Microbiology Viruses

Synthesis of DNA Viruses

-DNA viruses replicate their DNA in the nucleus of the host using viral enzymes -Synthesize capsid in the cytoplasm using host cell enzymes -Proteins migrate into the nucleus and are joined with the newly synthesized DNA to form virions -Examples: papovaviruses, adenoviruses and herpesviruses

Filterable virus

-Early researchers found that when fluids from host organisms passed through porcelain filters designed to trap bacteria, the filtrate remained infectious -This showed that an infection could be caused by a fluid containing agents smaller than bacteria

True or False: Bacteria are the most abundant microbes on Earth

False, viruses are

Replication of Animal Viruses: 4. Assembly/Maturation

-First step is the assembly of the protein capsid; this appears to be a spontaneous process -The capsid of many animal viruses are enclosed by an envelope made of protein, lipid, and carbohydrate -Envelope protein is encoded by the viral genes and is incorporated into the plasma membrane of the host cell

True or False: Most of the emerging infectious diseases today are caused by viruses

True

True or False: Skies of the flu viruses can change

True

True or False: Viruses are the smallest infectious agents

True

True or False: Viruses greatly outnumber bacteria

True

Neuraminidase (NA) spikes

act as enzymes which function in the release of mature viruses from the host cell

Naked viruses have no...

envelope

Hemagglutinin protein (HA) spikes

spikescalled this because they bind virions to red blood cells and cause the cells to clump together - a process called hemagglutination; overall attach virus to cells

Latency

the state of existing but not yet being developed

Virion

complete, fully developed viral particle capable of infection

Cytopathic effects

defined as virus-induced damage to the cell that alters its microscopic appearance

A viral genome may be either ________ or ________

linear, circular

Lysogenic Conversion examples

-Corynebacterium diptheriae toxin is encoded on a prophage -Other bacteria are made virulent by their prophages such as Vibrio cholerae and Clostridium botulinum

Antigenic drift

-refers to accumulation of HA and NA mutations within a single strain of virus in a given geographic area -Provides relatively minor changes in the virus's antigenicity -This is one reason why we have to get a flu vaccine each year

Viruses and Cancer

-13% of cancers are caused by viruses 1. Transformation: the effect of oncogenic, or cancer-causing viruses: Some viruses carry genes that directly cause cancer Other viruses produce proteins that induce a loss of growth regulation, leading to cancer -Transformed cells: Increased rate of growth Changes in their chromosomes Changes in cell's surface molecules Capacity to divide indefinitely 3. Oncoviruses: mammalian viruses capable of initiating tumors: Papillomaviruses Herpesviruses Hepatitis B virus HTLV-I

Prions disease: Chronic Wasting Disease

-A prion disease that always fatal, contagious, neurological disease -affecting deer species, elk, and moose -It causes a characteristic spongy degeneration of the brains of infected animals resulting in emaciation, abnormal behavior, loss of bodily functions and death

Capsid Morphology: 1. Icosahedral Capsid

-A three-dimensional 20-sided figure with 12 evenly spaced corners -The arrangements of the capsomeres vary from one virus to another

Host Range: 1. Tissue tropism

-A virus usually attacks only certain specific cells and or tissues of that host -Example: hepatitis B infects only liver cells of humans

Latency in Animal Viruses examples

-AIDS, chicken pox, hepatitis B, and herpes viruses -Some viruses such as herpes and chicken pox become latent in nerve cells

Synthesis of RNA Viruses: 1. +ssRNA Viruses

-An RNA viral genome that can act directly as mRNA is called a positive strand RNA -Use RNA polymerase to replicate their genomes -It uses the plus-strand RNA as a template to make a minus strand which can be used as a template for making more plus-strand RNA for progeny virions -Translation of positive strand RNA (as mRNA) begins right away making capsid proteins and the enzyme RNA polymerase -Example: poliovirus

Replication of Animal Viruses: 1. Adsorption/Attachment

-Animal viruses enter a host cell shortly after attachment -Even though entry is not well understood as well as entry of bacteriophages, there are three different mechanisms: 1. Direct penetration 2. Membrane fusion 3. Endocytosis -Prophage is expressing genes that can be harmful to people

Lysogeny: Induction

-At some point the prophage may be excised from the chromosome by a process called induction -Induction may be caused by inductive agents such as ultraviolet light, X-rays, and carcinogenic agents -Once induction occurs, the phage picks up the lytic cycle

Virions: 2. Intracellular (inside the host cell):

-Capsid removed -Virus exists as nucleic acid

Capsid Morphology

-Capsid: the protein coat that provides both protection for the viral nucleic acid and means by which many viruses attach to a host's cells -Composed of proteinaceous subunits called capsomeres -The assembly of the capsomeres results in two most common types: icosahedral and helical

Capsid Morphology: 2. Helical Viruses

-Have rod-shaped capsomers that bond together to form a series of hollow discs resembling a bracelet -Capsomers that bond together in a spiral fashion to form a tube around the nucleic acid -During the formation of the nucleocapsid these discs link with other discs to form a continuous helix into which the nucleic acid strand is coiled

Bacteriophages

-In 1917, Canadian biologist Felix d'Herelle published a paper announcing the discovery of a bacteriophage that preyed on bacteria -"Devourer of bacteria" -Half the bacteria on Earth succumb to phages every two days -Phage therapy was used in the early 1900's to combat dysentery, typhus, and cholera, but was largely abandoned in the 1940's due to the development of antibiotics

Damage to Host Cell

-Individual cells can become distorted, undergo gross changes in size or shape, or develop intracellular changes -It is common to see inclusion bodies, or compacted masses of viruses or damaged cell organelles in the nucleus or cytoplasm

Host Range: 2. Generalists

-Infect many kinds of cells in many different hosts -Example: rabies

Fungal Viruses

-Little is known about them -We do know that fungal viruses exist only within cells; they have no extracellular state -Because fusion of cells is typically part of the fungal life cycle, viral infections can easily be spread from an infected fungus to an uninfected fungus -Only intracellular

Who termed the word "virus?"

-Louis Pasteur -Virus is latin for "poison"

Lysogenic vs Lytic cycle

-Lysogenic cycle: involves the incorporation of the viral genome into the host cell genome, infecting it from within -Lysogenic cycle leads to integration of phage into the host genome. -Lytic cycle: involves the reproduction of viruses using a host cell to manufacture more viruses; the viruses then burst out of the cell -Lytic cycle leads to the death of the host, whereas the

Replication of Animal Viruses: 2. Entry

-Many viruses enter into eukaryotic cells by receptor-mediated endocytosis -The cell's plasma membrane folds inward to form vesicles -If a virion attaches to the plasma membrane of a potential host cell, the host cell may enfold the virion and form a vesicle

Synthesis of RNA Viruses: 2. -ssRNA Viruses

-Minus-strand RNA can't participate directly in gene expression -It is transcribed into plus-strand RNA -This is accomplished by RNA-dependent RNA transcriptase (present in the virion) -Plus strand can be used to make proteins for the capsid and as a template to make more negative strand RNA genomes -Examples include rabies and flu viruses

Biggest Virus to Date

-Pithovirus sibericum -was found deep within the Siberian permafrost, nearly 100 feet underneath the frozen ground -It exclusively infects amoebae -Survived for 30,000 years in the cold

Virions: 1. In the Extracellular (outside the host cell)

-Protein coat (capsid) surrounding nucleic acid -Protects DNA -Nucleic acid and capsid also called nucleocapsid -Some have phospholipid envelope -Outermost layer provides protection and recognition sites for host cells

Role of NA and HA

-SPIKES OF THE INFLUENZA VIRUS -hemagglutinin (HA), neuraminidase (NA)

Latency in Animal Viruses

-Some animal viruses may remain dormant in cell in a process known as latency -First the viruses cause a primary infection that leads to the symptoms of the disease -Then the virus becomes latent and there are no symptoms -Later the virus may be reactivated and cause other symptoms -Latency is permanent when the virus DNA is incorporated in the host DNA as with HIV; there is no induction as with bacteriophages

Viral Spikes

-Some of the proteins are virally coded glycoproteins (proteins bound to a carbohydrate), which appear as spikes protruding from the envelope's surface -Are essential for attachment of viruses to the host cell receptors

Latent Viral Infections

-Some viral infections build up gradually over a long period -Usually are fatal -Example: Several years after causing measles, the measles virus can be responsible for a rare form of encephalitis called subacute sclerosing panencephalitis -In this viral infection, detectable infectious virus builds up over a long period of time, rather than appearing suddenly

Replication of Animal Viruses: 3. Synthesis

-Synthesis and replicative phases are highly regulated and extremely complex at the molecular level -Replication depends on what type of nucleic acid is involved - DNA or RNA; single or double-stranded

What type of bacteriophages infect E. coli?

-T-even bacteriophages -T1 to T7 bacteriophages attack E. coli -T means type

Lysogenic Conversion

-The ability of some phages to survive in a bacterium as a result of the integration of their DNA into the host chromosome -The integrated dna is termed a prophage -When there is a change in the phenotype of the bacterium as a consequence of the specific prophage it carries -Sometimes, phage genes carried by a bacterial chromosome code for toxins or enzymes that cause pathology in the human

Replication of Animal Viruses: 5. Release

-The envelope is acquired during release -Enveloped viruses are often released via budding -Each virion acquires a portion of membrane which becomes the viral envelope -Viral glycoproteins are inserted into cellular membrane which stick out of the surface of the viral envelope -Viral cell may remain alive for some time

Replication of Animal Viruses: 2c. Uncoating

-The removal of the capsid to release the genome Enveloped viruses that penetrate the host cell with their capsids intact must remove them to release the genome - this is called uncoating Varies from virus to virus Some accomplish uncoating by the action of lysosomal enzymes of the host cell

Host Range

-The spectrum of different organisms a particular virus infects -Specificity is due to the precise affinity of viral surface proteins or glycoproteins for complementary proteins or glycoproteins on the surface of the host cell They have to match

Retroviruses

-These are special + ssRNA viruses -Reverse transcriptase forms DNA which then serves as a template to make more + RNA molecules, which act both as mRNA and as genomes for new virions

How are bacteriophages able to find and attach to host cells?

-They use their tail fibers to propel forward until they find a host cell, and then the capsid aids in attachment -They randomly collide with host cells, and then viral tail fibers attach to receptors on the host cell surface -They are non-motile -They have special receptors that sense host cells and fibers that aid in attachment

Antigenic Shift

-This is a major antigenic change in influenza A resulting from the reassortment of genomes from different influenza virus strains infecting the same host cells -Asia is the major site of antigenic shift and the source of most pandemic strains because of the very high density of humans and domesticated birds and pigs there

Viral Multiplication

-Viruses are dependent on their host's organelles and enzymes to produce new virions -Replication cycle usually results in the death and lysis of the host cell -3 types of viral replication

Reverse Transcriptase

-a polymerase (enzyme) that catalyzes the formation of DNA using RNA as a template -encoded by certain viruses (retroviruses)

Prions

-a proteinaceous infectious agent that lacks nucleic acid -Composed primarily of protein (no nucleic acid) -Exact mode of infection is still being investigated -Deposited as long protein fibrils in the brain tissue of humans and animals

Lysogeny

-a state in which phage DNA is incorporated into the host cell without lysis -Method of replication for bacteriophages -Some bacteriophages have a modified type of replication in which the infected cell grows and reproduces normally for many generations before they lyse

Virus

-a tiny, acellular, infectious agent -also known as an obligate intracellular parasite -having one or several pieces of nucleic acid - either DNA or RNA, but never both -wrapped in a protein coat

Replication of Animal Viruses: 2b. Membrane fusion

-an alternate method of entry -The viral envelope and host cell membrane fuse, releasing the capsid into the cell's cytoplasm -Occurs in some enveloped viruses such as measles and mumps -In addition is direct penetration which is accomplished by some naked viruses; the viral capsid attaches and sinks into the cytoplasmic membrane creating a pore through which the genome alone enters the cell

Antigenic drift vs Antigenic shift

-antigenic drift- point mutations- over time -Antigenic shift- overnight- reassortment and jump***ALSO TAKES PLACE WITH B**** no longer A exclusively***

Some viruses are enveloped, which means they have...

-covered their capsid with some of the membrane system taken from a host cell -usually an animal cell

Viruses and Cancer: 2. Oncoviruses

-mammalian viruses capable of initiating tumors: Papillomaviruses Herpesviruses Hepatitis B virus HTLV-I

Viruses and Cancer: 1. Transformation

-the effect of oncogenic, or cancer-causing viruses: -Some viruses carry genes that directly cause cancer -Other viruses produce proteins that induce a loss of growth regulation, leading to cancer -Transformed cells: a. Increased rate of growth b. Changes in their chromosomes c. Changes in cell's surface molecules d. Capacity to divide indefinitely

-Virus families are written with _______ on the end of the name and genera end with ________ -Specific epithets are their common English designations written in ________

-viridae, -virus -italics

Viral Multiplication- Major Sequential Events

1) Adsorption or attachment - a recognition process between a virus and the host cell that results in virus attachment to the external surface of the host cell 2) Penetration - entrance of the virion or its nucleic acid into the host cell 3) Biosynthesis or Replication - copying and expression of the viral genome at the expense of the host's synthetic equipment, resulting in production of the various virus components 4) Assembly or maturation - the individual viral parts come together into whole, intact virions 5) Release - escape from the host cell, of active, infectious viral particles

Viruses, Viroids, and Prions

1. Acellular agents 2. Most lack a plasma membrane 3. Lack most of the characteristics of life 4. Cannot carry out metabolic pathways 5. Can neither grow nor respond to the environment 6. Cannot reproduce independently 7. Utilize chemical and structural components of host cell

Characteristics of Viruses

1. Are obligate intracellular parasites 2. Are ubiquitous in nature 3. Are not cells (acellular) 4. Do not independently fulfill the characteristics of life 5. Basic structure consists of protein shell (capsid) surrounding nucleic acid core 6. Nucleic acid can be either DNA or RNA, but not both 7. Molecules on virus surfaces give them high specificity for attachment to host cell 8. Lack enzymes for most metabolic processes 9. Lack machinery for synthesizing proteins

T-Even Bacteriophages: The Lytic Cycle

1. Attachment: phage attaches by the tail fibers to the host cell 2. Penetration: phage lysozyme opens the cell wall; tail sheath contracts to force the tail core and DNA into the cell 3. Biosynthesis: production of phage DNA and proteins 4. Maturation: assembly of phage particles 5. Release: phage lysozyme breaks the cell wall

Replication of Animal Viruses: Synthesis of RNA Viruses 4 types of RNA viruses

1. Positive single-stranded RNA 2. Retroviruses 3. Negative single stranded RNA 4. Double-stranded RNA

3 types of viral replication

1. Lytic replication in bacteriophages 2. Lysogenic replication in bacteriophages 3. Replication of animal viruses

Virions contain

1. Nucleic acid—DNA or RNA can be single- or double-stranded; linear or circular 2. Capsid—protein coat made of capsomeres (subunits) 3. Nucleocapsid - capsid plus nucleic acid 4. Envelope—lipid, protein, and carbohydrate coating on some viruses 5. Spikes—projections from outer surface 6. Naked virus - has no envelope, just nucleocapsid

Types of Horizontal gene transfer

1. Transformation 2. Transduction 3. Conjugation

Types of Prions in diseases: 1. Creutzfeldt-Jakob disease 2. Bovine spongiform encephalopathy ("mad cow disease") 3. Shy-Drager syndrome or multiple system atrophy

1. afflicts the central nervous system and causes degeneration and death 3. resembles Parkinson's disease

Viruses can have what kind of genomes?

1. double stranded DNA 2. single stranded DNA 3. double stranded RNA 4. single stranded RNA

The particular shape of a virus depends on...

1. how the capsomeres are arranged 2. how many kinds of capsomeres there are

What percentage of human DNA comes from viruses?

8%

Virions

A complete viral particle

Bacteriophages and small pox are examples of...

Complex viruses

Central Dogma

DNA-transcription-RNA-translation-protein

How do viruses multiply?

Multiply by taking control of host cell's genetic material and regulating the synthesis and assembly of new viruses

What type of mutations are responsible for new strains of the flu virus?

Mutations of HA and NA

Eclipse period

New virions appear as they are assembled just before and during the burst period. The period when no intact virions is present

What are the smallest type of viruses?

Parvoviruses

What is Lysogenic Conversion also known as?

Phage conversion

What was the first disease to be globally eradicated?

Small pox

Burst time

The period of time required to complete the entire process, from attachment to release

What is the key to host range?

The virus has to fit into the receptor sites that are made out of proteins or glycoproteins

Horizontal gene transfer vs Vertical gene transfer

Vertical gene transfer: involves the passing of genetic material from one generation to the next through sexual or asexual reproduction -Horizontal gene transfer: involves an existing organism gaining new genetic material and does not result in the formation of progeny

Antigen

a toxin or other foreign substance which induces an immune response in the body, especially the production of antibodies

Genome

all of an organism's genetic material

Complex Viruses...

have capsids of many different shapes and do not readily fit into either of the other two categories

The influenza virus have their lipid envelope covered with prominent glycoprotein spikes composed of ________ and ________

hemagglutinin (HA), neuraminidase (NA)

Burst size

the number of new virions released from each lysed bacterial cell