Viruses A “borrowed life”

Characteristics of Life All living things exhibit the following four characteristics: 1.Carry out metabolic activities to meet their energy needs 2.Grow and develop 3.Reproduce 4.Adapt to changes in their environments

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 (~25X smaller than smallest cells) are considered infectious particles or biochemical entities

“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 As such, viruses are often called obligate (bound by restrictive conditions) intracellular parasites

Anatomy of a Virus At its simplest, 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.

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) a)Tobacco Mosaic Virus (plant host) b) Adenovirus (animal host) c) Influenza Virus (animal host) d) Bacteriophage (bacteria host)

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

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

Transcription and manufacture of capsid proteins Self-assembly of new virus particles and their exit from the cell Entry and uncoating Fig VIRUS DNA Capsid 4 Replication HOST CELL Viral DNA mRNA Capsid proteins Viral DNA

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

Fig Capsid RNA Envelope (with glycoproteins) Capsid and viral genome enter the cell HOST CELL Viral genome (RNA) Template mRNA ER Glyco- proteins Capsid proteins Copy of genome (RNA) New virus