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Doesn’t belong to any kingdom -It’s not a plant or an animal.
-It’s not a fungi, protist, or bacteria.
WHAT IS A VIRUS?
How many characteristics of life do viruses possess?
*Genetic Material
Are viruses living?
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I. Viral Structure and Characteristics
A. Characteristics: – NOT ALIVE!
– Has DNA or RNA as its genetic material
– Cannot replicate or carry out life processes outside a HOST cell.
– Obligate intracellular parasite.
• A virus is an infectious agent made up of nucleic acid (DNA or RNA) wrapped in a protein coat called a capsid.
• Viruses have NO nucleus, NO organelles, NO cytoplasm or NO cell membrane
• This is why it does NOT belong to any kingdom.
vs
B. Viral Structure
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C. Viral shapes Polyhedral Bacteriophage
Helical
Enveloped
Viral shape
• Refers to the shape of the capsid that contains the viral DNA.
– Polyhedral
– Helical
– Phages (we seen these before Bacteriophages)
– Enveloped - have a “Cloak” of membrane from the host cell covering the virus. The membrane coat helps the virus go undetected and evade detection of the hosts immune system.
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Viral Morphology
• Note that regardless of shape, all viruses have the same composition
– Genetic material (DNA or RNA) – we’ll get to this
– Protein capsule (capsid)
– Some RNA viruses also carry an enzyme, reverse transcriptase. We’ll get to this in more detail later.
Classification of Viruses
• Viral classification is based on type of genetic material the virus has:
– DNA Viruses:
• (ssDNA) single stranded linear DNA
• (dsDNA) double stranded DNA can be either linear or circular.
– RNA Viruses:
• All are (ssRNA) single stranded DNA and linear
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HIV Infected Cell (This is the reason why HIV is so incurable.)
Viral Replication Viruses with RNA that is transcribed into DNA inside the host are called retroviruses.
• Viruses are parasites—an organism that depends entirely upon another living organism (a host) for its existence in such a way that it harms that organism.
• A flea is a parasite to a dog and is harmful to the dog.
Capsid
Nucleic Acid
Bacteriophage—a virus that infects bacteria (bacteria is the host)
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3. DNA/RNA is copied.
2. DNA/RNA injected into cell. 1. Virus attaches to
cell.
4. Virus copies itself.
5. Cell bursts (lyses) and releases new
viruses.
Step 1 Step 2 Step 3
Step 4 Step 5
Replication: • How a virus spreads.
• A virus CANNOT reproduce by itself—it must invade a host cell and take over the cell activities, eventually causing destruction of the cell and killing it. (The virus enters a cell, makes copies of itself and causes the cell to burst releasing more viruses.)
Retroviruses
• ssRNA viruses must copy their RNA genome into a DNA strand before it can be integrated into the host cells genome.
• Reverse Transcriptase is the enzyme responsible for this.
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Replication is either Lytic or Lysogenic
• Lytic phase– 1. Virus infects host cell. 2. Virus takes over host cell,
replicates its genome and viral proteins.
3. New viruses are assembled, and cause cell to burst (LYSES), releasing 100’s- 1000’s new virus particles.
4. Some viruses are only lytic.
• Lysogenic Phase - 1. Virus infects host cell 2. Viral genome is integrated
into the host cells genome. 3. Here the viral genes will
remain dormant until activated.
4. If the host cell replicates, it also replicates the inserted viral genome, producing more infected cells.
5. After activation, steps 2 & three of the lytic phase will occur.
Lytic/Lysogenic Phases Start here!
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Specificity
It’s like the pieces of a puzzle. The ends have to match up so only certain pieces fit.
Surface Markers
Receptor Sites
• Most viruses are specific to the species of host and even the type of tissue they infect.
Example: The rabies virus only attacks brain or nervous cells.
Virus
Cell
Surface Markers
Receptor Sites
Virus
A virus recognizes cells it can infect by matching its surface marker with a receptor site on a cell.
Cell
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Viral Disease
• VECTOR – is an intermediate host. An organism that is infected but is not affected by the virus (asymtomatic)
• Example: West Nile Virus – the vector is mosquitos. The carry the virus from host to host without being effected.
Importance: *Harmful Causes disease—pathogenic Disease producing agent—pathogen
•Human Diseases: Warts, common cold, Influenza (flu), Smallpox, Ebola, Herpes, AIDS, Chicken pox, Rabies •Viruses disrupt the body’s normal equilibrium/balance
•Viruses can be prevented with vaccines, but NOT treated with antibiotics. (antibiotics treat bacteria)
Beneficial: Genetic Engineering—harmless virus can carry good genes into specific cells.
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HIV/AIDS • Human immunodeficiency Virus (lysogenic
phase) • Acquired Immune Deficiency Syndrome (Lytic
Phase) – Is a viral infection that specifically attacks the T-
lymphocytes (white blood cells) of the immune system.
– In its lysogenic phase it is undetected buy the immune system.
– In its lytic phase, the cells that would respond are the ones compromised.
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RNA or DNA core (center), protein coat (capsid)
Copies itself only inside host cell--REPLICATION
DNA or RNA
NO
NO
NO
NO
Cell membrane, cytoplasm, genetic material, organelles
Asexual or Sexual
DNA and RNA
YES—Multicellular Organisms
YES
YES
YES
Structure
Reproduction
Genetic Material
Growth and Development
Response to Environment
Change over time
Obtain and Use Energy
Viroids and Prions • Viroids
– ssRNA genome and the smallest known pathogens – Affects plants
• Prions – Infectious particles that are entirely protein – No nucleic acid – Highly heat resistant – Animal disease that affects the nervous system – Affects nervous tissue and results in
• BSE – bovine spongiforum encephalitis (Mad Cow Disease) • Scrapie in sheep • Kuru and Creutzfeldt Jakob disease in humans
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Pirons - Nonviral particle Has protein only, no DNA or RNA (cause of mad cow disease and Creutfeldt-Jacob disease in humans)—Prions (affect the brain and are always fatal)
No DNA or RNA!
