General characteristics of viruses

on 15.9.05 with 1 comment



  1. Viruses contain only RNA or only DNA, but never both.

  2. They are obligate intracellular parasites and they replicate by producing subunits and then assembling within their host cells.


WE finally have several anti-viral agents today after much research.


  1. Viruses, like chlamydia do not have the necessary metabolic machinery to replicate outside of a cell.

  2. Viruses have no cell wall.

  3. Viruses are filterable agents—much smaller than bacteria

  4. Obligate intracellular parasites

  5. Can not make energy or proteins inedependent of a host cell

  6. Have a capsid or envelope morphology.


-The capsid is a shell that formed of subunits of proteins that are called capsomeres. The capsid protects the genome of the viral genome and are coded for by the genome.


  1. Viruses are not living.

  2. Viruses must be infectious to endure in nature.

  3. Viruses must be able to utilize host cell processes to produce their components (viral mRNA, protein identical copies of the geneome).

  4. Viruses must encode any required process not provided by the cell.

  5. Viral components must self-assemble


The largest of the viruses are almost the size of chlamydia—they are the pox viruses. Of the mammalian viruses, they are the largest viruses.


DNA viruses—herpes virus, adenoviruses, papiloviruses, pox viruses.

The pox virus vaccine was discontinued because of the side effects—there were more problems with the vaccine than there were cases of the disease.


Piconovirus—very small include the rhinoviruses and polio viruses.


We think that RNA was the first genetic material because of its catalytic activity. There are three major theories as to how they evolved---

  1. there was a free-living bacteria that moved into the cell, and lost its ability to carry out many cellular functions—it became adapted to intracellular life, and then it became a virus as it evolved.

  2. The host cell genome there were multiple self-replicating RNA sequences that became so self-sufficient, that they became the virus.

  3. They could have evolved as complex polymers that became self-replicating RNAs that gave rise to viruses and cells independently and simultaneously.


Stucture—nucleic acid is in the capsid center, proteins surround it (if it is a naked virus that is all it has). Enveloped viruses have the same structure but it is covered in lipid membrane that it may have stolen from its host cell. Within that membrane are proteins that allow for the attachment and infection of the host cell. You need to have something in the virus that will interact with host cell to allow it to infect it. If the capsid or the envelope don’t, then the virus can’t infect. Most viruses are neurotrophs. Some are hepatotrophic or enterotrophic—this is all determined by the binding proteins or molecules of the virus.


What ever glycoprotiens that are in the envelope are encoded by the viral genome.


Remember that disease is not the same as infection.


You have a virus that is either
DNA or RNA and structural proteins that are in the capsid. Some viruses have nucleic acid binding proteins needed to ensure replication. The nucleocapsid—same as the naked capsid virus. Some have the additional glycoprotein/lipid bilayer membrane. Virion—the infectious unit for a naked virus that has to have the nucleic acid binding proteins and other enzymes needed—it is complete.


There are some fibers that come out of the capside structure called the penton fibers—these are the attachment sites for viruses that have a naked capsid. This capsid has repeating units of the same type. It is called a Icosahedron. The geometry is very similar to the millennium dome in London. THE reason for that is because the repeating subunits with this shape gives you the maximum volume with minimum amount of genetic information.


Viruses always maximize their genetic information by using information-efficient design.


Picornia viruses assemble in the following way:

  1. they make a long linear protein that then gets cut up into subunits that then form pentomers.

  2. The pentomers then start to congregate

  3. The final cleavage to release a final protein results in the completion of the capsid

-one of the drugs that we have prevents the action of the last protein.


As picornia viruses replicate inside the cell and as you look at the virus particles in the cell under EM, you see that some of them don’t get the nucleic acid and are defective. They can continue the infection because there are so many of them.


The herpes virus has an envelope.


The helical configuration—the nucleic acid is assembled in a helical configuration and the capsomeres are as well as they surround the nucleic acid. The ones that you see the most are the adenovirus.


The pox virus is a larger virus that is more complex. They have a nucleoid and some other structures not seen in most viruses. This is one of the reason that their mode of replication suggests that they used to be viruses. They are complex.


To summarize, we can have either DNA or RNA, naked or enveloped viruses.


Naked capsids—the capsid is protein and is environmentally stable and they are not released from the cell by budding, but must lyse the cell to get out. They can be spread easily. Picornoviruses—don’t go swimming in the Bay because of the sewage output—they survive the treatment of the sewage. They are acid stable, heat stable, resist detergents etc. They can survive inside the stomach. They can illicit a humoral response from the immune system.


The enveloped viruses—have the lipid membrane with glycoproteins. They are easily disturbed by heat, acid and drying. They must stay wet, can’t survive in the GI tract, spread in large droplets. They have to bud to get out of the cell. They initiate a cell-medaited response from the immune system. They don’t have to kill their host cell to spread. They also elicit an antibody mediated response. Pathogeneis of the virus is the hypersensitivity reaction that they illicit.


Three are many types of DNA viruses:

  1. double stranded DNA viruses

  2. Single stranded DNA viruses—


The replication of the viral DNA replication occurs in the nucleosome—the exception to this rule is the pox virus. They replicate in the cytoplasm of the host cell!

-the linear DNA of the virus is indicative of its origins (mammalian cells)


RNA viruses—Rheoviruses are double stranded RNA viruses—You have to pay attention to the polarity of the RNA—

  1. Positive strand RNA viruses have the same polarity of the mRNA. IF you are a positive strand virus, they go and directly start making proteins once they get into the cell. These virions depend on the host cell’s DNA-dependent DNA polymerase as well as the host cell’s DNA-dependent RNA polymerase.

  2. Negative strand RNA viruses are the opposite polarity of the mRNA. Once they enter the cell, you have to do something before you can be transcribed. These viruses have to make a copy of the RNA that is in the right polarity. There is no enzyme in the cell that can do that. These viruses carry within their viral particle-an enzyme that makes a copy of the negative strand into the positive strand.

-these virions carry a RNA-dependent RNA polymerase.

-some have the RNA-depent DNA polymerase—also called reverse-transcriptase.


MORE PATHOGENS (smaller than viruses)

Viroids- contain only RNA (mostly plant pathogens) no protein

Prions-contain only proteins (proteinaceous infectious particles)


Portal of entry

Fecal-oral route

Respiratory tract

Urogenital

Conjunctiva

Skin important anatomical barrier

Arthorpods can be vectors for viroins.

Cow pox and rabies are zoonotic diseases. WE got vaccines from the cow pox because of its low virulence in humans and the homology to small pox.


Anatomical barriers—lysozyme in the tears, tears flush the eyes and the lysozyme protects against bacteria. The skin is an important barrier and has antimicrobial secretions. Langerhan’s cells participate in monitoring the skin as well.

-mucous membranes can prevent the spread and adherence of viruses to the tissue.

-the stomach acidity is a great deterrent to viral infections—only a few can survive here.

-the native flora can compete and keep out infectious agents.

-the urogential tract provides an acidic environment and continous flushing.


Reactions of host cell to virus—most viruses kill the host cell. Some however, make the host cell divide—such as the papilloma virus that can cause cervical cancer.

-some viruses such as HIV can cause cells to conglomerate to produce giant multinucleated cells.

-some cause histological cahges such as the rubella virus and some adenoviruses.

-RNA containing retroviruses are capable of changing the cell morphology as they can incorporate into the host genome.


Viral inclusion bodies—particular for certain disesases—these bodies are rapid means to diagnose the disease such as the rabies virus. They traditionally produce the includsion bodies in (negri bodies) in the brain of the infected animal. These are cytoplasmic.


Cowdry bodies are found in cells infected with herpes virus. These are nuclear inclusions.


Guanieri bodies are found in the cytoplasm of infected cells that bear the small pox virus.



Determinants of viral disease—the target tissue, the portal of entry, the access to target tissue, the attachment to the tissue, and the strain of the virus all contribute to the disease process and determine its course.


The severity of the disease is determined by innoculum size, immunopathology, cytopathic ability of the virus, length of time before resolution of infection, general health of the individual, and other diseases that the host may have. Age and genetic constitution of the host also influence the course and severity of the disease.


Category: Microbiology Notes

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1 comments:

Anonymous said...
February 5, 2011 at 1:40 PM

cn u gve me an explanation of this characteristics of virus??

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