Viruses

composed of a nucleic acid (RNA or DNA) and protein coat Very small particles that do not posess all the properties of life but can greatly affect living systems composed of a nucleic acid (RNA or DNA) and protein coat May also contain a phospholipid bilayer membrane (enveloped virus) Viruses infect cells, take over transcriptional and translational machinery of the host & often kills the host

Virology Study of viruses

Viruses probably arise by accident DNA or RNA synthesized incorrectly and a set of genes escapes from a cell.

Figure 18.1 Comparing the size of a virus, a bacterium, and a eukaryotic cell

How big is a virus? http://cellsalive.com/howbig.htm Viruses range in size from 20nm – 400nm and contain 5000 – 200000 base pairs of nucleic acids

There are 3 main shapes of viruses, although the diversity of viruses is huge and their specificity of host is very exact

Main shapes: Isohedral Helical - thintube Complex – combination of shapes &/or additional structures (‘moonlanders’)

Viral Structure Protein coat is called a capsid Envelope is located outside of the protein coat in some viruses Glycoproteins-used to attach to a host cell

Viral Replication They are intracellular parasites Bacteriophages-viruses that infect bacteria (Prokaryotes) Viruses attack eukaryotic cells

Viruses and bacteriophages invade cells and use the host cell's machinery to synthesize more of their own macromolecules. Once inside the host the bacteriophage or virus will either go into a Lytic Cycle - destroying the host cell during reproduction. or It will go into a Lysogenic Cycle - a parasitic type of partnership with the cell

Attachment The glycoproteins will attach to specific receptor proteins on the host cell Some infect certain species or certain cells because chemical interactions control attachment

Injection Virus punctures cell and its nucleic acid in the cytoplasm

Gene Expression Gene expression and genome assembly proteins ‘enslave’ transcriptional and translational machinery of host, forcing its own genes to be expressed Most viruses cause the complete shutdown of host gene expression

Assembly & Release Viral genome is packed in the protein capsid and new viral particles escape the host Enzymes that degrade host macromolecules cause cells to weaken and burst

The Lytic Cycle These viruses are VIRULENT because they cause disease Lysis-cell disintegration

The Lytic Cycle

The Lysogenic Cycle Viruses that stay in a host for a period of time go through the lysogenic cycle They are called temperate viruses because they do not kill the host immediately The virus DNA integrates itself with the host’s DNA It may later enter the lytic cycle

The Lysogenic Cycle http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter17/animation_quiz_2.html

Shingles Herpes Zoster Caused by same virus as chicken pox Lies dormant in nervous system and is reactivated by things such as stress, immun deficiencies, or cancers

Smallpox Caused by several different Variola virus strains Transmitted via person to person contact Varies in severity Said to be eradicated world wide b/c of vaccines

How does your body respond? Immunobiology, 5th ed. Janeway

What are Vaccines? A vaccine is a substance that stimulates the body’s immune response The goal is to prevent or control an infection

Retrovirus Reverse transcriptase – enzyme that converts RNA to DNA ssRNA → ssDNA → dsDNA → mRNA

HIV is a retrovirus

HIV doesn’t target just any cell, it goes right for the cells that want to kill it. “Helper" T cells are HIV's primary target. These cells help direct the immune system's response to various pathogens.

Click here for Life Cycle of HIV HIV destroys the immune system by depleting T cells which protects against disease HIV can infect 10 billion cells a day, yet only 1.8 billion can be replaced daily After many years of a constant battle, the body has insufficient numbers of T-Cells to mount an immune response against infections. At the point when the body is unable to fight off infections, a person is said to have the disease AIDS. It is not the virus or the disease that ultimately kills a person; it is the inability to fight off something as minor as the common cold. Click here for Life Cycle of HIV