Viruses

  • Virus

    • obligate intracellular parasite that sub-microscopic and has a specific rang of hosts

  • Viral structure

    • Nucleic acid

      • DNA

      • RNA

    • Capsid

      • outer protein cover protecting genetic material

        • basic protein subunits

          • capsomeres

        • protect nucleic acid

        • interact with host cell surface

        • facilitate nucleic acid penetration into cell

    • Capsids determine shape

      • icosahedron symmetry

  • Some viruses have

    • Envelope

      • Lipids and proteins

        • helps with entry

        • does not protect

    • Spikes

      • proteins in envelope that assist in recognition and attachment of host cells

      • do not penetrate

  • Classifying Viruses

    • Type of host

      • animal viruses, human viruses

    • Nucleic acid content

      • Viral genomes include single stranded RNA, single stranded DNA, double strand RNA, single strand RNA

    • Envelope presence

      • Enveloped viruses

      • Naked viruses

        • non-enveloped viruses

    • Infection type

      • lytic, persistent

  • Steps in Replication

    • Attachment

      • use capsids or envelope spikes to attach to host cell receptors

        • outside host cell

          • virion

    • Entry/penetration

      • virus injects genetic info into host cell

      • envelope is lost

    • Uncoating

      • capsid sheds

    • synthesis/replication

      • viral genome is replicated and capsid proteins produced

        • hijack host genetic machinery

          • Single Strand DNA

            • use host enzymes to make a complementary strand

          • Double strand DNA

            • replicated in the nucleus

          • Positive sense single strand RNA

            • genomes acting directly as mRNA

          • negative sense single strand RNA

          • Double stranded RNA

            • use host enzymes to make proteins from positive-sense stands

          • Retroviruses

            • use reverse transcriptase

      • Assembly

        • Assembling protein capsid around the nucleic acid core

      • Release

        • Replicated viruses are released

          • budding-enveloped

          • lysis-naked

        • Viruses acquire the envelope during the release stage

  • Retroviruses

    • Retrovirus

      • replicated by converting its single stranded RNA genome to double stranded DNA and integrating into host genome

        • contain genes producing reverse transcriptase

    • Reverse Transcriptase

      • DNA-RNA hybrid forms

      • RNA is removed

      • DNA stand templates and makes DS DNA

      • Integrase integrates dna

        • Provirus/Prophage

      • HIV is a retrovirus

  • Viral Pathogenicity

  • Cell penetration ability

    • wounds, insect bites, mucous membranes

    • mucosal immunity

  • Intracellular growth ability

  • Defense mechanism resistance

  • Host damage ability

    • lysis

    • tranforming genome

    • etc

  • How Viruses Enter Human body

    • Mucous membrane

      • respiratory most common

      • alveolar macrophages

        • important immune cells fighting infection in alveoli and protecting against pathogens

      • genital, conjunctiva (Eyelids)

      • Skin

        • parenteral route

          • non oral infection

            • insect bites, surgical trauma

  • Key points

    • Entry point is not the infection site

    • Entry point is not the exit point

      • Fecal-0ral route

        • entering via mouth exiting via feces

    • Characteristic entry patterns exist

  • How Viruses spread

    • Local spread over epithelial surface

    • Neurotrophic viruses

      • nervous system cells

    • Blood-brain barrier

    • Lymphotropic viruses

      • spread through lymphatic system

    • Viremia

      • viruses present in blood

    • Skin penetration

    • Congenital disease

      • viruses pass mother to fetus via placent

  • How Viruses Shed

    • Virus shedding

      • virus exit methods

  • Consequences of a viral infection

  • Types of viral infections

    • lytic infection

      • standard viral infection

        • enter, hijack, destroys via lysis

  • Abortive

    • Enter but The virus cannot replicate

  • Productive

    • Virus can replicate

  • Persistent Infecvtion

    • The virus does not immediately kill the hosts

    • Host cells continue to produce

      • result in chronic infections

  • Latent infections

    • Viruses remain dormant until triggers activate replication

      • Cold sores

  • Transforming infections

    • viruses affect host genetics, causing mutations

      • oncogenes, which cause cancer

  • Virus-Induced changes in cells

    • Cytopathic effects refer to microscope-visible effects of viral infections

      • Viral plaque formation

        • visible areas in Petri dishes where viruses destroyed infected cells in culture

      • Synctia

        • formation involves the creation of a single cell with multiple nuclei

  • One Step Growth Cycle

    • A

      • Attachment and penetration

        •  receptor-mediated endocytosis

          • Forming endosomes

          • Low ph causes uncoating and fusion of the virus to an endosome

    • B

      • Eclipse

        • No detectable viruses exist, but components are produced

    • B

      • Yield/maturation

        • Burst/Rise

          • Infectious viruses increase as assembly begins

    • C

      • release

      • Viral particle numbers remain high

      • viruses liberated through budding or lysis

  • Viral Syndromes

    • Syndrome

      • Collection of signs and symptoms

        • Signs

          • objective manifestations

            • ie rashes

          • Symptoms

            • internal manifestations

    • Viral signs and symptoms

      • oral infections, RT infection, rashes, eye infections, fevers, hepatitis, STDS, etc

  • Mechanisms for Viral Reproduction

    • Lytic Cycle

      • Type of phage replication cycle resulting in the release of new phages by lysis (and death) of the host cell

        • Mechanism

          • Phage binds to surface receptor, injects DNA

          • DNA of host is degraded

          • Host’s metabolic machinery makes DNA and protein of the phage

          • The phage produces lysozymes, cell lyses

          • new phages are released

    • Lysogenic cycle

      • Integration of the bacteriophage’s nucleic acid into host genome

        • no cell death

          • Phage binds to receptor

          • DNA of phage takes a round shape

            • Prophage

          • Induction

            • environmental cue causes the prophage to synthesize new phages

              • new phages enter lytic and kill host

    • Temperate Phage

      • Phage capable of establishing lysogeny

      • Lysogen

        • bacterium that contains a virus

    • Lysogenic conversion

      • Bacterial host acquires a new trait as a direct result of expression of a gene or lysogen

  • How Viruses are Grown in Lab

    • Viruses can be cultured in

    • cell culture

    • lab animals

      • Primary culture

        • group of cells removed from an organism and placed in suitable environment

      • Monolayer

        • Layer or homogenized cells growing side by side of one another

    • embryonated eggs

  • Quantifying viruses

    • Plaque Assay Technique

      • place a few drops of host cells on agar plate

      • Allow host cells to grow

      • Infect bacteria with virus

      • Count number of plaques, or clear areas where the virus has killed a group of cells through lysis

    • Hemagglutination Assay

      • Hemagglutinin causes red blood cells to agglutinate, or clump up

      • Expose solution of viruses to cells and observed agglutination

    • Tissue culture assay technique

      • uses human or animal cells

  • Viruses and Cancer

    • Viruses have the ability to cause cancer

      • Hep B/C

        • liver cancer

      • HPV

        • cervical

      • Neoplasia

        • Any new abnormal cell growth/reproduction due to loss of regulation of cell cycle

          • Neoplasia leads to formation of tumors

          • Tumors can become dangerous when they metastasize

          • Anaplasia

            • loss of cell differentiation that allows cancer to spread

      • Carcinogenesis

        • Process by which can forms

          • Oncogenes

            • cancer causing genes

  • Prions

    • Infectious proteins

      • BSE/Mad cow

      • Scrapie (sheep)

      • Creutzfeldt-Jakob

    • Proteins have PrP

      • Disease causing protein turns PrP into disease causing protein

  • Viroids

    • Infectious agents consisting of closed, single-stranded RNA

    • Require host cell to replicate

    • Cause diseases in plants