Viruses

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

• These twilight creatures are parasites responsible for causing many diseases in living things (herpes and HIV in humans, for example).

• Viruses are found everywhere.

Vocabulary to know

• Nucleic acid Lysogenic cycle• Genome genes• DNA lysis• RNA replicate• Protein capsid• Bacteriophage (phage) vaccine• Lytic cycle pathogen• Oncogenic

What is a virus?

A virus is a non-cellular piece of genetic material (nucleic acid – either DNA or RNA) surrounded by a protein coat (capsid).

• Non-living (do not meet all 8 characteristics)• 1/100 of the width of a cell• Pathogen (causes disease) in plants, animals and bacteria (bacteriophage)• Highly host specific (can only enter certain cells)

Classification

• 2 main groups based on nucleic acid– DNA viruses more stable, vaccines can last for

years. Measles, mumps, chicken pox– RNA viruses – mutations occur. New vaccines

each year. Flu

8 Characteristics• Viruses are not made of cells • Viruses do not grow or develop• Viruses do not respond • Viruses do not need materials or energy• Viruses do not need to maintain homeostasis• Viruses do contain DNA or RNA• Viruses are able to evolve• Viruses are able to reproduce by taking over a host cell

and causing host cell to make more viruses.• So, are they alive? Or not?

Viral Structure

Viral structure

Viral “Reproduction”

• Viruses can only reproduce inside a host cell• Viruses invade host cell and use cell processes

to make more viruses• 2 methods of reproducing:– Short cycle: Lytic Cycle– Long cycle: Lysogenic Cycle

Lytic Cycle

Lytic Cycle

• Virus recognizes host cell receptor protein• Virus attaches to host cell• Virus injects DNA or RNA into host cell• Virual DNA takes over host cell processes– Has cell copy viral DNA & protein coat– Viruses assembled– Viruses lysis (burst) out of cell, killing cell to attack

other cells.

Lysogenic Cycle

Lysogenic Cycle

• Virus recognizes host cell protein receptor sites• Virus attaches to host cell• Viral DNA or RNA injected into host cell• Viral DNA attaches to host cell DNA– Each time host cell copies itself, Viral DNA copied

along with host cell DNA.• A stimulus (stress) signals viral DNA to begin the

lytic cycle.

Lytic Cycle begins…

• Host cell copies Virual DNA & protein coat• Viruses assembled• Cell lysis (bursts) releasing viruses.

Lysogenic Cycle

Viral Diseases in Humans

• Polio Herpes HIV/AIDs-1986• Measles Cold sores Ebola- 1976• Mumps Hepatitis Encephalitis• Chicken pox Flu**West Nile - 1999• Yellow fever ColdsSARS*-02• Rabies Oncogenic Bird flu

• Cervical cancer*found in bats

**1918 – killed 40 million people¾ new human infections came from animals

Prevention

• Vaccines – dead or weakened virus injected into host– Fools immune system into thinking it is being invaded– Host builds antibodies to destroy virus (time of

immunity varies)

– HIV attacks Helper T cells that triggers immune response. No immune response. HIV doesn’t kill you, other infections that the body can’t fight do.

Immune system

• Antibodies- proteins that bind to surface of viruses & flag them for death• White blood cells – engulf (eat) virus infected

cells

• Memory cells recognize virus if it reenters the body.