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Viruses, Viroids, and Prions
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Viruses Living or not living? Inert outside of living host
Cause infection and disease Obligate intracellular parasites Require hosts to multiply Very small structures
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Viruses vs. Bacteria
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Viruses Contain a single type of nucleic acid
DNA RNA Contain a protein coat (sometimes envelope of lipids, proteins, and carbs Multiply inside living cells by using the synthesizing machinery of the cell Cause the synthesis of specialized structures that can transfer viral nucleic acid to other cells
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Viruses Lack enzymes for protein synthesis
Antiviral drugs interfere with cellular multiplication Few or no enzymes present Cannot generate ATP
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Viruses Host range Spectrum of host cells the virus can infect
Invertebrates, vertebrates, plants, fungi and bacteria Bacteriophage (phage) Virus that infects a bacterium Determined by Virus’s requirements for attachment to host cell Outer surface of virus chemically interact with host receptor Availability within host cell of factors required for replication
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Viral size Require EM Viruses vary in size
Some are close to the size of smaller bacteria such as Mycoplasma Rickettsias Chlamydias Range from nm
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Viral size
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Viral Structure Virion Complete Fully developed Infectious
Viral particle Composed of nucleic acid Surrounded by protein coat Classification Protection Vehicle fro transmission
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Viral structure Nucleic acid DNA or RNA Single or double stranded
Linear or circular Several separate segments Influenza Few thousand to 250,000 nucleotides Bacteria have 4 million
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Viral Structure Capsid Protein coat surrounding nucleic acid
Most of mass of virus Composed of capsomeres Arrangement characteristic for a particular virus Single protein type Several protein types
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Viral structure Envelope Covers capsid in some viruses Combination of
Lipids Proteins Carbohydrates Can be derived from host cells plasma membrane Components can determined by viral nucleic acid
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Viral Structure Spikes Carbohydrate – protein complexes
Project from envelope Attachment mechanism Means of identification Hemagglutination Clumping of RBC’s
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Viral structure Non – enveloped viruses Capsid
protects nucleic acid from nuclease enzymes Promotes virus attachment Progeny of mutant viruses have different surface proteins Elude immune system
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Morphology of viruses Helical viruses Rigid or flexible
Nucleic acid within hollow cylindrical, helical Rabies Ebola Hemorrhagic fever
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Morphology of viruses Polyhedral viruses Icosahedron Polio virus
20 triangular faces 12 corners Polio virus
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Morphology of viruses Enveloped viruses Roughly spherical
Enveloped helical Influenza virus Enveloped polyhedral Herpes simplex virus
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Morphology of viruses Complex viruses Bacterial viruses Bacteriophages
Additional structures attached Capsid Polyhedral Sheath Helical
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Morphology of viruses Complex viruses Pox viruses
Lack clearly identifiable capsids Several coats around nucleic acids
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Viral isolation Difficulties
Viruses cannot multiply outside a living host Require living cells not chemical medium Animals and plants difficult to maintain Expensive to maintain
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Viral isolation Bacteriophages Easy to grow
Used to study models of viruses Multiplication Host cell invasion New virus particle production
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Viral Multiplication Virus requires host cell to multiply
Utilizes host cell’s metabolic machinery Single virion can produce several to thousands of new viruses in a single host cell
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Viral Multiplication Two types of multiplication Lytic cycle
Ends with death and lysis of cell T-even bacteriophage Lysogenic cycle Host cell remains alive Bacteriophage λ
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Lytic Cycle Lytic cycle T-even bacteriophage Large virus Complex virus
Nonenveloped Head and tail
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Lytic cycle Lytic cycle 5 stages Attachment Penetration Biosynthesis
Maturation Release
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Lytic cycle Attachment Also called adsorption
Phage particle and bacteria collide Attachment site on virus attaches to complementary receptor on plasma membrane Chemical interaction Weak chemical bonds form
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Lytic cycle Penetration Phage injects DNA into bacterium
Phage’s tail releases phage lysozyme Breaks down portion of bacterial cell wall Tail sheath contracts Tail core driven through cell membrane Capsid remains outside of cell
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Lytic cycle Biosynthesis Viral DNA reaches cytoplasm
Host protein synthesis halts due to degradation of host DNA Viral proteins interfere with transcription or repression of translation
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Lytic cycle Biosynthesis
Phage uses host DNA to synthesize numerous copies of phage DNA RNA transcribed is for phage enzymes and capsids Eclipse period Period where complete viral particles are not yet present
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Lytic cycle
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Lytic Cycle Maturation Bacteriophage DNA and capsids are assembled
Phage tails and head are separately assembled from protein subunits DNA filled head attached to tail
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Lytic cycle Release Virions released from host cell
Lysis of cell occurs as plasma membrane opens up Enzyme lysozyme causes bacterial cell wall breakdown Cell releases new viral particles
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Lytic Cycle
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One – Step Growth Experiment
Burst time Time elapse from attachment to release 20-40 minutes Burst size Number synthesized particle released 50-200
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One – Step Growth Curve
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Lysogenic cycle Lysogenic cycle Bacteriophage λ as model
Lysogenic phages Temperate phages Does not lead to death of host cell May proceed to the lytic cycle Capable of incorporating their DNA into host cell’s DNA Lysogeny Phage remains inactive (latent) Bacterial cell are called lysogenic cells
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Lysogenic cycle Lytic cycle 1 – penetration of E. coli cell
2 – linear phage DNA forms circle 3A – circle can multiply and be transcribed 4 A– new phage production and to cell lysis
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Lysogenic cycle Lysogenic cycle
3B – circle of phage DNA combines with bacterial chromosome Called prophage Prophage genes Repressed by two repressor proteins Products of phage genes Stops transcription of all other phage genes Phage genes turned off
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Lysogenic phase Bacterial chromosome
Chromosome replicates, prophage replicates Prophage remains latent within progeny cells
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Lysogenic cycle Occasionally
Rarely Spontaneously UV light Certain chemicals Excision of phage DNA, and lytic cycle initiated
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Lysogeny Results of lysogeny Lysogenic cells immune to reinfection
Not resistant to other phage types Phage conversion Host cell may exhibit new characteristics Some bacteria may only produce toxin when temperate phage is present Cronybacterium diphtheriae – diptheria Streptococci sp – scarlet fever Clostridium botulinum – botulism Vibrio cholerae - cholera
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Lysogeny Results of lysogeny Specialized transduction
Prophage excised from host cell May carry part of bacterial chromosome Galactose metabolism
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Multiplication in animals
Animal viruses can remain latent in cells for long periods of times Disease may become inserted in a host chromosome in a repressed state Cancer – causing viruses may be latent
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Multiplication in Animals
Similar to bacteriophage Some differences Mechanism of entering cell (penetration) Synthesis and assembly may differ Prokaryote versus eukaryote Animal viruses may have different enzymes than those found in phages
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Multiplication of animal viruses
Attachment Attach to complementary receptors Receptors are glycoproteins of plasma membrane Animal viruses lack tail fibers
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Multiplication of animal viruses
Attachment Attachment sites all over the viruses Attachment may be spikes Receptor sites are inherited traits Varies between individuals
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Multiplication of animal viruses
Attachment Monoclonal antibodies Combine with virus attachment sites Prevent binding with cell’s receptors Treatment for AIDS?
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Multiplication of Animal Viruses
Penetration Endocytosis Active cellular process Virus in vesicle Envelope is destroyed Capsid is digested Enveloped viruses may enter by fusion Viral envelope fuses with plasma membrane Capsid released into cytoplasm HIV
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Multiplication in Animal Viruses
Uncoating Separation of viral nucleic acid and protein coat Lysosomal enzymes Degrade protein coat Specific enzyme coded by viral DNA Synthesized soon after infection May occur while virus is still attached to plasma membrane poliovirus
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Multiplication of animal viruses
Biosynthesis of DNA viruses Replicate viral DNA in nucleus Synthesize capsid proteins in cytoplasm Proteins migrate to nucleus Join with DNA in nucleus New virions formed Transported along ER Some exceptions
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Multiplication of Animal Viruses
Overview 1- attachment 2- penetration and uncoating 3- transcription of “early” viral genes Translation Enzymes require to multiply viral DNA Most viruses utilize host transcriptase (RNA polymerase) Pox viruses Utilize own transcriptase
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Multiplication of Animal Viruses
Overview 4- transcription and translation of “late” viral genes Capsid proteins Other structural proteins 5- synthesis of capsid proteins in cytoplasm
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Multiplication of Animal Viruses
Overview 6- capsid migrates to nucleus Maturation occurs Viral DNA Complete assembly of virions 7 – virions released from the cell
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Multiplication of Animal Viruses
Maturation and release Capsid assembled Enveloped viruses may be released by budding Does not kill host cell Non enveloped viruses Released in ruptures in host cell plasma membrane
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The reproductive cycle of an enveloped virus
Attachment (adsorption) Entry and Uncoating Viral replication and protein synthesis Assembly (maturation) Release
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Entry and Uncoating of Animal Viruses
Figure 13.12a-b
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Entry and Uncoating of Animal Viruses
Figure 13.12c
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Release of Enveloped Viruses by Budding
Figure 13.13
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DNA Viral Families Adenoviridae Herpesviridae
One of many causes of common colds Herpesviridae 100 herpesviruses Simplex virus HHV1 and 2 Chicken pox HHV-3 Mononucleosis HHV-4 Cytomegalovirus HHV-5 Kaposi’s sarcoma HHV-8 AIDS patients
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DNA viral families Papoviridae Hepadnaviridae Papillomas – warts
Papillomavirus (HPV) May cause cervical cancer Polyomas – tumors Vaculoation – vacuoles Hepadnaviridae Cause hepatitis Hepatitis B Others are RNA
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Biosynthesis of RNA viruses
Several different mechanisms of mRNA formation Depends on virus Four examples of four different RNA viruses Picornoviridae Rhabdoviridae Reoviridae Retroviridae
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Picornaviridae Picornaviridae Poliovirus Single stranded RNA
Very small viruses RNA virion Sense strand (+ strand) Can act as mRNA
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Picornaviridae After penetration RNA is translated into two proteins
Inhibit host cell synthesis of RNA RNA –dependent RNA polymerase Catalyzes synthesis of complementary RNA Anti sense strand (- strand) Template to form more + strands
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Rhabdoviridae Rhabdoviridae Rabies virus Contain single – strand
RNA – dependent RNA polymerase present in virus - strand is template for + strand
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Reoviridae Reoviridae Respiratory and enteric system
Double stranded RNA Viral mRNA produced in cytoplasm - strands produced + and – strands join Capsids form around
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Retroviridae Retroviridae HIV 1 and HIV 2 Unique mechanism
1 – contain own polymerase RNA dependent DNA polymerase Reverse transcriptase (RNA DNA)
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Retroviridae Retroviridae 2-Reverse transcriptase uses RNA of virus
Synthesize complementary DNA DNA replicates to form double stranded DNA Degrades original viral RNA DNA transcribed into mRNA for viral protein synthesis
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Retroviridae Retroviridae
3 – viral DNA integrated in host DNA and chromosome This is called a provirus Never exits chromosome Unlike prophage Protects virus from immune system
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Retroviridae Retroviridae Outcomes of integration of provirus
Provirus remains latent Replicates when cell replicates Provirus is expressed Produce new viruses Provirus can convert host cell to tumor cell
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HIV, a retrovirus
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HIV reproduction Play movie
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Consequences of Viral Infection
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Viruses and Cancer Viruses may cause cancer Often undetected
Most particles of viruses do not induce cancer Cancer might develop until long after viral infection Cancers do not seem contagious like viruses
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Oncogenes Oncogene Cancer causing alteration to cellular DNA
Activated to abnormal functioning by variety agents Mutagenic chemicals High energy radiation Viruses Oncoviruses – viruses that can cause cancer
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Cancer Sarcomas Carcinomas Cancers of connective tissue
Chicken sarcoma Carcinomas Cancers of glandular epithelium Adenocarcinomas in mice Human cancer causing viruses
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Oncoviruses Oncoviruses
Genetic material integrates into host cell’s DNA Mechanism is similar to lysogeny May alter host cell characteristic Transformation Tumor cells undergo change Acquire properties different than uninfected cells that do not produce tumors
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Transformation After transformation by virus
Tumor cell contain virus specific antigen on their cell surface Tumor – specific transplantation antigen (TSTA) Or antigen in their nucleus (T antigen) Tumor cells tend to be less round than normal Tumor cells exhibit chromosomal abnormalities
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DNA oncogenic viruses Adenoviridae, Herpesviridae, Poxviridae, Papoviridae, and Hepadnaviridae Papillomaviruses Uterine (cervical) cancer
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Epstein – Barr virus Epstein – Barr virus Causes mononucleosis
Burkitts lymphoma Rare cancer of lymphatic system Children in Africa Nasopharyngeal carcinoma Hodgkin’s lymphoma 90% of U.S. population carry EB virus (latent)
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RNA oncogenic viruses Retroviridae Human T cell leukemia viruses
(HTLV-1 and HTLV-2) Causes Adult T – cell leukemia Lymphoma Feline leukemia virus (FeLV) Contagious Causes leukemia and lymphoma in cats Related to presence of reverse transcriptase Some contain promoters that turn on other oncogenes
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Latent Viral Infections
May live within cell and not cause disease for a long time = latent infection Herpesvirus Herpes simplex virus May remain in host for life of individual Resides in nerve cells Activated by fever or sunburn Fever blister Chickenpox Virus remain latent in nerves Changes in immune response activate latent viruses Shingles
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Persistent Viral Infections
Usually fatal Measles Several years later Subacute sclerosing panencephalitis (SSPE) Differ from latent Persistent virus gradually builds up over a long period of time Latent virus particles appear suddenly
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Prions Proteinaceous infectious particle (prion)
Cause spongiform encephalopathies – large vacuoles in brain Scrapie in sheep CWD – chronic wasting disease in deer Mad Cow Disease – bovine spongiform encephalitis (BSE) Cruetzfeldt-Jakob disease (CJD)
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Prions Prions Present in nervous tissue Transmitted
Surgical instruments Consumption of infected meat/ nervous tissue Cause conversion of a normal host glycoprotein PrPc to PrPSc
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Prion infection Gene for PrPc located on chromosome 20
PrPc produced by cells and secreted to cell surface PrPSc reacts with PrPc on the cell surface converting PrPc to PrPSc PrPSc is taken in by endocytosis and accumulates in lysosomes & cause death Plaques form but are not cause of damage Cause of damage
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Plant viruses and viroids
Plant viruses similar to plants in many respects Some plant viruses can multiply in insects Plant viruses Bean mosaic virus Wound tumor virus Potato yellow dwarf virus Can cause Color change, deformed growth, wilting, stunted growth, symptomless
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Plant viruses
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Plant viruses Cell wall protects plant cells
Viruses enter through wound, parasites (nematodes), and sucking insects
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Viroids Viroids Short pieces of naked RNA 300-400 nucleotides
Often paired No protein coat RNA does not code for proteins Derived form introns Potato spindle tuber viroid (PSTV)
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