Viruses By Roman Boychuk, Luke Caccamo, Robert Morris
What is a virus? • Viruses are small intracellular structures containing DNA or RNA surrounded by a protein shell • Viruses lack any organelles, and thus can only survive and replicate within a host cell
Living or Non-Living? Viruses exhibit some living traits: • Contain a genome • Can replicate • Can mutate and evolve over time But lack others: • Cannot metabolize sugars • Cannot independently form proteins • Therefore, they are not included in the traditional phylogenetic “tree of life”
Classification • Viruses are primarily classified in 3 ways: • Morphology (shape) • Chemical composition (DNA, RNA) • Mode of replication (Lytic vs. Lysogenic Cycle)
1. Morphology Basic structures found in all viruses: • Nucleic Acid: DNA or RNA that is either single- or double-straned • Capsid: A protein coat that surrounds the virus and protects its genome
Morphology Additional Structures: • Envelope: A layer of lipids from the host cell serving as an extra layer of protection for animal viruses • Bacteria-affecting viruses (bacteriophages) have additional structures that allow them to attach to and inject their genes into their host
Symmetry Virus structures follow one of three symmetry patterns: • Polyhedral: Very geometric and round • Helical: Following a spiral pattern • Complex: Having additional structures
2. Chemical Composition • The genes of a virus are either DNA or RNA • Can be single- or double-stranded • The type of chemical composition of a virus can affect many factors of the virus, such as the mutation rate (higher in RNA viruses)
3. Replication Viruses go though one of two replication cycles: • Lytic Cycle: The virus immediately begins replicating itself after entering the host cell • Lysogenic Cycle: The virus enters the host cell’s chromosomes, becomes dormant, replicates with the cell, and later activates to produce more viruses
Viral Lifecycle The virus lifecycle can be summarized in 5 steps: • Attachment • Penetration • Uncoating • Replication • Release
Attachment • Bond between viral proteins and receptors on the host cell • Viral proteins determine which cells the virus can effect • Protein and receptor must be compatible • (i.e. a cold virus cannot affect your nerve cells)
Penetration • The virus penetrates the nuclear membrane • Enter through the nuclear pore complex
Uncoating • Once in the nucleus the virus uncoats • The viral genome is released from the capsid
Replication • Enzymes from the host are used during viral DNA replication • New genomes and proteins are produced • They are then assembled to create new virions
Release • The new virions leave the cell by: • Cell rupture: The cell bursts releasing the virus particles • Budding: takes part of the cell membrane on the way out as the envelope (host cell is not destroyed) • Exocytosis: new virions exported in vacuoles
Lytic Cycle • Virus attaches to host cell and injects its nucleic acid • The new virus’ genes and proteins are replicated in the host cell • The virions are assembled, creating new virus particles • More and more virions are created until the host cell bursts, releasing them into the environment (e.g. A cold sore)
Lysogenic Cycle • Virus attaches to host cell and injects its genes into the host cell which joins the host’s chromosomes • The host cell replicates while the viral genes are dormant for a prolonged period • The viral nucleic acid activates, separates from the host’s chromosomes, and begins the lytic cycle • The new virions are assembled, and the host is destroyed
HIV (Human Immunodeficiency Virus) • Transmitted through bodily fluids • Attacks the immune system, leaving the body susceptible to other micro-organisms • Has a long incubation period (because of lysogenic cycle) and is often initially undetected
Type: Retrovirus • Based on single-stranded RNA • Contains an enzyme called reverse transcriptase, which copies the HIV RNA into the host cell’s DNA • Includes an envelope and leaves host cell primarily by budding
Viral Mutations: Antigenic Drift • Eg. Yearly strains of Influenza • Viruses based on RNA do not have a proofreading process as DNA • Mutations are much more common • Changes occur gradually and accumulate over time • This makes it difficult for vaccines and natural immunities to keep up with new strains of Influenza every year
Viral Mutations: Antigenic Shift • Eg. H1N1 • A sudden change in the genome of a virus • An intermediate host such as a bird or pig is infected by two different strains of a virus • The genes of these two strains combine, creating an entirely new subtype of virus • This entirely new virus can lead to a pandemic similar to the 2009 H1N1 flu pandemic.
Treatments Vaccines Experimental Treatments The Immune System
The Immune System • Macrophages destroys foreign microorganisms • In the event of a viral infection, the body fights back with B and T lymphocytes • B cells produce antibodies; which bind to a virus to stop it from replicating, and mark viruses so that other blood cells know to attack them
The Immune System • T cells alarm the body when they detect invading viruses, kill virus infected cells, and help B cells produce antibodies. • Once a virus has been eliminated, some of the B and T cells will remember the virus. • This ability of the immune system to retain information about and defend against destroyed viruses is what led to the creation of the vaccine.
Vaccination • Vaccines use dead or weak strains of a certain virus in order to prime the immune system and simulate long-term resistance, hopefully without causing the actual infection. • Different kinds of vaccines include; Live/attenuated vaccines, Killed or Inactivated vaccines, toxoids, and subunit and conjugate vaccines.
Experimental Treatment: DRACO • Researchers at MIT have created a drug that can identify and terminate cells that have been infected by any type of virus. • DRACO targets a type of RNA produced only in cells infected by viruses. • Has already been proven to cure dozens of viruses, including influenza.