Viruses

Relative sizes  Viruses are one of the smallest biological structures known  Between 20 and 50 nanometers in size.  The average animal cell would be about 10,000 times bigger 0.25  m Virus Animal cell Bacterium Animal cell nucleus

T4 bacteriophage infecting an E. coli cell Viruses are cellular terrorists The Latin root for virus means “poison” They take over host cells, but they are VERY specific in the types of cells that they attack Bacteriophage= a type of virus that attacks bacteria 0.5  m

Are they alive? In pairs, look at the set of pictures and diagrams related to viruses and make a hypothesis to answer the question: Are viruses alive? Why or why not? H1N1 video Tobacco Mosaic Virus 

Viral Structure The head also called a capsid, is made mostly of protein and protects the genetic material inside The remaining portion is called the tail and it acts in recognition of the host cell and attachment Some viruses are covered in a fatty envelope

Viral structure 18  250 mm 70–90 nm (diameter) 80–200 nm (diameter) 80  225 nm 20 nm50 nm (a) Tobacco mosaic virus, a rod shaped, helical virus (b) Adenoviruses, an icosahedral virus with 20 triangular faces (c) Influenza viruses, have a viral envelope surrounding them (d) Bacteriophage T4, attack bacteria RNA Capsomere of capsid DNA Capsomere Glycoprotein Membranous envelope Capsid DNA Head Tail fiber Tail sheath

Diversity (of genetic material)  Viruses can have DNA (double stranded or single stranded) or RNA as their genetic material  Not all viruses cause disease.  What is a disease causing agent called?  A pathogen Influenza 

Classes of Animal Viruses

Specificity  Viruses have a restricted host range.  Some animal viruses will have a broad host range. For instance the rabies virus can infect many mammalian species  Some have a narrow host range. For instance the human cold virus tends to infect the cells of the upper respiratory tract. The AIDS virus affects the T cells of the human immune system

Viral Replication Purpose of replication = to make more viruses Two requirements for replication, since viruses have 2 components. More protein is needed for the outside and more genetic material is needed for the inside. Two possible lifecycles:  The lytic cycle  The lysogenic cycle

The Lytic Cycle: Viral Lifecycle 1.Attachment: Virus recognizes host cell and attaches 2.Entry: Genetic material is inserted inside 3.Synthesis: Viral DNA takes over the hosts cellular machinery for: o Making genetic material by replication o Making more capsids by protein synthesis 4.Assembly: Hundreds of new viruses come together (cell becomes virus factory!) 5.Release: Virus ruptures the host (lysis) Called a virulent virus E.g. influenza

The lytic cycle of phage T4, a virulent phage Attachment. The T4 phage uses its tail fibers to bind to specific receptor sites on the outside of the E. coli cell. Entry of DNA DNA injected into the cell and leaving an empty capsid outside. The cell’s DNA may be broken up. Making components. The phage DNA tells the cell how to make proteins and copies of the genetic material. Assembly. Proteins form themselves into phage heads, tails, and tail fibers. The phage genome is packaged inside the capsid. Release. The phage damages the bacterial cell wall, allowing fluid to enter. The cell swells and finally bursts, releasing 100 to 200 phage particles Phage assembly Head Tails Tail fibers

The Lysogenic Cycle: Viral Lifecycle 1.Attachment: Virus recognizes host cell and attaches 2.Entry: Genetic material is inserted inside Enters lysogeny: Virus integrates into the hosts DNA and becomes dormant. Now each time the host divides, each of its daughter cells will have a copy of the virus. At some point, it re-enters the lytic cycle… 3. Synthesis: Viral DNA takes over the hosts cellular machinery 4. Assembly: Hundreds of new viruses come together 5. Release: Virus ruptures the host (lysis) Called a temperate virus E.g. Herpes

The lytic and lysogenic cycles of phage, a temperate phage Many cell divisions produce a large population of bacteria infected with the prophage. The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells. Phage DNA integrates into the bacterial chromosome, becoming a prophage. New phage DNA and proteins are synthesized and assembled into phages. Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Certain factors determine whether The phage attaches to a host cell and injects its DNA. Phage DNA circularizes The cell lyses, releasing phages. Lytic cycle is induced Lysogenic cycle is entered Lysogenic cycleLytic cycle or Prophage Bacterial chromosome Phage DNA

Reproductive strategies Difference between temperate and virulent phages? Which strategy is more successful for reproduction? In the short term? In the long term?

Back to the start… Living? Yes, its alive… Interacts with its environment Reproduces Adapts or evolves No, its not alive… No cells No metabolism No homeostasis No growth or development No response to stimuli  Is it a cellular parasite?

Human Health What makes viruses so challenging? 1. Most are difficult to treat and are not destroyed by sulfa drugs or antibiotics that have been so effective at treating bacterial illnesses. 2. Some viruses remain dormant for years before symptoms appear (due to the lysogenic cycle)

Vaccines  Are used to prevent some viral diseases.  Contain weaken or killed virus, or parts of the virus eg. Polio, smallpox, hepatitis B Mode of action: The body reacts to the vaccine as if it were a real virus and produces antibodies, and as a result make us immune to the disease.

Homework Worksheet on Viruses and Human Health