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.

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.

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).

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.

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.

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.

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!

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.

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

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.

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.

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!

Viral Reproduction  Occurs two ways… The Lytic Cycle The Lysogenic Cycle

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  Penetration Synythesis  Maturation  Release

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.

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.