1. Packaged Genes – “Ready for Delivery”
Viruses
Packaged Genes – “Ready for Delivery”

2. Characteristics of Life
All living things exhibit the following four characteristics:
Carry out metabolic activities to meet their energy needs
Grow and develop
Reproduce
Adapt to changes in their environments

3. Are Viruses Alive?
Based on the characteristics of life, viruses do not fit the criteria
Technically speaking, cells are the smallest entities that retain the four characteristics of life – viruses are acellular (have no cellular parts)
Viruses (~25X smaller than smallest cells) are considered infectious particles or biochemical entities

4. “A Borrowed Life”
On their own, viruses do not possess the previously described characteristics of life; however, they do have the ability to enter cells and take over the host cells’ metabolic functions in order to reproduce and avoid detection by host defenses
Because they cannot function or reproduce independently and often harm the host cell, viruses are frequently called dependent intracellular parasites

5. Anatomy of a Virus
In simplest terms, viruses can be thought of as “genes in a package” – a virus consists of nothing more than a nucleic acid genome (entire collection of genes) encapsulated by a protein shell/coat known as a capsid, built from subunits called capsomeres.

6. Viral Varieties
Viruses can vary with regard to the type of nucleic acid they possess (DNA vs. RNA and single-stranded vs. double-stranded)
Viral capsids also have various structures
Furthermore, some viruses have a membranous envelope that surrounds the capsid (derived from the host cell’s membrane)
Helical (non-eveloped)
Ex: Tobacco Mosaic Virus
(plant host)
b) Polyhedral
(non-enveloped)
Ex: Adenovirus (animal host)
c) Spherical (enveloped)
Ex: Influenza Virus
(animal host)
d) Complex (non-enveloped)
Ex: Bacteriophage
(bacteria host)

7. Viral Entry into Host Cell
Viruses rely on proteins or glycoproteins (proteins with carbohydrate residues covalently attached) to interact with receptors on the surface of the host cell in order for virus to “fuse” to cell and allow entry of nucleic acid genome into the host intracellular environment
The specific recognition between viral proteins/glycoproteins and host cell receptors greatly restricts the range of host cells that a particular virus can infect

8. Once a viral genome has entered a cell, the cell begins to manufacture viral proteins and copy the viral nucleic acid
The virus makes use of host cell’s “machinery” (enzymes and ribosomes) and molecules (amino acids and nucleotides)
Viral nucleic acid molecules and capsomeres spontaneously self-assemble into new viruses
Mature viruses, or virions, then leave the host cell, often causing cellular destruction (lysis) in the process

9. VIRUS Entry and uncoating DNA Capsid Transcription and manufacture
Fig. 19-4
VIRUS
Entry and
uncoating 1
DNA
Capsid
Transcription
and manufacture
of capsid proteins 3 2
Replication
HOST CELL
Viral DNA
mRNA
Viral DNA
Capsid
proteins
Figure 19.4 A simplified viral reproductive cycle
Self-assembly of
new virus particles
and their exit from
the cell 4

10. Viral Envelopes
Many viruses, but not all, that infect animals have a membranous envelope
Viral glycoproteins on the envelope bind to specific receptor molecules on the surface of a host cell
Some viral envelopes are formed from the host cell’s plasma membrane as the viral capsids exit
Other viral membranes form from the host’s nuclear envelope and are then replaced by an envelope made from Golgi apparatus membrane

11. Capsid and viral genome enter the cell Capsid
Fig. 19-7
Capsid and viral genome
enter the cell
Capsid
RNA
HOST CELL
Envelope (with
glycoproteins)
Viral genome (RNA)
Template
mRNA
Capsid
proteins ER
Copy of
genome (RNA)
Glyco-
proteins
Figure 19.7 The reproductive cycle of an enveloped RNA virus
New virus