viral envelopes - Membranous envelope –Host phospholipids and membrane proteins –Viral proteins & glycoproteins 2 Fig. 19.3

Virus Structure Nucleic acids –Double-stranded DNA –Single stranded DNA –Double-stranded RNA –Single-stranded RNA Single linear or circular molecule –4 to ~ 1000 genes 3

Virus Reproduction Obligate intercellular parasites Host range –Limited range of host cells each type of virus can infect –~ one cell type in multicellular organisms Host specificity –Recognition proteins “lock & key” 4

Virus Reproduction Recognition –Virus binds to host Entry –DNA only or entire virus Injection Endocytosis Fusion of viral envelope with host membrane Reproduction –Genome copied DNA virus - host’s DNA polymerase RNA virus – virally encoded polymerases –Genome transcribed/translated 5 Fig. 19.4

Reproductive Cycles of Phages Lytic cycle –Results in the release of new phases by lysis of the host cell –New phages can infect other cells Virulent phage –Reproduces only by lytic cycle 6 Fig. 19.5

Reproductive Cycles of Phages Bacterial defense against phages –Natural selection for different receptor proteins –Restriction enzymes Enzymes that cut DNA at specific sequences Host DNA methylated, so virus DNA recognized as foreign –Co-evolution 7

Reproductive Cycles of Phages Lysogenic cycle –Viral genome becomes incorporated into the bacterial host chromosome as a prophage and does not kill the host Prophage –A phage genome that has been inserted into a specific site on a bacterial chromosome 8 Fig. 19.6

Reproductive Cycles of Phages Temperate phages –Use both modes of reproduction Environmental cues stimulate switch from lysogenic to lytic cycle 9 Fig. 19.6

Reproductive Cycles of Phages Prophage mostly silent –One gene codes to block transcription of most other genes Some genes may be transcribed and alter bacteria –Prophage genes code for toxins in bacteria causing diphtheria, botulism and scarlet fever –Prophage in harmless E. coli strain causes food poisoning 10

Animal Viruses Many animal viruses have envelopes Glycoproteins in envelope bind to receptor proteins of host Production of new glycoproteins –Coded by viral genome –Proteins made in ER –Sugars added in ER & GA –Transported to host plasma membrane New capsids w/viral DNA bud from host cell May not kill the host cell 11 Fig. 19.7

Animal Viruses Single-stranded RNA viruses –Serves as RNA Can be translated into proteins right away –Template for mRNA synthesis –Template for DNA synthesis –See table 19.1 RNA-RNA synthesis requires viral enzymes –Carried with genome in capsid 12

Animal Viruses Retrovirus An RNA virus that reproduces by transcribing its RNA into DNA and then inserting the DNA into a cellular chromosome Reverse transcriptase –Enzyme that transcribes RNA template to DNA 13 Fig. 19.8

Animal Viruses Retrovirus E.g., HIV (human immunodeficiency virus) –Capsid contains two molecules of ssRNA and two molecules of reverse transcriptase Provirus –Integrated viral DNA –Never leaves cell - latency 14 Fig. 19.8

Read Pg. 390 Evolution of Viruses 15

Viral Diseases Viral diseases –Cell death –Cell releases toxins –Immune responses to infected cells –Ability of cells to regenerate Polio - mature nerve cells don’t divide Vaccine –A harmless variant or derivative of a pathogen that stimulates a host’s immune system to mount defenses against the pathogen –E.g., smallpox vaccines 16

Emerging Viruses Ability of virus to mutate –RNA viruses have very high mutation rate – copy errors are not corrected by proofreading Dissemination from small isolated human populations Virus spreads from other animals to human population –“Spanish flu” pandemic of million people died 17

Other Infectious Agents Viroids –Molecule of naked, circular RNA a few hundred nucleotides long –Specific to plants –Replicate in host cells –Do not encode proteins –Interfere with plant growth regulatory system 18

Other Infectious Agents Prions –Infectious agent that is a misfolded version of a normal cellular protein –Cause degenerative brain diseases E.g., scrapie (sheep), mad cow disease, Creutzfeldt-Jakob disease (human) –Can be transmitted by eating infected meat 19

Other Infectious Agents Prions Long incubation period (~10 years) Not destroyed by normal cooking temperatures Prion propagation –Prion converts normal proteins to misfolded proteins and forms aggregates 20 Fig