Chapter 19: Viruses 1. Viral Structure & Reproduction 2. Bacteriophages 3. Animal Viruses 4. Viroids & Prions 1. Viral Structure & Reproduction Chapter Reading pp. 393-396 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) 1
What s a Virus made of? All viruses consist of at least 2 components: Genetic Material usually a single or RNA molecule can be single or double stranded, linear or circular contains the viral genes A 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 The Viral s are hollow, protein shells that: are an array of protein subunits called capsomeres consist of >1 type of protein house the genetic material (/RNA) are frequently involved in host recognition & entry vary in shape, size among viruses 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 2
Viral Genetic Material Viral genomes range from ~4000 to 250,000 bp (or nt) and can be: 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 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 Variety of Viral Structure RNA Capsomere of capsid Capsomere Glycoprotein Membranous envelope Glycoproteins RNA Head Tail sheath Tail fiber 18 250 nm 70 90 nm (diameter) 80 200 nm (diameter) 80 225 nm 20 nm 50 nm 50 nm 50 nm (a) Tobacco mosaic virus (b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4 3
1 2 The Viral Reproductive Cycle Attachment & Entry VIRUS Replication of HOST Viral Gemone CELL Viral Viral mrna 3 proteins Expression of Viral Genes 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 2. Bacteriophages Chapter Reading pp. 396-398 What s a Bacteriophage? A bacteriophage is a virus that infects and destroys bacterial cells. Bacteriophages are of many different types (some w/ 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!*** 4
Bacteriophage Reproductive Cycle T-even bacteriophages have an exclusively lytic reproductive cycle such as this. 5 Release 1 Attachment 2 Entry of phage and degradation of host Phage assembly Head Tail Tail fibers 4 Assembly 3 Synthesis of viral genomes and proteins Lytic vs Lysogenic Cycle Bacteriophage l has 2 options: lytic vs lysogenic Phage Phage The phage injects its. Bacterial chromosome Phage circularizes. Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Daughter cell with prophage Cell divisions produce a population of bacteria infected with the prophage. Lytic cycle Lysogenic cycle Certain factors The cell lyses, releasing phages. determine whether lytic cycle or lysogenic cycle Prophage is induced is entered The bacterium reproduces, copying the prophage and transmitting it to daughter cells. New phage and proteins are synthesized and assembled into phages. Phage integrates into the bacterial chromosome, becoming a prophage. 3. Animal Viruses Chapter Reading pp. 398-401 5
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, RNA) 2) replication of viral genome in nucleus ( or RNA) or cytoplasm (RNA) 3) expression of viral genes can occur entirely in the cytoplasm for some RNA viruses 4) assembly RNA viruses typically assemble in cytoplasm viruses typically assemble in nucleus 5) release via lysis (rupture of plasma membrane), budding or exocytosis host cell is not necessarily killed Reproductive Cycle of a Virus 6
Unique Features of RNA Viruses Copying of viral RNA poses a unique problem: 1) viral RNA must be converted to 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 from an RNA template all other RNA viruses use RNA-dependent RNA polymerase to transcribe from an RNA template Reproductive Cycle of RNA Viruses RNA Envelope (with glycoproteins) 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 Viral genome (RNA) proteins Copy of genome (RNA) New virus Glycoprotein Reverse transcriptase HIV Viral envelope RNA (two identical strands) Viral RNA Retroviral Cycle HOST CELL Reverse transcriptase HIV Membrane of white blood cell RNA- hybrid NUCLEUS Provirus Chromosomal RNA genome for the next viral mrna generation 0.25 m HIV entering a cell New virus New HIV leaving a cell 7
Latent Viral Infections Some viruses and retroviruses integrate viral into host cell chromosomal : analogous to the lysogeny of bacteriophage l the inserted viral 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 Important Animal Viruses 4. Viroids & Prions Chapter Reading pp. 402-406 8
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 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 ( 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 Model of Prion based Illness contact between PrP C & PrP SC induces PrP SC insoluble PrP SC accumulates, kills cells 9
Prion Pathology Normal vs Abberrant Prp Spongiform Encephalopathy Prp C Prp SC 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 10