Viruses II Flashcards
What are viruses?
Noncellular ( do not have a cellular structure) parasitic
- not classified in any kingdom.
- Infects: bacteria, plants & animals.
- lack metabolism, don’t grow, can’t divide by cell division.
Virus discovery
Adolph Meyer:
Studied tobacco mosaic disease.
Found it could spread from sick plants to healthy ones using liquid plant extracts.
Dmitri Ivanowski:
Showed that the disease could still spread even after all bacteria were removed with the Chamberland-Pasteur filter.
Proved that the cause of the disease wasn’t bacteria, but something smaller—later identified as a virus
Virions
Virus Particles
- Most, very small
- 20 to 250 nanometers in diameter.
Some viruses from amoebae can be up to 1000 nm in diameter.
Large Virions: Poxvirus and other large DNA viruses.
Viruses are too small to be seen with a light microscope, except for large ones.
Electron microscope
Developed in the late 1930s,
Allows us to see
structure of viruses
Scanning and Transmission
Electron Microscopes
Scanning: Seeing surface structure of virions.
Transmission: Seeing both surface and internal structures of viruses.
Virus theories
Devolution/ regressive:
viruses evolve from free living cells
Escapist/ progressive :
- Virus have RNA/ DNA Genome
- Originates from RNA & DNA molecules that escaped from host cell
Self replication:
- System of self-replication
- studies of some plant pathogens support this
Virion Structure:
- Nucleic Acid Core: DNA or RNA.
- Capsid: Outer protein coating.
- Outer Envelope: Sometimes present, made of protein and phospholipid membranes from the host cell.
Additional Proteins: Enzymes may be inside the capsid or attached to the genome.
What are capsids
- Protective layer protein layer made up protein sun bunits AKA capsomeres
Viral Morphology
- Different viral families have different structures.
- Complexity of virus does not always match the complexity of its host.
- All viruses have a nucleic acid genome covered by a capsid.
Capsid Shapes
- Simple forms: Helical or polyhedral shapes.
- Complex forms: More intricate structures.
Virus core
Contains the genome/ total genetic content of a virus
Virus genomes
- Small
- Contain only genes that encodes proteins that virus cannot get from host cell
Single, double stranded, linear, circular
- Some viruses have segmented genomes ( e.g influenza virus) which contributes to its rapid evolution.
DNA viruses
Replication: Host cell’s proteins replicate the viral genome, transcribe and translate it into viral proteins.
Diseases: Chickenpox, hepatitis B, adenoviruses, herpes, and HPV
- Low mutation rate
RNA Viruses
Replication: RNA viruses need to encode enzymes to replicate RNA into RNA or, in the case of retroviruses, into DNA.
Mutation Rate: Higher
mutation rates due to errors in RNA replication.
Diseases: Influenza, hepatitis C, measles, rabies, HIV (an RNA retrovirus).
Bacteriophages
- Viruses that target bacteria
- DNA containing head
- Tail fibers that attach to bacteria
Viral Attachment to Host Cells
Receptors:
-Viruses use molecules on host cells to attach.
- needed for the virus to enter the cell and replicate.
Example: HIV uses the CD4 receptor on T lymphocytes. CD4 helps immune cells communicate.
Glycoproteins: Many viruses use proteins to attach to host cells.
Penetration: virus penetrating the host cell after attachment to replicate.
Adenovirus:
non-enveloped virus that causes respiratory illness in humans, using glycoprotein spikes for attachment.
Other non-enveloped viruses: Poliovirus, Papillomavirus (plantar warts), Hepatitis A virus.
Enveloped Viruses
viruses have a phospholipid bilayer envelope with virus-encoded proteins.
Influenza virus, Chickenpox, HIV, Mumps.
uses glycoproteins
Entry : Receptor mediated endoc or fusion
- many uses endocytosis
- Fusion only occurs in HIV with enveloped viruses
Special fusion proteins in envelopes fuses with plasma membrane of the cell then releases genome & capsid in cytoplasm
Matrix proteins: stabilizing envelope and help in assembling new virions
More fragile sensitive to changes in temp, ph. and disinfects
Classification by Genome
RNA
DNA
Single-stranded
Double-stranded
Linear
Circular
Non-segmented
Segmented
Rabies virus, retroviruses
Herpesviruses, smallpox virus
virus, retroviruses
Herpesviruses, smallpox virus
Rabies virus, retroviruses, herpesviruses, smallpox virus
Papillomaviruses, many bacteriophages
genome consists of a single segment of genetic material Parainfluenza viruses
genome is divided into multiple segments Influenza viruses
Baltimore classification
- Double-stranded DNA
- viral DNA is transcribed into mRNA
Herpes simplex (herpesvirus)
- Single-stranded DNA
Virus sDNA enters the nucleus -> host DNA converts to dsDNA -> host RNA transcribes the dsDNA into mRNA -> ribosomes translates it into viral proteins
Canine parvovirus (parvovirus)
- Double-stranded RNA mRNA is transcribed from the RNA genomeChildhood gastroenteritis (rotavirus)
- Single stranded RNA (+)
Rna is translalatd by the ribosomes to produce the protiens & enzymes needed for replication
Common cold (pircornavirus) - Single stranded RNA (−)
RNA genome is used to make mRNA by transcription
which is then translated into proteins
Rabies (rhabdovirus)
- Single-stranded RNA with reverse transcriptase
RNA genome is turned into DNA by reverse transcription, and then the DNA is used to make mRNA by transcription.
Example: HIV.
- Double-stranded DNA with reverse transcriptase
DNA is transcribed into RNA then the RNA is turned back into DNA by reverse transcription before making mRNA.
Example: Hepatitis B virus.
Non- Enveloped Animal viruses NAKED
No cell wall penetration
Two entry way
Receptor-Mediated Endocytosis:
The virus binds to a receptor on the host cell surface.
The cell “takes in” the virus through a vesicle, as part of its normal cell process of taking in substances.
Capsid Proteins Create Channels:
or
The capsid (protein shell) changes shape, creating channels in the cell membrane.
The virus “injects” its genetic material through the channel into the host cell.
Viriods
- Single-stranded RNA (genetic material) that cause infections in plants.
- No capsid & outer envelope
- Simpler than viruses.
- Can only reproduce inside a host cell,
Viroids don’t make proteins; they only create one specific RNA molecule.
Causes serious damage to plants, especially in agriculture, as they can disrupt plant growth or development.
Example: A viroid called PSTV can infect potatoes, causing damage.
