Virus Notes

Louis Pasteur

Postulated that rabies was caused by a virus and developed the first vaccine for rabies.

Ivanovski and Beijerinck

Showed that a disease in tobacco was caused by a virus.

Virology

A multifaceted discipline that studies viruses, which are noncellular particles with a definite size, shape, and chemical composition that can infect every type of cell.

Virus Size

Viruses are ultramicroscopic and vary in size.

General Characteristics of Viruses

Viruses are non-living entities composed of nucleic acid surrounded by a protein coat. They can only replicate inside living cells and are obligate intracellular parasites.

Bacteriophage

Viruses that infect bacteria.

E. Coli

only the nucleic acid enters the cytoplasm

Viral Architecture

Viruses lack protein-synthesizing machinery and contain only the parts needed to invade and control a host cell. They are surrounded by a protective protein coat called a capsid, and some viruses have an external covering called an envelope.

Viral Genome

The sum total of genetic information carried by an organism. Viruses contain either DNA or RNA, but never both. The genome carries genes necessary to invade host cells and redirect their activity to make new viruses.

Replication Cycle

Viruses can only multiply in living cells and must possess the genetic information to encode proteins required for viral protein coat synthesis, viral nucleic acid replication, and movement in and out of the host cell.

Bacteriophage Replication

Bacteriophages multiply inside the cells they invade and escape by lysing the host cell. Some bacteriophages integrate their DNA into the host's genome and replicate when the host replicates.

Stages of Phage Replication

Adsorption, Penetration, Replication, Assembly, Maturation, Lysis & Release.

Lysogeny

The silent virus infection. Some phages undergo adsorption and penetration but don't replicate or lyse the host cell. The viral genome inserts into the bacterial genome and becomes an inactive prophage, resulting in the spread of the virus without killing the host cell.

Animal Viruses

Animal viruses have a structure composed of capsid proteins, nucleic acid (DNA or RNA), and may or may not have a viral envelope. They can cause various diseases in animals.

Phases of Animal Virus Replication

Adsorption, Penetration, Uncoating, Synthesis, Assembly, Release.

Damage to Host Cells

Viruses can induce cytopathic effects like

Change in size and shape

cells fuse to form multinucleated cells

alter DNA

form inclusion bodies

mass of virus or damaged organelles

transform cells into cancerous cells

cause cell lysis.

Persistent Infections

Viruses or their genomes are continually present in the body, and virions are released from infected cells by budding. The virus can last weeks or a lifetime. There are three major categories of persistent infections:latent infections, chronic infections, and slow infections.

Viruses and Tumors

Some animal viruses can permanently alter the genetic material of host cells, resulting in cancer and the formation of tumors. Transformed cells have an increased growth rate, alternate chromosomes, and the capacity to divide all resulting in tumors.

Medical Importance of Viruses

Viruses are the most common cause of acute infections and can have high mortality rates. They may also be connected to chronic afflictions of unknown cause and can have several billion viral infections per year.

Detection and Treatment

Detection of viruses is more difficult than other agents and requires appropriate sampling, cell culture or tissue culture methods, and screening for viral antigens or immune response. Antiviral drugs can cause serious side effects.

Prions

Composed primarily of infectious protein with no nucleic acids, prions cause diseases such as Creutzfeldt-Jakob disease: affects central nervous system

bovine spongiform encephalopathy (mad cow disease)

and Shy-Drager syndrome, which resembles parkinson’s diseases.

Oncovirus

tumors

examples are papillomavirus, herpesvirus, and hepatitis B. virus

Bacteriophage

viruses that infect bacteria

Characteristics of viruses

non-living entities that are bits of nucleic acid with a protein coat

they can only replicate inside living cells

nucleocaspid

A viral caspid with nucleic acid

Caspid

A protein coat that protects the nucleic acid when the virus is outside of the host cell

envelope

Viruses that have an external covering

Function of envelope

helps bind the virus to a cell surface and assist with the penetration of the viral DNA or RNA into a suitable host cell.

Function of caspid and envelope

stimulate host’s immune system antibodies

naked viruses

those lacking an envelope

Icosahedral shape

virus can have 20 sides with 12 corners

helical shape

gives the virus a filamentous or rod-like appearance.

complex shape

some have icosahedral head

caspid with a long helical protein component called

1. sheath

2. tail pins

3. tail fibers

   (looks like a spider and injects into the bacteria)

Lytic phage

when viruses multiply inside the cells they invade then escape by bursting the host cell

Temperate phage

when the viruses integrate their DNA into the host’s genome and replicate when the host replicates

Latent infections

persistent infections that are asymptomatic until itis reactivated with more symptoms. Infectious particle cannot be detected unless the disease is reactivated.

Chronic infection

The infectious virus can be detected at all times. Symptoms can be present or absent for an extended period of time.

Slow Infection

The infectious agent gradually increases over a very long time during which no symptoms are apparent. Eventually a slow progressive lethal disease occurs.