Viruses

Discovery of Viruses◦ Virus: nonliving particle made up of nucleic acid

and a protein coat or nucleic acid and a lipoprotein coat

◦ Cause many diseases in living organisms◦ Useful tools for genetic research

Discovery of Viruses◦ Late 1800s◦ Living or nonliving?◦ 1935, Wendell Stanley crystallized the tobacco

mosaic virus (TMV)◦ Concluded: infective agent that could be

crystallized–––>not made of cells

Characteristics of Viruses◦ Nonliving b/c:

No cytoplasm or organelles No metabolism or homeostasis Do not grow by dividing in two Need other cells to reproduce

Viral Size & Structure◦ Very small◦ Shape is the result of genome or protein coat

Helix Shape: capsid, protein coat, forms the helix TMV, Rabies, Measles

Icosahedron: 20 triangular faces & 12 corners Adenovirus, herpes simplex, chickenpox, polio

Spherical: circular Influenza

◦ Envelopes: bilipid membrane that surrounds membrane Helps new viruses to recognize host cells

Classification of Viruses1. RNA or DNA2. Genome single-stranded/double-stranded &

linear OR circular3. Nature of capsid & presence or absence of an

envelope

Replication in DNA Viruses1. Inserts DNA into host cell’s chromosome

Provirus2. Transcribe mRNA Translate viral proteins3. Use cell’s enzymes to make new viral DNA4. Assembled to make new viruses

Replication in RNA Viruses◦ Some: enter host cell & serve directly as mRNA◦ Others: Transcribed & is a template for the

synthesis of mRNA & more copies of viral genome

Replication in RNA Viruses, continued◦ Retroviruses: use reverse transcriptase, which

uses RNA as a template to make viral DNA, which then creates proteins that become part of the new viruses

Replication in Viruses that Infect Prokaryotes◦ Lytic Cycle

Invades a host cell, produces new viruses, and ruptures (lyses) the host cell when releasing newly formed viruses

Virulent: viruses that reproduce only by the lytic cycle

Destroy the cells that they infect Ex: T phages

Replication in Viruses that Infect Prokaryotes◦ Lysogenic Cycle

Hide in their host cell for days, months, or years

Called Temperate Viruses

Viruses: Tools for Biotechnology◦ Replace large pieces of the DNA of a phage with

DNA of a particular interest, such as the human gene for cystic fibrosis

◦ Insert recombinant DNA into empty phage heads and allow them to infect bacteria

◦ Bacteria produces millions of copies◦ Invaluable tools for medical research

The Origin of Viruses◦ Evolved from early cells◦ Naked pieces of nucleic acid that could travel

from one cell to another◦ Entered cells through damaged membranes◦ Genes evolved to allow for protective protein

coats and recognition proteins to attack healthy cells

◦ Example: Influenza virus and HIV

Vectors of Viral Disease◦ What is a vector?

Intermediate host that transfers a pathogen or a parasite to another organism

Include: humans, animals, mosquitoes, ticks, and fleas

◦ West Nile Virus: Mosquito---bird(WNV)---human, the virus can spread

to human

Human Viral Diseases◦ Can affect:

Brain Liver Heart Lungs Skin

Viruses & Cancer◦ What is cancer???

Cells divide at an uncontrolled rate and form a tumor that invades surrounding tissue

◦ Viruses contain viral oncogenes: genes that cause cancer by blocking the normal

controls on cell reproduction◦ Viral DNA inserts itself into a host’s

chromosome near a proto-oncogene --- controls cell growth Converted to a oncogene HPV: cervical cancer Hep. B: liver cancer

Emerging Viral Diseases◦ Illnesses caused by new or reappearing infectious

agents that typically exist in animal populations--often in isolated habitats--and can infect humans who interact w/ these animals Ebola: hemorrhagic fever Hantavirus: caused outbreak of pneumonia SARS: avian flu

Vaccinations◦ Solution with a harmless version of a virus,

bacterium, or toxin◦ Causes an immune response body forms an

antibody Inactivated virus: not able to replicate in a host Attenuated virus: weakened form of the virus that

cannot cause disease Attenuated viruses provide greater protection from

disease

Vector Control◦ Mosquito-control programs◦ Rabies Vaccinations

Drug Therapy◦ Antiviral drugs interfere w/ viral nucleic acid

synthesis◦ Small number of these drugs, WHY?

Viroids◦ Smallest know particles that can replicate◦ Made of RNA◦ Infect plants

Prions◦ Infectious protein particles that have no

genome◦ Abnormal forms of a natural brain protein,

that convert normal brain proteins into prions◦ Clump together inside of cells---death

BSE: cattle Scrapie: sheep CJD/kuru: humans

CH 24: Viruses

What are viruses made of?a. Enzymes and fatsb. Carbohydrates & ATPc. Protein and nucleic acidsd. Mitochondria & lysosomes

How do viroids differ from virusesa. Viroids are larger in sizeb. Viroids do not have a capsidc. Viroids do not have nucleic acidsd. Viroids can cause disease in plants

During which of the following processes does a phage kill its host?

a. Conjugationb. Transcriptionc. The lytic cycled. The lysogenic cycle

Which of the following is one reason why viruses are not considered living organisms?

a. Viruses are able to growb. Viruses do not metabolizec. Viruses can reproduce by splitting d. Viruses are too small to be easily observed

Which of the following does the diagram represent?

a. A virusb. A prionc. A virod d. A bacterium

To which of the following is label X pointing?

a. Envelopeb. Nucleic acidc. Protein coatd. Cell membrane

X

Complete the following analogy:Skin : Person :: Capsid :

a. Virus b. Insectc. Fungi d. Bacterium

Which of these four illnesses/diseases are caused by a virus?

a. Malaria, West Nile, Yellow Fever, Lymeb. HPV, HIV, Influenza, Chickenpoxc. Gang green, West Nile, Measles, Smallpoxd. Heart disease, Tuberculosis, DMV, AIDS

Genes that cause cancer by blocking the normal controls of cell reproduction are called

a. Protease inhibitorsb. Viroidsc. Virulentd. Oncogenes