Viruses Viroids And Prions Chapter 13 Microbiology Homework


1 Chapter 19: Viruses 1. Viral Structure & Reproduction 2. Bacteriophages 3. Animal Viruses 4. Viroids & Prions

2 1. Viral Structure & Reproduction Chapter Reading pp

3 What exactly is a Virus? Viruses are extremely small entities that are obligate intracellular parasites with no metabolic capacity of their own. have none of the characteristics of living cells do NOT reproduce or metabolize on their own do NOT respond to their environment or maintain homeostasis in any way **It s hard to kill something that s not really alive, so antibiotics that kill bacteria, fungi, etc, do NOT harm viruses** depend on host cells for their reproduction (which are typically destroyed in the process)

4 What s a Virus made of? All viruses consist of at least 2 components: Genetic Material usually a single DNA or RNA molecule can be single or double stranded, linear or circular contains the viral genes A Capsid a hollow protein capsule which houses the genetic material Some viruses also contain: An Envelope membrane from host cell with viral proteins (spikes) that surrounds the capsid

5 The Viral Capsid Capsids are hollow, protein shells that: are an array of protein subunits called capsomeres consist of >1 type of protein house the genetic material (DNA/RNA) are frequently involved in host recognition & entry vary in shape, size among viruses

6 The capsid of some viruses is enclosed in a phospholipid membrane called an envelope containing viral proteins called spikes : membrane comes from host cell spike proteins involved in attachment and entry into host cell The Viral Envelope

7 Viral Genetic Material Viral genomes range from ~4000 to 250,000 bp (or nt) and can be: DNA or RNA Double- (ds) or single-stranded (ss) if single-stranded, it is referred to as + or + strand = sense or coding strand strand = antisense or template strand

8 Viral Morphology Viruses come in 4 basic morphological types: 2. Helical Viruses 1. Icosahedral Viruses capsomeres in capsid have a helical arrangement 3. Enveloped Viruses capsomeres in capsid have a polyhedral arrangement 4. Complex Viruses consist of multiple types of structures

9 Variety of Viral Structure RNA Capsomere of capsid Capsomere DNA Membranous envelope RNA Capsid Head DNA Tail sheath Tail fiber Glycoprotein Glycoproteins nm nm (diameter) nm (diameter) nm 20 nm 50 nm 50 nm 50 nm (a) Tobacco mosaic virus (b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4

10 The Viral Reproductive Cycle 1 Attachment & Entry DNA Capsid VIRUS 3 Expression of Viral Genes 2 Replication of Viral Gemone Viral DNA HOST CELL Viral DNA mrna Capsid proteins attachment requires highly specific interactions between viral capsid or envelope proteins and host cell surface molecules attachment determines the host range of virus 5 Release from the host cell 4 Self-assembly of new virus particles

11 2. Bacteriophages Chapter Reading pp

12 What s a Bacteriophage? A bacteriophage is a virus that infects and destroys bacterial cells. Bacteriophages are of many different types (some w/dna or RNA, etc), however 2 types are of particular interest due to decades of study: 0.5 mm T-even bacteriophages (T2, T4, T6) bacteriophage lambda (l) ***More is known about the biology of these viruses than any other type of virus!***

13 Bacteriophage Reproductive Cycle T-even bacteriophages have an exclusively lytic reproductive cycle such as this. 5 Release 1 Attachment 2 Entry of phage DNA and degradation of host DNA Phage assembly Head Tail Tail fibers 4 Assembly 3 Synthesis of viral genomes and proteins

14 Lytic vs Lysogenic Cycle Bacteriophage l has 2 options: lytic vs lysogenic Phage DNA The phage injects its DNA. Daughter cell with prophage Phage Bacterial chromosome Phage DNA circularizes. Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Cell divisions produce a population of bacteria infected with the prophage. The cell lyses, releasing phages. Lytic cycle lytic cycle is induced Certain factors determine whether or lysogenic cycle is entered Lysogenic cycle Prophage The bacterium reproduces, copying the prophage and transmitting it to daughter cells. New phage DNA and proteins are synthesized and assembled into phages. Phage DNA integrates into the bacterial chromosome, becoming a prophage.

15 3. Animal Viruses Chapter Reading pp

16 Life Cycle of Animal Viruses The basic life cycle stages of animal viruses differ from bacteriophages in some key ways: 1) attachment and entry requires specific interactions between host cell membrane proteins & viral spike proteins (enveloped) or capsid proteins (non-enveloped) entry by direct penetration, endocytosis or fusion of the envelope with the cytoplasmic membrane uncoating of the virus (release of DNA, RNA) 2) replication of viral genome in nucleus (DNA or RNA) or cytoplasm (RNA)

17 3) expression of viral genes can occur entirely in the cytoplasm for some RNA viruses 4) assembly RNA viruses typically assemble in cytoplasm DNA viruses typically assemble in nucleus 5) release via lysis (rupture of plasma membrane), budding or exocytosis host cell is not necessarily killed

18 Reproductive Cycle of a DNA Virus

19 Unique Features of RNA Viruses Copying of viral RNA poses a unique problem: 1) viral RNA must be converted to DNA which can then be transcribed to produce more RNA OR 2) viral RNA must somehow serve as a template to produce more RNA In reality, RNA viruses use both approaches: retroviruses use reverse transcriptase to make DNA from an RNA template all other RNA viruses use RNA-dependent RNA polymerase to transcribe from an RNA template

20 Reproductive Cycle of RNA Viruses Capsid RNA Envelope (with glycoproteins) Capsid and viral genome enter the cell HOST CELL copying of the viral RNA genome requires RNA-dependent RNA polymerase is commonly packaged in viral capsid (essential for RNA strand viruses) mrna ER Glycoproteins Template Capsid proteins Viral genome (RNA) Copy of genome (RNA) New virus

21 Glycoprotein Reverse transcriptase HIV Viral envelope Capsid RNA (two identical strands) Viral RNA Retroviral HOST CELL Reverse transcriptase Cycle HIV Membrane of white blood cell RNA-DNA hybrid DNA RNA genome for the next viral generation Chromosomal DNA mrna NUCLEUS Provirus 0.25 m HIV entering a cell New virus New HIV leaving a cell

22 Latent Viral Infections Some DNA viruses and retroviruses integrate viral DNA into host cell chromosomal DNA : analogous to the lysogeny of bacteriophage l the inserted viral DNA is considered a provirus which can remain dormant indefinitely such an infection is considered to be latent the provirus can become active due to various conditions of stress in the cell and re-enter the standard viral life cycle

23 Important Animal Viruses

24 4. Viroids & Prions Chapter Reading pp

25 What are Viroids? Viroids are very small, circular RNA molecules (hundreds of nucleotides) that infect plants: viroid RNA does NOT encode any proteins the viroid RNA is copied in the host cell RNA silencing via mirna is thought to perturb host plant gene expression causing the pathology of viroid infection transmission appears to be mechanical: grazing animals cutting tools

26 What are Prions? Prions are unique, infectious proteins that cause spongiform encephalopathies: e.g., mad cow disease, kuru, Creutzfelt-Jacob disease (CJD), scrapie involves NO nucleic acid (DNA or RNA) involves aberrant folding of a normal protein (PrP) expressed in neural tissue normal = PrP C ; aberrant & infectious = PrP SC PrP SC is extremely stable, forms insoluble aggregates consumed PrP SC induces host PrP C to become PrP SC

27 Model of Prion based Illness contact between PrP C & PrP SC induces PrP SC insoluble PrP SC accumulates, kills cells

28 Prion Pathology Normal vs Abberrant Prp Spongiform Encephalopathy Prp C Prp SC

29 Key Terms for Chapter 19 capsid, envelope, spikes icosahedral, helical, enveloped, complex viruses attachment, entry, release, budding bacteriophage, lytic vs lysogenic prophage, lysogen uncoating, latent, provirus RNA-dependent RNA polymerase reverse transcriptase, retrovirus viroid, prion, spongiform encephalopathy Relevant Chapter Questions 1, 4-6, 8

В этот момент Сьюзан поймала себя на том, что готова взвалить на Хейла вину за все свои неприятности. За Цифровую крепость, волнения из-за Дэвида, зато, что не поехала в Смоуки-Маунтинс, - хотя он был ко всему этому не причастен.

Единственная его вина заключалась в том, что она испытывала к нему неприязнь. Сьюзан важно было ощущать свое старшинство. В ее обязанности в качестве главного криптографа входило поддерживать в шифровалке мирную атмосферу - воспитывать .


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