Viruses and Viral Like Agents

What is a virus? • Organism that causes diseases

– Common cold– Flu– AIDS– Bird flu– Polio

Electron micrographs of viruses.

HIV Aeromonas virus 31 Influenza virus

Orf virusHerpes simplex virusSmallopx virus

Viruses are non-living•In the 1930s, it was discovered that viruses are nonliving agents

composed of nucleic acids and proteins.

FIGURE 2: Size relationships among microorganisms and viruses

What are viruses???• About 5, 000 viruses have been identified, but there is believed

to be around 400, 000 different viruses.• Believed to be the most abundant biological entity on Earth!!• Viruses are very small- they do not “grow,” just reproduce• They cannot reproduce on their own (obligate, intracellular

particles)• Most can only be seen with the electron microscope, to survive

they must infect and take over a host cell in order to reproduce.– They lack the chemical machinery to generate energy and synthesize

large molecules.• Viruses do not have organelles, have no cytoplasm, and no cell

nucleus or nucleoid.• They do have: nucleic acid core and a protein coat.

What are Viruses???• The viral genome contains either DNA or

RNA, but does not have both (like we do).• They have a protein coat called a capsid that

gives shape to the virus.• The capsid is what provides protection to the cell.

– The amino acids make it resistant to temperature, pH and other environmental fluctuations.

• Some capsids have special proteins called spikes, that help the virus attach to host cells and penetrate cell membranes.

• Most viruses have a membrane that surrounds the nucleocapsid that is flexible called an envelope.

Viral Shape• Viruses are grouped by the shape of their

nucleocapsid symmetry

FIGURE 6: Various viral shapes

From Medical Microbiology, 5th ed., Murray, Rosenthal & Pfaller, Mosby Inc., 2005, Fig. 6-4.

Structures compared

Properties of Viruses

• Various morphologiespolyhedralhelicalsphericalfilamentouscomplex

Ebola virus Rabies virus

Poliovirus Herpes virus Coronavirus Lassa virus

Viral Structure

1) Nucleic acid core• the genetic material of the virus.

• instructions to make all the viral parts and viral enzymes.• either DNA or RNA.

2) Capsid layer• made of protein that surrounds and protects the

nucleic acid core. • can also be involved in attaching the virus to the

cell membrane of its host.

Structure Continued• 3) Envelope- not seen in all viruses.- made of lipid or fat.- surrounds the capsid layer.- can also be used to attach to the CM of the host cell.• 4) Spike proteins- not seen in all viruses.- embedded in the envelope of the virus.- have a specific shape.- used to attach the virus to the host’s CM.

FIGURE 5: The components of viruses