Viruses

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Viruses with Envelopes

-          Some viruses have a viral envelope outside the capsid that helps the parasite enter the host cell.

-          Membrane is generally a lipid bilayer with glycoproteins.  These glycoproteins form spikes that help bind the viruses to specific receptor molecules on the host cell.

-          The membrane then fuses with the host’s plasma membrane and transports the capsid and viral genome inside. 

-          After enzymes remove the capsid, the viral genome can replicate and direct synthesis of proteins including those for new envelopes.

-          The ER of the host makes the membrane proteins, which are transported to the plasma membrane where they are clustered to serve as exit points for the viral offspring. 

-          The viruses bud from the cell like exocytosis, wrapping themselves in membrane as they go. 

-          The enveloped viruses are now free to spread the infection.

-          Other viruses have membranes that are derived from the nuclear membrane of the host. 

-          The genomes of herpesviruses are double-stranded DNA and the viruses reproduce within the cell nucleus using a combo of viral and cellular enzymes to replicate.

-          While in the nucleus the virus DNA may integrate with the cell genome to form a provirus, which is similar to a bacterial prophage in the lysogenic cycle.

-          Once acquired, these viruses tend to be latent during a period and then reoccur like with cold sores.

 RNA Viruses

 -          These are referred to as retroviruses (Retro = backwards) and refer to the reverse direction in which genetic information flows in these viruses. 

-          These viruses are equipped with a reverse transcriptase, which can transcribe DNA from an RNA template. 

-          The newly formed DNA then integrates as a provirus into the chromosomes within the nucleus of the animal cell.

-          The host’s RNA polymerase then transcribes the viral DNA into the RNA molecules, which can function as mRNA for the synthesis of viral proteins and new genomes for viral offspring.

-          One retrovirus of importance is HIV that causes AIDS.

 Important Viral Diseases in Animals

 -          Some viruses damage or kill cells by causing the release of hydrolytic enzymes from lysosomes

-          Some causes infected cells to produce toxins that lead to disease symptoms and some have toxic components themselves like envelope proteins.

-          The amount of damage depends on the ability of the infected tissue to regenerate by cell division. For example, we bounce back from a cold because our respiratory tracks are able to repair themselves rapidly but polio attacks the nerve cells, which cannot be replaced.

-          Many of the symptoms associated with viruses are caused by our own bodies such as fever, aches, and inflammation, which may result from the body defending itself.

-          Vaccines are harmless variants or derivatives of pathogenic microbes that stimulate the immune system to mount defenses against an actual pathogen

-          Vaccine is derived from vacca – Latin for cow because the first vaccine consisted of the cowpox virus.

-          Edward Jenner, in the late 1700’s, discovered that milkmaids that were infected by the cowpox virus were resistant to the smallpox virus.

-          This is because the two viruses are so similar that the immune system cannot tell them apart.

-          Vaccines may help prevent viruses but once they attack, there is very little that can be done to cure them.  There are two antiviral drugs that may be used arabinoside which inhibits the synthesis of host nucleic DNA and acyclovir which inhibits herpesvirus DNA synthesis

Emerging Viruses

-          Three processes contribute to the emergence of viral diseases:

1.          An existing virus can evolve and cause disease in individuals who had developed immunity to the ancestral virus. An example is the flu virus that changes each year.

2.          An existing virus can spread from one host species to another.  An example is the monkeypox virus, which spread from African monkeys to Asian monkeys in the 1950s when the two species were transported together to a laboratory situation.  In the 1970s, the virus was first seen in humans although it is thought that it did not come directly from monkey but from game animals that harbored the virus without falling victim to it.

3.          An existing virus can disseminate from a small population to become more widespread.  In 1993, the Hantavirus made the news but it was not a new virus.  Deer mice harbored the disease and transferred it through their urine or fecal matter.  The Navajo had known about the disease for years.  In 1993, the population of deer mice increased and so did the virus.  The same is true with HIV.  Technology, social behaviors, and economy have increased the numbers affected by the disease.

 Viruses and Cancer

 -          Scientists have recognized that some viruses cause cancer is some animals.

-          These viruses are found in the retrovirus, papovavirus, adenovirus, and herpesvirus groups.

-          When certain tumor viruses infect animal cells, the cells undergo transformation into a cancerous state and abandon their normal growth.

-          There is strong evidence that viruses cause certain types of human cancer.  For example, the virus that causes Hepatitis B seems to cause liver cancer in patients with chronic hepatitis.  Another is the Epstein-Barr virus that causes infectious mononucleosis has been linked to Burkitt’s lymphoma.  Papilloma viruses have been associated with cancer of the cervix.  HTLV-1 causes a type of adult leukemia. 

-          All tumor viruses transform cells through the integration of viral nucleic acid into the host cell DNA.

-          Scientists have identified several viral genes directly involved in triggering these characteristics and are called oncogenes and are not unique to tumor viruses or tumor cells.  These genes can be found in normal cells of many species.  These genes seem to code for cellular growth factors.  In some cases, the tumor virus lacks the oncogenes so it turns on or speeds up the expression of those in the cell’s DNA.  Normally, more than one oncogene must be activated to transform the cell in a truly cancerous state.  Many are only effective in combination with other events like exposure to carcinogens (nonviral, cancer causing agents).

 Plant viruses

            -          Stunt plant growth

-          Diminish crop yields

-          Most are RNA viruses and have rod shaped capsids with the proteins arranged in a spiral

-          Major routes of infection:

-             Horizontal transmission – plant is infected from an external source of the virus.  Virus must get past the thick epidermis so it is more susceptible if the plant has been damaged.  Insects are a double threat because they damage the plant and carry viruses

-             Vertical transmission – Plant inherits a viral infection form a parent.  Occurs with asexual propagation or in sexual reproduction via infected seeds. 

 Viroids and Prions

 -          Viroids are tiny molecules of naked RNA only several hundred nucleotides long. 

-          They disrupt the metabolism of the plant cells and stunt the growth of the entire plant.

-          The nucleotide sequence is similar to sequences of introns found within those genes.

-          Viroids originated as “escaped introns.”

-          They cause errors in the regulatory systems that control the genes of the cell and are associated with abnormal development and stunted growth.

-          They are nucleic acids and their self-directed replication prevents them from being diluted when they are transmitted.

-          Prions are infectious agents that are proteins.

-          May be responsible for some degenerative diseases of the nervous system.

-          It is a defective form of a protein normally present in specific cells.

-          When the prion infects a cell with the normal protein, the prion catalyzes conversion of the normal protein to the prion version.  If this is correct, the prion can trigger a chain reaction that increases the number of prions that allows it to spread through a host population without dilution.

 Virus Evolution

 -          It is reasonable to assume that viruses are not the descendants of precellular prototypes of life but that they evolved after the first cells.

-          Perhaps the earliest viruses were naked bits of nucleic acid, similar to viroids.

-          The most likely candidates for these genomes are plastids and transposons.

-          Plasmids are small, circular DNA molecules that are separate from chromosomes found in bacteria and yeasts.  They can replicate independently of the rest of the cell’s genome and are occasionally transferred between cells.

-          Transposons are DNA segments that can move from one location to another in the cell’s genome.