Download Viruses Lecture 16 Fall 2008

January 15, 2018 | Author: Anonymous | Category: , Science, Health Science, Infectious Disease
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Viruses Lecture 16 Fall 2008

Viruses • What is a virus? • Are viruses alive?

• Read Discovery of Viruses pgs. 381-382 and Fig. 19.2

1

Virus Structure • Capsid - protein shell – Capsomere – protein subunits

• Viral envelopes - Membranous envelope – Host phospholipids and membrane proteins – Viral proteins & glycoproteins

Fig. 19.3

2

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

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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”

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 Fig. 19.4

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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

Fig. 19.5

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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

Reproductive Cycles of Phages

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• 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

Fig. 19.6

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Reproductive Cycles of Phages • Temperate phages – Use both modes of reproduction

• Environmental cues stimulate switch from lysogenic to lytic cycle

Fig. 19.6

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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

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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

Fig. 19.7

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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

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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 Fig. 19.8

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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

Fig. 19.8

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• Read Pg. 390 Evolution of Viruses

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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

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 1918 • 40 million people died

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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

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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

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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

Fig. 19.11

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