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• Too small to see with a light microscope

Chapter 10: Viruses
Lecture Exam #3 Wednesday, November 22nd (This lecture WILL be on Exam #3)

• Visible with electron microscopy

• Not cells: no nucleus, organelles, or cytoplasm • Obligate intracellular parasites • Can only reproduce inside a living cell

Dr. Amy Rogers Office Hours: MW 9-10 AM

• Viruses are on the border between living & nonliving things • Have either DNA or RNA, never both • Often cause death of the host cell • Genus/species names not used in viral classification

Compenents of a virus
(lipid bilayer)

Viral components: Nucleic Acid
• Viral genomes can be either DNA or RNA • This genome, once inside a host cell, directs synthesis of new viral proteins, and also replication of new viral genomes • Viral genomes come in all kinds: • Single- or double-stranded, RNA or DNA • Linear or circular • One piece or segmented (in several fragments)


(DNA or RNA)


Viral components: Capsid
• A protein shell that surrounds & protects the nucleic acid • Determines the shape of the virus • Made up of many subunits called capsomeres • Capsomeres may be all the same, or virus may have several different proteins in its capsid • Composition & arrangement of capsomeres is characteristic of each virus (use for identification)

Viral components: Envelope
• Not all viruses have an envelope
• Enveloped virus: has one • Naked virus: does NOT have an envelope

• Envelope is a Lipid bilayer membrane
• acquired from a host cell membrane when virus “buds” (plasma membrane) or passes through a membrane-bound organelle (such as the nucleus) • Composition of envelope resembles that of the cell membrane from which it came

• Some envelopes have spikes
• Glycoproteins (proteins bound to carbohydrates) that stick out from the envelope • Spikes often are important for attachment to host cells


Viral components: Envelope
Enveloped viruses: Advantages
• Membrane “looks” like cell, hides virus from the immune system • Helps virus infect new cells by membrane fusion with a new host cell

Viruses: Size & Shape
• Like bacteria, viruses come in a range of sizes & shapes – though they are all very small, average about 100 nm

Enveloped viruses: Disadvantage
• Enveloped viruses are fragile
– Conditions that damage membranes, will damage the envelope (heat, freezing, pH change, lipid solvents, chemical disinfectants like chlorine & hydrogen peroxide)

• Shape is determined by the capsid or envelope, often beautifully symmetric • Enveloped viruses tend to be roughly spherical

• Naked viruses are generally tougher

Right: coronavirus
Phage: more complex structure

Below: T4 phage; Marburg virus; tobacco mosaic virus

Polyhedral shapes

Virus host range & specificity
By now you are getting the idea that at the very small level of life, molecular interactions tend to be highly specific. This is especially true for viruses.

Replication of bacteriophages (virulent & temperate/lysogenic)

Host range:

all life gets infected by some kind of virus, but each virus type can only infect a specific range of host organisms. • Some viruses infect only one host; others have a broader range

Viral specificity:

molecules on the surface of a virus determine whether it can attach to a particular cell type; cell must have correct surface receptors, plus other internal factors, for virus to infect it. • Some viruses infect only one cell type in a single host species; others can infect many cell types


Animal viruses work somewhat differently
1. Attachment (adsorption) of virus to target cell
• • Bacteriophage: “tail” clings to bacterial cell wall Animal virus: spikes, capsid, or envelope bind to proteins in the plasma membrane of target cell

Animal viruses work somewhat differently 2. Penetration
• • Bacteriophage: DNA gets injected into the cell Animal virus: virus genome + capsid enters the cell...
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