Viruses Rotavirus (causes stomach flu) HIV virus

What is a virus? A virus is a microscopic, infectious agent that may infect any type of living cell. Viruses must infect living cells in order to make more copies of themselves. The study of viruses is known as virology.

Living or Non-living? Viruses do not fall into the classification of the 3 domains of living organisms. It is debatable whether to consider viruses living or non-living.

What does it mean to be “living?” Living organisms must: a) reproduce independently b) respire (carry out cellular respiration) c) metabolize d) have cellular structures

Why are viruses not classified as living? Viruses have no functional organelles. Their structures are mainly protein coat and nucleic acid (DNA or RNA). A virus must infect a living cell in order to reproduce. It uses the cell’s machinery to make more of itself, and in doing so, destroys the cell.

Why are viruses not classified as living? Viruses do not metabolize. No chemical reactions occur within the virus particle itself. Any reactions involved in producing more virus occurs in the cells they infect.

Why are viruses not classified as living? Viruses do not respire. They do not require oxygen or any other gases to perform cellular functions, as they do not have cells.

Where did viruses come from? The origins of viruses in the evolutionary history of life are unclear. Some may have evolved from plasmids – pieces of DNA that can move between cells – while others may have evolved from bacteria.

Viral Structure A virus particle is often much smaller than a bacterial cell. In order to infect a living cell, the virus must be smaller than that cell. The average size of a virus particle, or virion, is between 0.03 and 0.3 microns (1 micron is 1/1000 of a millimeter)

All viruses have 2 main parts: A capsid or protein coat. This coat surrounds the core of the virus and protects that material contained within it. It makes up about 95% of the virus structure. The protein coat determines the shape of the virus (rod shaped, spherical, cubic, polyhedral, etc.)

All viruses have 2 main parts: The core. This contains only the nucleic acid. Viruses may contain DNA (double or single stranded) or RNA (single stranded) as their nucleic acid. The nucleic acid contains between 10-100 genes that code for information used to produce more identical virus particles.

T4 Bacteriophage

HIV virion

Ebola virus

Viral Classification Viruses are classified based on a number of different criteria: Capsid shape Nucleic acid type Organisms they infect Methods of reproduction

Viral Reproduction Depending on the type of virus, once it infects a cell, the virus will reproduce itself in one of two ways:

Lytic Cycle In this method, the virus immediately makes copies of itself as soon as it enters the cell. A. Adsorption - the virus attaches to the surface of the cell. B. Entry – The virus injects it’s nucleic acid into the cell.

Lytic Cycle C. Replication - The virus takes over the cell. Its nucleic acid is read by the cell and used to produce copies of the capsid proteins and viral nucleic acid. D. Assembly - The pieces of capsid and nucleic acid are assembled into new viral particles.

Lytic Cycle E. Release/Lysis - When the cell is full of virus, the viral nucleic acid signals the cell to produce enzymes (chemicals) that cause the cell membrane of the infected cell to tear open (a process called lysis). The newly formed viral particles are released.New viral particles can now infect surrounding cells.

Lytic Cycle

Lytic Cycle This method of reproduction is used by viruses that make you sick immediately after you contact them, like colds and influenza. SARS Common cold Influenza Rabies Tobacco mosaic virus

Lytic Cycle Lytic cycle animation of a bacteriophage infecting an E. Coli bacterium Lytic cycle animation 2 List and draw a labeled diagram of the 5 phases of the Lytic cycle and briefly describe each phase.

Lysogenic cycle In this reproductive method, the viral nucleic acid incorporates itself into the infected cell’s DNA. The virus may then go dormant inside the cell. It does not cause the cell to produce virus, but instead the viral nucleic acid is read and copied as if it were part of the cell’s own DNA.

Lysogenic Cycle When the cell divides to produce identical daughter cells, the viral DNA is passed along with the cellular DNA to the daughter cells. Each daughter cell will carry the virus.

Lysogenic Cycle The viral nucleic acid incorporates itself into the infected cell’s DNA and then goes dormant. At some point, the viral nucleic acid is activated. At this point, the cell is triggered to begin producing viral parts, and a lytic phase follows.

Lysogenic Cycle

Lysogenic Cycle Lysogenic cycle animation Lysogenic cycle animation 2

Lysogenic viruses HIV Herpes Hepatitis B Varicella-Zoster (chicken pox)

Transduction In our discussion of DNA transfer mechanisms in bacteria, the process of transduction was discussed. If a virus infects a bacterium and goes into the lysogenic cycle, it incorporates itself into the bacterial DNA.

Transduction When the virus becomes active again, small pieces of the bacterial DNA may be copied along with the viral DNA. These pieces of bacterial DNA may then be passed on the new bacteria that are infected by the new virus produced at the end of the lysogenic cycle.

How can viruses cause cancer? Virus brings in an oncogene (gene that turns a normal cell into a tumour cell) as part of its genome Viral nucleic acid integrates into host DNA, disrupts cell cycle regulation, or activates a latent (inactive) oncogene

Do viruses cause cancer in humans? Maybe 1- Some animal viruses have been shown to cause cancer in animals (e.g. experimentally shown that "Rous sarcoma virus" causes cancer in chickens). 2- In humans evidence is circumstantial. i.e. some viruses are found in association with some cancers

TUMOUR VIRUSES HEPATITIS B (DNA): Hepatocellular carcinoma (liver cancer) EPSTEIN BARR (Herpes): Burkitt's lymphoma, Nasopharyngeal carcinoma HERPES SIMPLEX 2 ? : cervical cancers? PAPILLOMA VIRUS : skin and cervical cancers (HPV = Human Papilloma Virus – you may have been vaccinated in grade 8 or 9 for this RETROVIRUS : T-cell leukemia