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Viruses

Viruses. Viruses. What is a virus? Defined by their inability to replicate/multiply without utilizing a host cells reproductive mechanisms. Only contain ONE nucleic acid, NOT both. DNA OR RNA Smaller than a bacterial cell! Typically too small to be seen under a light microscope.

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Viruses

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  1. Viruses

  2. Viruses • What is a virus? • Defined by their inability to replicate/multiply without utilizing a host cells reproductive mechanisms. • Only contain ONE nucleic acid, NOT both. • DNA OR RNA • Smaller than a bacterial cell! Typically too small to be seen under a light microscope. • What does a virus do? • “Hijacks” the host cell and uses the hosts organelles to synthesize components required to create new viruses. • The infected cell can produce up to thousands of new virus particles before cell death occurs. • Viral infections cause tissue damage as a result of this cellular death.

  3. Viral Structure • Viral Components: • Nucleic Acid Core: The location of the DNA or RNA of the virus. • Protective Coating: Either a capsid protein coating or a bilayered membrane envelope surrounds the genetic material. • Lipid Envelope: Fatty envelope providing extra protection. • Spikes: Protein spikes that act as antigens – these trigger out immune response to the invading virus. • The two types are known as hemagglutinin spikes and neuraminidase spikes. • Viron: A complete virus particle.

  4. Nucleic Acids • Genome: The genetic information a virus uses to replicate itself in the host cell. • Results in a disruption of host cell activities and eventually cell death. • Nucleic Acid: The total genetic material of a virus. • Can be DNA (deoxyribonucleic acid) or RNA (ribonucleic acid). • Can be Single-stranded or Double-stranded. • Can be Linear (straight), Circular, or Segmented (in fragments).

  5. Capsids • Capsids: The outer structure that encloses & protects the nucleic acid of a viron. • Determines the shape of the virus. • Plays a key role in the attachment of the virus to the host cell. • Capsomeres: Protein subunits that combine to compose of the capsid.

  6. Envelope • Envelope: The bilayer membrane either outside the capsid or acting as the sole protector of the nucleic acid of a virus. • Produced as the virus buds (moves through) the membrane system of the host cell. • Spikes: Glycoproteins extending from the envelope that help to attach virons to receptor sites on host cells. • Naked Virus: A virus that contains only an envelope & no capsid layer.

  7. Envelope • Advantages: • Help protect the virus from the hosts immune system since it is acquired from the host cell membrane. • Help viruses infect new cells by fusion with the host’s cell membrane. • Disadvantages: • Can be easily damaged. • Environmental conditions can destroy the membranes. • E.g., extreme temperature, pH outside of normal range, chemicals, etc. • Naked viruses tend to be more resistant to extreme environments.

  8. Bacteriophages • Bacteriophages: Viruses that specifically infect bacteria. • Uses the bacteria’s structure to replicate new viruses. • When the bacteria is full of bacteriophages, the cell lyses (pops). • This releases endotoxins & causes us to feel ill!

  9. Viral Virulence • Host Range: The number and type of species that a virus is able to infect. • Most have a limited range of one or a small few potential hosts. • Viral Specificity: A property of viruses that refers to the specific kinds of cells that a virus can infect. Determined by 3 things.. • The ability of as virus to attach to a certain cell type based on the presence of specific receptor sites. • Whether the host cell has the proteins & machinery needed for the virus to replicate. • Whether the replicate viruses have a means of exiting the host cell.

  10. Classifying Viruses • Classification of viruses is dependent on… • Nucleic Acid Type • Capsid Shape • Envelope Presence • Viron Size

  11. Classifying Viruses – Nucleic Acid • DNA Viruses: Viruses containing DNA only. • Single-Stranded DNA (ssDNA): Viruses with a single strand of DNA. • Only one known to cause human disease – human papillomavirus (HPV). • Double-Stranded DNA (dsDNA): Viruses with a double-stranded DNA. • Can be linear (straight) or circular in shape.

  12. Classifying Viruses – Nucleic Acid • RNA Viruses: Viruses containing RNA only. Classified by the number of strands and whether it is segmented or in one piece. • Single-Stranded RNA (ssRNA): Viruses with a single strand of RNA – most common. • Positive Sense RNA: Contains RNA that can mimic eukaryotic mRNA – allows the host’s ribosomes to translate the viral RNA into protein. • Negative Sense RNA: Contains RNA that is not translatable by the host ribosomes – these viruses must carry a special RNA polymerase to change the negative strand into a positive strand, allowing RNA translation. • Double-Stranded RNA (dsRNA): Viruses with a double strand of RNA.

  13. Classifying Viruses – Retroviruses • Retrovirus: A type of virus that can translate its own RNA to form a complimentary strand of DNA. • Reverse Transcriptase: A specific enzyme that allows RNA translation & DNA synthesis. • The produced DNA strand is typically replicated to form dsDNA. • The dsDNA migrates to the host cell nucleus & incorporates into the host chromosome. • Retroviruses can be treated using antiviral medication. • HIV is a retrovirus!

  14. Viral Reproduction • Occurs two ways… • The Lytic Cycle • The Lysogenic Cycle

  15. Viral Reproduction – Lytic Cycle • Lytic Cycle: The main replicative cycle of all viruses. • A virus takes over a host cell’s machinery and reproduces new viruses. • The new viruses then exit the host cell & infect new cells. • Occurs in 5 steps: • Adsorption • PenetrationSynythesis • Maturation • Release

  16. Viral Reproduction – Lytic Cycle • Adsorption: The virus attaches to the host cell using a combination of virus & host cell recognition. • Penetration: The virus enters the cell. • Enveloped viruses enter through fusion. • Naked viruses enter through endocytosis (movement into the cell via a vesicle). • Synthesis: New viral genetic material & proteins are synthesized using the host cell mechanisms. • Maturation: Synthesized components are assembled into complete progeny viruses (virons). • Release: The new viruses are released from the cell. • Enveloped viruses can exit via budding in a controlled manner – this does not lyse the cell. • Viruses that do not follow this controlled release will lyse (pop) the cell, causing the clinical symptoms of endotoxin release.

  17. Viral Reproduction – Lysogenic Cycle • Lysogenic Cycle: A long-term, stable relationship between the host cell and the virus. • Involves the viral nucleic acid incorporating into the host chromosome. • Virus can stay dormant this way for long periods. • Dorman virus genes are passed on through each & every host cell division! • Induction: The process of the dormant virus genes becoming activated and initiating a lytic cycle. • Can be spontaneous or in response to an outside stimulus.

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