VIRUSES

18.2

Daily Objective 4/ 15

Describe common characteristics of Viruses.

Virus 101

• A virus is a nonliving strand of genetic material within a protein coat

• Why are they nonliving?– They have no organelles, to take in nutrients or

use energy, they cannot make proteins, move, or replicate on their own.

• Smaller and less complex than prokaryotes• Invade cells and can alter cellular functions

Viral structure• Non-cellular• Consist of an inner nucleic acid core and a

protein coat (capsid)• Viral shape is determined by the arrangement

of proteins making up the capsid

Viral Infection

• In order to replicate a virus must enter a host cell. The virus attaches to the host cell using specific receptors on the plasma membrane of the host.

• Once the virus successfully attaches to the host cell, the genetic material enters the cytoplasm of the host.

• The virus now uses the host cell to replicate by 1 of 2 cycles. The lytic or lysogenic cycle.

Viruses and You

• http://www.youtube.com/watch?v=Rpj0emEGShQ

Lytic Cycle (active)

• Step 1. The host cell makes many copies of the viral RNA or DNA. The virus takes over and all the host cell’s resources are used to make copies of the viruses, which eventually burst out of the host cell and kill it. A typical lytic cycle takes about 30 minutes and produces about 200 new viruses. They have a short incubation period and a short replication cycle.

Lysogenic Cycle (latent)

• Not all cells are killed by the virus immediatedly, sometimes the virus’s DNA/RNA become integrated into the host cells chromosomes.

• Once integrated, the infected cell will have the viral genes permanently. Each time the cell replicates , the provirus’s DNA/RNA is replicated as well. The viral genes may remain dormant for months or years.

Virus Release

• The virus can be released from the cell in two ways

1. Cell Lysis- the cell bursts and viruses move on to infect other cells

2. Excoytosis- the virus is surrounded by the cells plasma membrane and then excreted

Retrovirus

• HIV- (human immunodeficiency virus), the virus that causes AIDS, is retrovirus, which is a virus that contains RNA. HIV infects white blood cells, which are part of the body's disease fighting system, so as the disease progresses, the body’s inability to fight disease is much greater.

• Other retroviruses are thought to cause some forms of cancer. HPV can cause cervical cancer and Hepatitis B can cause liver cancer.

Plant viruses

• There are about 400 viruses found in plants that can cause up to 1,000 diseases. The tobacco mosaic virus infects tobacco plants. It is thought to be the 1st identified virus.

What the virus am I?……..is a highly contagious disease caused by primary infection with varicella- zoster virus (VZV) It usually starts with skin rash mainly on the body and head rather than at the periphery and becomes itchy, raw marks which mostly heal without scarring. On examination, the observer typically finds lesions at various stages of healing.

• It spreads easily through coughing or sneezing of ill individuals or through direct contact with secretions from the rash. A person is infectious one to two days before the rash appears. They remain contagious until all lesions have crusted over (this takes approximately six days). Immunocompromised patients are contagious during the entire period as new lesions keep appearing. Crusted lesions are not contagious.

• ………has been observed in other primates, including chimps and gorillas

What the virus am I? • …….. is a viral disease that causes acute encephalitis in warm- blooded

animals. The disease can be transmitted from one species to another, such as from dogs to humans, commonly by a bite from an infected animal. For a human, it is almost invariably fatal The virus infects the central nervous system, ultimately causing disease in the brain and death.

• The virus travels to the brain by following the peripheral nerves. The incubation period of the disease is usually a few months in humans, depending on the distance the virus must travel to reach the central nervous system .Once the virus reaches the central nervous system and symptoms begin to show, the infection is virtually untreatable and usually fatal within days.

• Early-stage symptoms are headache and fever, progressing to acute pain, violent movements, uncontrolled excitement, depression, and hydrophobia.

• …….. causes about 55,000 human deaths annually worldwide. 95% of human deaths due to rabies occur in Asia and Africa. Roughly 97% of human rabies cases result from dog bites. In the United States, animal control and vaccination programs have effectively eliminated domestic dogs as reservoirs of rabies.] In several countries, including Australia and Japan, rabies carried by terrestrial animals has been eliminated entirely. While classical rabies has been eradicated in the United Kingdom, bats infected with a related virus have been found in the country on rare occasions.

Rabies

What the virus am I?

• ……..is a potentially serious illness. Experts believe …..is established as a seasonal epidemic in North America that flares up in the summer and continues into the fall.

• Prevention measures consist of community-based mosquito control programs that are able to reduce vector populations, personal protection measures to reduce the likelihood of being bitten by infected mosquitoes, and the underlying surveillance programs that characterize spatial/temporal patterns in risk that allow health and vector control agencies to target their interventions and resources.

West Nile Virus

EXIT TICKET

• Scenario:• You have been told that a particular virus has

a long incubation period. What could you infer about the type of virus it is and what type of cycle it replicates by?