Viruses

Living or Non-Living?• Infectious particles of nucleic acid and proteins

• Cannot “live” (reproduce) outside a host

1st virus discovered-Tobacco Mosaic Virus (TMV)

Viruses -The Boundary of Life

At the boundary of life, between the macromolecules (which are not alive) and the prokaryotic cells (which are), lie the viruses and bacteriophages (phages).

Viruses are found everywhere.

Viruses consist of a core of nucleic acid, either DNA or RNA, and a protective coat of protein molecules and sometimes lipids.

Are viruses alive?

Cells and VirusesCharacteristic Cell Virus

Structure Cell membrane, cytoplasm; eukaryotes also contain nucleus and organelles

Reproduction Independent cell division either asexually or sexually

Genetic Code DNA

Growth and Development

Yes; in multicellular organisms, cells increase in number and differentiate

Obtain and Use Energy

yes

Response to Environment

yes

Change Over Time yes

Naming Viruses

• International Committee on Taxonomy of Viruses names them based on three characteristics:• Type of nucleic acid (DNA or

RNA)• Is the nucleic acid double or

single stranded• Presence or absence of nuclear

envelope

Prokaryotes Vs. Eukaryotes Vs. Viruses• No membrane

bound nucleus• Has a cell wall• Only a few

organelles or none at all.

• Has a capsule surrounding it

• Three main types.

• Nucleus with membrane

• Only plants have cell wall

• Contains many organelles

• Has a lipid bi-layer membrane surrounding it.

• Specialized by thousands of different sizes and shapes.

• No nucleus• No membranes• No organelles• Cannot

reproduce on its own

• Generally not considered alive by most standards

VIRUS STRUCTURE

Adenovirus, a naked virus, with a polyhedral capsid and a fiber at each corner

Influenza virus, surrounded by an envelope with spikes

Virion Structure

Nucleic Acid

Spike Projections

ProteinCapsid

Lipid Envelope

VirionAssociatedPolymerase

Types and shapes

HelicalRod like with capsid proteins winding around the core in a spiral

Tobacco Mosaic Virus

Polyhedral Has many sides

Most polyhedral capsids have 20 sides and 12 corners

Polyhedral capsid attached to a helical tail.

Flu virus

T4 Bacteriophage

HIV

A retrovirus injects the enzyme, reverse transcriptase into the cell to copy viral RNA into DNA.

Bacteriophages

Head

Tail fiber

DNA

Tail

Bacteriophages Have Multicomponent Particles

The head consists of an icosahedron that has very tightly packed DNA.

Nearly 20 proteins are found in the head. An equally complex tail sheath forms a helical

component. The head is connected to the tail sheath by a

neck that is composed of four to five proteins.

An end plate with lysozyme activity and pins at the base of the sheath contain several different proteins.

Tail fibers used to recognize receptor proteins on the surface of the bacterial cell consist of numerous additional proteins.

Phage assembly and infection processes require coordination of many genes.

Viral Infection

Viruses and Living Cells• Viruses must infect a living cell in

order to grow and reproduce• They also take advantage of the

host’s respiration, nutrition and all the other functions that occur in living things

• Therefore, viruses are considered to be parasites

Viral Reproduction• Steps of Lytic Cycle

• Attachment• Entry• Replication• Assembly• Lysis/Release (lyses the

cell)

How do viruses replicate?2 methods of replication:

1. Lytic Cycle – the virus enters the cell, replicates itself hundreds of times, and then bursts out of the cell, destroying it.

2. Lysogenic Cycle – the virus DNA integrates with the host DNA and the host’s cell helps create more virus DNA. An environmental change may cause the virus to enter the Lytic Cycle.

In the lytic cycle, the virus reproduces itself using the host cell's chemical machinery. The red spiral lines in the drawing indicate the virus's genetic material. The orange portion is the outer shell that protects it.

In the lysogenic cycle, the virus reproduces by first injecting its genetic material, indicated by the red line, into the host cell's genetic instructions.

Viruses Enter Living Cells

Viruses enter bacterial cells by punching a hole in the cells wall and injecting its DNA

Viruses Enter Living Cells

Viruses enter plant cells through tiny rips in the cell wall.

Viruses enter animal cells by endocytosis.

Viriods

• Much smaller than viruses• Just consist of small sRNA

molecule• No protein coat• Infect plants

Prions• Proteinaceous infectious agents• Contain only protein, no nucleic acid• Linked to number of fatal diseases in

humans and animals• Obligate intracellular parasite• How does it replicate if no nucleic acid?

• Prion protein converts host protein to prion protein

Prions• Cannot be killed by UV light or

nucleases, can be killed by proteases and heat

• Usually cannot be transmitted across species