Chapter 24: Viruses

Viruses consist of genetic material and capsid protein (Fig 24-1). Viruses can only reproduce inside other host cells ( they are obligate parasites ). Each virus usually infects particular species, or even particular cells within a species.

Bacteriophage - a virus that infects bacteria. It usually contains DNA and consists of a head, sheath and tail fibers. Video

Bacteriophages reproduce by : A) Lytic cycle Fig 24-2
1) Phage sticks to bacteria with its tail fibers.
2) It injects its DNA using the sheath.
3) The phage uses the bacterial ribosomes, enzymes etc to form copies of the phage.
4) It breaks open the bacterial cell, releasing the new phages

B) Lysogenic cycle Fig 24-3
1) Phage injects its DNA into the bacteria.
2) Phage DNA joins the bacterial DNA, forming a prophage.
3) The prophage stays dormant, and is copied every time the bacterial cell divides.
4) Eventually the prophage separates from the bacterial DNA and starts a lytic cycle.

Commercial use: Biotechnology companies are trying to develop phages that kill antibiotic resistant bacteria.

Animal viruses contain either DNA or RNA. DNA viruses cause diseases including herpes, chickenpox, warts, smallpox. Images

RNA viruses exists as 4 different types :
a) double stranded RNA : causes diarrhea
b) positive single stranded RNA : acts like mRNA and causes : polio, hepatitis, yellow fever
c) negative single stranded RNA : is a template for making mRNA. It causes : measles, rabies, flu (including bird flu and H1N1 "Swine" flu)
d) retroviruses have + RNA and reverse transcriptase enzyme. It causes AIDS.

Vaccination can prevent viral infection. Smallpox was eradicated in 1980 using vaccination. Polio might be eradicated soon. One problem with flu is that it changes fairly quickly: over 100 different strains are known. The regular "flu shot" vaccinates you against 3 strains, so you can still be infected with the others. Also, flu can pick up genes from pigs and birds, such as the H1N1 "Swine" flu. Most people have no resistance to this type of flu. Flu is spread by coughing, sneezing, and viral particles on doors and other objects.

HIV : Human Immunodeficiency Virus
HIV is a retrovirus with 10,000 base pairs of RNA containing 9 genes. It infects white blood cells, destroying the immune system. It is transmitted primarily through blood and semen.

HIV mutates very rapidly, so it is hard to find a cure. It uses two proteins on the cell membrane to enter white blood cells: CD4 and CKR5. A three drug cocktail ( protease inhibitor + 2 reverse transcriptase inhibitors ) helps to control (but not cure) HIV.
In the US : 1 million people are infected with HIV. Worldwide : 30 million. AIDS

West Nile Virus
West Nile virus first appeared in the US in 1999. The virus causes encephalitis (brain infection) and can kill people, horses and birds. In 1999, 60 people in New York were infected, 7 of these died. West Nile virus is spread by mosquito. West Nile virus.
In 2001 West Nile Virus reached Florida. CNN.

Viroids
Viroids are circular, single stranded RNA with no protein.They infect plants and replicate in the nucleus of the host cell. Example : Coconut cadang-cadang disease which kills whole plantations of coconut palm in the Philippines. Viroid

Prions
Prions are protein with no nucleic acid detectable. They convert normal protein PrP into a different shape : a prion PRP. Prion diseases. The diseases they cause are fatal, there is no cure; some prions survive cooking. Examples: BSE (mad cow disease)
Infected many cattle in Europe around 1990. The rate in cattle peaked in 1993. The disease was transmitted through high protein cattle feed, produced from the carcasses of slaughtered cows.

To date, over 4 million cows from infected herds have been slaughtered. So far, over 100 people in Britain have died from variant CJD (vCJD) which is apparently caused by eating infected beef. Like kuru, vCJD seems to have a long incubation period.
In the US, deer and elk suffer from a prion disease similar to "mad cow disease". "Mad elk" disease.

Last edited September 2009, by David Byres, dbyres@fscj.edu