Viruses

The Ultimate Parasite

•Characteristics of Viruses. . .

•They are not cellular

•They cannot carry on metabolic activities independently

•They contain either RNA or DNA- not both (true cells contain both RNA and DNA)

•They do not do protein synthesis - they lack the ribosomes and enzymes necessary for the process

•They reproduce only within the cells they infect

•Classification of viruses. . .

•They are not cells - or considered to be living

•They are not classified in any of the five kingdoms

•No classification system for viruses has been agreed upon

•Viruses are usually grouped according to size, shape, presence or absence of an outer envelope, and whether they contain DNA or RNA and whether it is single-stranded or double-stranded

•They are sometimes classified according to the diseases they cause or their mode of transmission

Viral Structure. . .

A virus is a tiny particle (virion) consisting of a core of DNA or RNA surrounded by a protein coat (capsid).

A virion is a single, infective virus particle. The envelope is a membranous layer surrounding some viruses. It contains proteins, lipids, carbohydrates and traces of metals. The nucleic acid core is either RNA or DNA.

Bacteriophages are viruses that infect bacteria. These “phages” are used widely in viral research. They are a good model to work with.

Virulent (lytic) viruses cause lytic infections. They cause the host cell to lyse.

Temperate (lysogenic) viruses remain relatively dormant within the host and do not kill the host cell.

Lysogenic viruses may become lytic.

•Viral Replication in Lytic Infections. . .

•Attachment - phage attaches to receptor sites on bacterial cell wall.

•Penetration - nucleic acid is injected through the cell membrane into the cytoplasm of the host cell; the capsid remains outside the cell.

•Replication - the virus takes over the host’s metabolic machinery and replicates its own macromolecules; viral genes contain all information necessary to produce new viruses.

•Assembly - viral components are assembled into new viruses.

•Release - lysozyme produced by viruses degrades the host cell wall, causing release of about 100 phages.

Lysogenic Infections. . .

When viral DNA is in the lysogenic mode it is called a prophage. In lysogenic infections, the temperate viruses integrate their DNA into the host cell’s DNA. When the bacterial DNA replicates, the viral DNA prophage also replicates.

Lysogenic Conversion is the display of new properties by bacterial cells containing temperate viruses; toxins are only produced by many disease-causing bacteria when infected with a phage.

Diphtheria, Scarlet Fever, and Botulism are all lysogenic bacteria toxins.

One form of coexistence is budding.

There are a few animal and bacterial viruses that release new viruses slowly without harming the host cell. The host cell carries on its own metabolic activities, using some energy to produce new viruses. The new viruses appear to exit the cell in a process that may be the reverse of penetration.

Plant Viruses and Viroids

The tobacco mosaic virus is one of several viruses that have certain characteristics. They consist of only an RNA core surrounded by a protein capsid. They are elongate and lack an outer envelope. They attach to the host’s ribosomes and are translated like mRNA.

Plant viruses cause serious agricultural losses and cures are not known. Infected plants are commonly burned. Virus resistant strains are in the process of being developed.

Viroids are infective agents that are smaller and simpler than viruses. They consist of a short strand of RNA (250-400 nucleotides). (This should be enough to code for a medium sized protein). They do not have a protein coat.

The host’s enzymes are used to replicate the viroid’s RNA. They are generally found in the nucleus and cause disease by either interfering with intron splicing or by interfering with the regulation of the host’s genes.

As far as we know, they only infect plants (potato spindle tuber disease and the stunting of chrysanthemums). They are suspected to cause some animal diseases.

Viruses that infect Animals

There are hundreds of different viruses that attack humans and other animals. Virtually every type of cell is susceptible to viral infection. Most viruses cannot survive very long outside of a living cell.

Viruses may be quite specific due to receptor sites and binding on surfaces. Some viruses are not only specific to the organism that they infect, but also to the tissue that they infect. (Poliovirus is specific to the spinal cord, throat, and intestinal cells.)

Viruses are associated with cancer. The virus, itself may contain oncogenes or may not contain oncogenes, but may activate cellular oncogenes.

Oncogenes are genes, which trigger the growth process in non-growing tissue. It is believed that oncogenes are a normal part of our genetic make-up. They can be triggered by viral infection.

Retroviruses are RNA containing viruses that also contain reverse transcriptase.

HTLV - retrovirus linked to certain leukemias.

HIV - the retrovirus that causes AIDS

DNA viruses have not been isolated from human tumors, but several are linked with human cancers:

Epstein-Barr virus - a type of herpes virus thought to cause Burkett’s lymphoma (a lymphatic cancer), infectious mononucleosis, and nasopharyngeal carcinoma among persons of Chinese ancestry.

Herpes simplex virus type 2 - linked with cervical cancer.

Hepatitis B virus - associated with liver cancer.

DNA viruses may play a role in other cancers also (Hodgkin’s Disease, breast cancer, Kaposi’s sarcoma).

The infection cycle of HIV - the AIDS-causing virus. . .

wThe HIV virus responsible for AIDS attaches to a CD4 receptor protein on the surface of a T4 lymphocyte cell, and enters the cell by endocytosis.

wThe viral RNA is released into the cell’s cytoplasm.

wA DNA copy is made of the virus RNA (with the assistance of reverse transcriptase).

wThe DNA copy enters the cell’s chromosomal DNA.

wAfter a long period (an average of 8 years), the virus genes initiate active transcription.

wBoth HIV RNA and HIV proteins are made.

wComplete HIV virus particles are assembled.

wSome cells are lysed, releasing free HIV, whereas others bud out HIV by exocytosis.

Prions - these are protein-like infectious particles. They are even smaller than a viroid. They consist of only glycoproteins with no nucleic acids. Prions polymerize in infected tissue.

“Mad Cow disease”

Treating viral infections. . .

•Vaccines can help prevent viral infections

•Antibiotics kill bacteria - not viruses

•Effective antiviral drugs have yet to be developed

•Acyclovir, used in the treatment of genital herpes, is the most effective antiviral drug to date; it doesn’t cure but it speeds healing.

Theories on the Origin of Viruses. . .

•Ancestors of viruses were primitive free-living heterotrophs in the primordial sea. As free nutrients were depleted, they switched to a parasitic mode.

•Viruses evolved from cellular ancestors and became highly specialized parasites. In the process, they lost their cellular components.

•Viruses are bits of nucleic acids, which “escaped” from cellular organisms. Some are from plants and some are from animals. This would help explain host specificity. (Most likely).