Viruses

What is a Virus? Particles of nucleic acid and protein reproduce by infecting living cells

What do all viruses have in common? They enter living cells and, once inside, use the machinery of the infected cell to produce more viruses.

Comparing the size of a virus, a bacterium, and an animal cell Most viruses are so small they can only be seen with the aid of a powerful electron microscope A virus is smaller than a bacterium 0.25 m Virus Animal cell Bacterium Animal cell nucleus

Viral Structure A typical virus is composed of a core of either DNA or RNA surrounded by a protein coat Capsid – made of protein DNA or RNA sheath T4 Bacteriophage

(a) Tobacco mosaic virus Viral structure Virus consists of a protein shell called a Capsid The capsid may have many shapes 18  250 mm 70–90 nm (diameter) 80–200 nm (diameter) 80  225 nm 20 nm 50 nm (a) Tobacco mosaic virus (b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4 RNA Capsomere of capsid DNA Capsomere Glycoprotein Membranous envelope Capsid Head Tail fiber Tail sheath Respiratory infections

How does a virus get inside a cell The capsid proteins bind to receptors on the host cell It “tricks” the cell into allowing it inside.

Once inside the viral genes are expressed. The host cell makes more viruses and the host cell is destroyed.

Viruses are very specific to the cells they infect a particular virus can only infect a specific cell. Bacteriophage – infects bacteria

Reproductive Cycles of Phages Two types Lytic cycle or Virulent Host cell is lysed (bursts) and immediately destroyed Lysogenic cycle or Temperate The virus embeds its DNA into the host’s DNA The virus’s DNA is replicated along with the host cell’s DNA A host cell makes copies of the virus indefinitely.

The lytic cycle of phage T4, a virulent phage Bacteriophage attaches to The bacterium’s cell wall Bacteriophage injects its DNA into the cell The cell makes mRNA from the Bacteriophage’s genes New viruses are assembled. The Virus wrecks the cell causing it to burst The bursting of the cell Releases new bacteriophage particles 1 2 4 3 5 Phage assembly Head Tails Tail fibers

Lysogenic Infection The host cell is not immediately destroyed Viral DNA inserted into the host cell as a prophage

A comparison of the lytic and lysogenic cycles of a bacteriophage 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. The viral DNA is embedded in the Host’s DNA 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 or Prophage Bacterial chromosome Phage DNA

Retroviruses Contain RNA as their genetic information They produce a DNA copy of their RNA RNA produces DNA (this is the reverse of what normally happens in healthy cells) This DNA is inserted into the DNA of the host cell Example….AIDS

Is a Virus a living thing? Some biologists do not consider viruses to be alive because they can’t reproduce independently. Viruses do have some characteristics of a living organism once it infects a host cell, so they are at the borderline of living and non-living things.

Viruses and Cells Virus Cell DNA or RNA, core, capsid Structure DNA or RNA, core, capsid Reproduction only within a host cell Genetic Code – DNA or RNA Growth and Development – NO Obtain and Use Energy – NO Response to the Environment NO Change Over Time - YES Structure cell membrane, cytoplams;eukaryotes also contain nucleus and organelles Reproduction Independent cell division either asexually or sexually Genetic Code – DNA Growth and Development – YES Obtain and Use Energy - YES Response to the Environment - YES Change Over Time - YES

Viral Diseases rubella AIDS HPV influenza Hand, foot and mouth mumps Ebola Cold sore Chicken pox rubella AIDS Small pox

Viral Disease Diseases in Humans Polio, measles, AIDS, mumps, influenza, yellow fever, smallpox, the common cold, and many others

Viral Diseases in Humans Disrupt the bodies normal equilibrium Symptoms caused by Toxins produced by infected cells Toxic components of the viruses themselves Cells killed or damaged by the virus Body defense mechanisms fighting the infection

Viral Damage Depends on the ability of infected tissue to repair itself Epithelium of respiratory tract heals fast so we get over colds Polio –damage to nerve cells is permanent

Cancer causing viruses Oncogenic viruses Examples: Human papilloma virus Human herpes virus Hepatitis B

Viral Diseases in Animals Rabies Parvo Hoof and mouth disease

Viral Diseases in Plants Have a difficult time invading plants due to the cell wall Plant injuries make a plant susceptible to viral disease Insects act as carriers

The simplest infectious agents Viroids – small infectious molecules of single stranded RNA that have no surrounding capsids. Affect plant cells Cause stunted plant growth

Prions – protein infectious particles Linked to several degenerative brain diseases Ex. Mad cow, Creutzfeldt-Jacob Transferred in food Prions are nearly indestructible

Ways to protect against viruses Vaccines Variants or derivatives of pathogens Stimulate the immune system to react against the actual disease Few drugs treat or cure viral diseases Antiviral drugs that are effective interfere with viral DNA or RNA synthesis Example polio, smallpox, rubella, mumps, Hepatitis B

Most viral infections cannot be cured once they occur Antibiotics are powerless against viruses Viruses do not contain the enzymes which antibiotics work against. How the body attacks a virus http://www.bbc.co.uk/science/0/22028517