Viruses

A.What is a Virus? All common colds and flu are a result of having an invasion of viruses in your body  Virus: nonliving particles (one particle is a “virion”) composed of nucleic acids enclosed in a protein coat which are smaller than any bacterium

oConsidered nonliving because they don’t exhibit all criteria for life  No respiration  No growth or development oMain job is replication – making copies of themselves, and since they aren’t living, they need help from living cells called hosts  Host cell: a cell in which a virus replicates

oViruses are named after the disease they cause or the organ system they infect since they are nonliving, and scientists don’t name nonliving things the same way as living things

 Common viruses:  Polio virus  Rabies virus  HIV virus  Influenza virus DISEASES Polio virus Rabies virus HIVInfluenza virus

 Adenovirus (adenoid tissue between throat and nasal cavity = common cold)  Hepatitis (hepatic organ = liver)  Bacteriophage (bacteria) adenovirus hepatitis bacteriophage

All viruses have similar structure, although they may look different  nucleic acid  capsid  envelope

 Nucleic acid inner core oInside of a virus contains nucleic acid, either DNA or RNA oThis contains the instructions for making more viruses oViruses may contain as little as 4 genes, or as many as 100s of genes

 Capsid oOuter protein coat is called a capsid, and is what is detected by the host to determine if it is a foreign object oArrangement of the protein in capsid will determine:  the shape of the virus  which cell can be infected  how the virus infects the cell

 Envelope oIf the virus is a large one like the human flu virus, it may also have an outer envelope that surrounds the capsid

 Five major shapes of viruses oPolyhedral (icosahedral)  Non-enveloped with geometric shape with flat sides  Human papilloma virus (HPV)

oHelical  Non-enveloped, with the proteins arranged like a spiral staircase with the nucleic acids in the middle  Ebola virus

oEnveloped polyhedral  Polyhedral shaped with a spherical envelope making it round  Herpes virus

oEnveloped helical  Helical shaped with a spherical envelope making it round  Often times have extra protein projection studs or spikes  Influenza virus  Human immunodeficiency virus (HIV)

oComplex  Polyhedral head with cylindrical tail and leg-like fibers  T4 bacteriophage

Virus Types

All viruses have to attach to the host cell before it can enter and begin with replication  Has to recognize particular host cell, so must recognize specific receptor sites on host cell’s plasma membrane o One of its proteins interlocks with receptor site, kind of like spaceships docking together for refueling

 Each virus has specifically shaped attachment protein, so it can usually only infect a few types of cells  T4 bacteriophages have tail-like protein fibers that attach to certain E. coli bacteria  It can’t infect an animal or plant cell, so humans don’t have to worry about them  Polio viruses can only infect human intestinal or nerve cells

 Due to the fact that certain viruses are species specific, it makes it possible to control some  By 1980, human small pox virus was eradicated, simply because we could control it with vaccines and isolation

 Certain influenza viruses infect both humans and other animals, so we can’t control it  West Nile virus infects mainly birds, horses, and humans, so we can’t control it either

B.Viral Replication Cycles Once attached to host cell, virus takes over cell’s metabolism and it can replicate Two methods of entering host cells:  injection  endocytosis

 Injection (non-enveloped viruses) o Virus attaches to cell and injects nucleic acid into host cell

 Endocytosis (enveloped viruses) o Virus attaches to cell, and host cell’s membrane surrounds virus, becoming a vacuole that enters the cell. Once in the vacuole, the virus bursts out, and enters cell

Once inside the host cell, the virus will replicate itself through a cycle, and this replication can destroy the cell immediately, or leave it intact for awhile. As a result, there are two types of replication cycles:  Lytic cycle – host cell dies  Lysogenic cycle – host cell lives

 Lytic Cycle oViral genes alter the host cell to force it to make new viruses  Host cell uses own enzymes, proteins, nuclear bases, and energy to make more viruses, which then burst from host cell, which kills it  New viruses infect other cells, repeating process  Disadvantage: will eventually kill all host cells, and thereby killing themselves

LYTIC CYCLE

 Lysogenic Cycle oVirus inserts own nucleic acid into host cell’s nucleic acid, so when host cell undergoes own replication, new viruses are made.  viral DNA that is integrated into a host cell’s chromosome in this fashion is called a provirus

 A provirus does not necessarily affect host cell’s activities, so the host cell continues on with own duties  this can continue for years, but a provirus can become lytic at any time, and start killing the host cells

 long duration of lysogenic cycle explains how an organism can be infected with the same virus for life, but only see the symptoms every once in a while  Are many examples of human viruses that can alternate between the lysogenic and lytic cycles for a lifetime

 herpes simplex I  virus causes cold sores and canker sores  always in your body (lysogenic cycle), but when certain viruses become lytic, they kill those certain cells which shows up as a sore Cold sore or fever blister Canker sore

 herpes simplex II  virus causes genital warts  lytic cycle shows up as warts that go away, but may come back every time the virus goes from lysogenic cycle to lytic cycle

 hepatitis B  virus causes hepatitis B  Disease attacks the liver, causing liver cancer, cirrhosis (scarring), and liver failure

Cirrhosis of the Liver

 Varicella  virus that causes chicken pox and shingles  chicken pox is lytic cycle of virus, and once the person is well, it’s really the virus going into lysogenic cycle

 person always has virus, but it now affects nerve cells, and may become lytic again but this time causing disease shingles, a painful infection of nerve cells

most complex replication cycle is that of a retrovirus  retrovirus: type of viral replication where a virus uses reverse transcriptase enzyme to make DNA from viral RNA; the retroviral DNA is then integrated into the host cells chromosome

ocells can normally only make RNA from DNA (transcription), and can’t go in the reverse A C G U C U RNA A G T C G A DNA trxn

othese viruses contain RNA only, and RNA can not be integrated into host cell’s DNA since it is not the same nucleic acid  have the capability of making own DNA from their RNA using enzyme reverse transcriptase that takes RNA and transcribes it backwards, into DNA A C G U C U RNA A G T C G A DNA Reverse transcription

 virus can now insert viral DNA into host DNA, and begin lysogenic or lytic cycle, thereby becoming a provirus  ex: HIV

 virus now infects white blood cells, those that are responsible for generating immune response  new viruses get released from white blood cells via exocytosis, and are free to infect other white blood cells  infected person may not feel sick or out of sorts for 2 – 5 years, but they are still able to infect another person

 HIV becomes AIDS when HIV enters lytic cycle, and white blood cells are killed, preventing body from fighting smallest disease, like a cold  Human immunodeficiency virus (HIV) = lysogenic cycle  Acquired immunodeficiency syndrome (AIDS) = lytic cycle

Retroviral Replication Cycle

How HIV Drugs Work

C.Cancer and Viruses Some viruses have been linked to certain cancers in animals including humans  Viruses disrupt the normal cell cycles (mitosis or meiosis), causing abnormal growth and creating tumors

oHepatitis viruses have been known to cause liver cancer

oHPV has been known to be a cause of cervical cancer and genital cancers

oEpstein-Barr virus (EBV) is the cause of mononucleosis, but can also increase risk of nasopharyngeal cancer, lymphomas, Hodgkin Disease, and even stomach cancers EBV

 Don’t freak out! – Most people infected with these viruses will NOT develop the cancer, but the reality is that those people who DO develop the cancer, the MAJORITY of them have the virus

Researchers have found two virus-like particles that can also cause infectious diseases  Prions  Viroids

 Prions oPrions are proteins that occur naturally in the brains of animals and people, but they do not have nucleic acids.

 Normally, these proteins are harmless, but when they're misshapen they can cause devastating illnesses.  Thought that they cause infections by folding incorrectly, resulting in improper functioning  Common in many animal diseases like mad cow disease, but also in human equivalent Creutzfeldt- Jakob disease (CJD)

CJ Disease

 Viroids oComposed of a single strand of RNA with no protein coat  Common in many plant diseases

About 1000 plant diseases are caused by about 400 viruses  First discovered was tobacco mosaic virus, which causes disease in tobacco plants TMV

oViruses can cause stunted growth, and overall damage to crops which are rendered useless  Require wounds or insect bites to enter plant host  Do not undergo lytic or lysogenic cycles Insect inserting proboscis in cactus leaves a wound that allows virus to enter, causing “lipstick” marks where virus is located

 Do not necessarily cause disease, just a disruption in the normal plant cell cycle o Example is a tulip breaking virus which causes a color streaking in the petals, sometimes making them more valuable Yellow striping in red tulip is result of a virus “breaking” or streaking the color

D.Origin of Viruses Where did viruses come from?  Probably originated from host cells since they need host to replicate o Maybe the nucleic acid broke free from host cell, but still retains ability to replicate