MICROBIOLOGY: UNIT I Viruses
List the 6 major steps of the lytic cycle.
1. Attachment - the virion attaches itself to a specific receptor site on the surface of the host cell 2. Penetration - viral nucleic acid enters host cell (3 different ways depending on whether virus is naked or enveloped) 3. Uncoating - removal of the capsid & envelope; only occurs if virus is enveloped. 4. Biosynthesis - viral components (nucleic acids & proteins for capsids) are synthesized by the host cell. 5. Assembly/Maturation - components are assembled into new viruses (nucleic acids are inserted into capsids) 6. Release - hundreds of intact virions exit host cell (2 different ways lysis or budding)
How does the presence of an envelope increase a virus's disease causing capacity? (2 ways)
1.) Makes it difficult for the host's immune system to recognize the foreign virus. 2.) Makes it easy for the enveloped virus to enter another cell (membrane fusion: cell membranes fuse, dumping the virus into the new host cell)
What is a syncytium?
A multicleated cell formed by the fusion of the plasma membranes of adjacent cells.
Most animal viruses are RNA viruses (RNA or DNA). Why do these viruses have a higher mutation rate?
Because of the error rate of the enzymes involved in RNA replication
What are 2 ways virions released from a host cell?
Burst/Lyses - If the virus is the naked type, an encoded protein, lysozyme, dissolves the cell membrane &/or cell wall of the host cell, causing the cell to lyse & releasing the hundreds of viruses inside it Budding - If the virus is the enveloped type, it pushes out the cell membrane, forming a bud that encloses the virus - then the bud pinches off behind, resealing the host cell; as a result the host cell is not lysed
Why are viruses considered obligate intracellular parasites?
Can only reproduce/replicate if inside a host cell.
What cytopathic effect does measles virus and RSV have on host cells?
Causes plasma membranes of neighboring cells to fuse, creating giant, multinucleated cells called syncytia.
Continuous cell lines are usually derived from cancer cells. Why?
Cells keep dividing out of control, supplying an endless supply of cells for use in research.
How are viral species named?
Common English names
Examples of latent (lysogenic) animal viruses include:
Ex. Typical of DNA viruses belonging to Herpesvirus family - herpes simplex 1 (causes fever blisters) causes a symptomless latent infection of nerve cells of mouth & lips - infection can be reactivated by a fever, a cold, too much sun, or stress. Ex. Varicella Zoster (another Herpesvirus) causes chickenpox as the primary infection & shingles as the reactivation. Ex. HIV (Human Immunodeficiency Virus) - belongs to the Retrovirus family; causes AIDS (Acquired Immune Deficiency Syndrome). Ex. HPV (Human Papilloma Virus) - causes warts and cervical cancer.
Provide 3 examples of viruses that form syncytia.
HIV, Measles virus, Respiratiory Syncytial Virus (RSV)
How was this problem solved? In the 1950's cell culture & tissue culture methods were developed from cancer cells.
How was this problem solved? In the 1950's cell culture & tissue culture methods were developed from cancer cells.
Why are the Retroviruses referred to as the "backward viruses?"
In the process of transcription, our cells make RNA from our DNA. The retroviruses are RNA viruses, so DNA must first be made from their RNA before they can be replicated. Since their process is opposite of ours, we call them "backwards."
What is the dilemma of growing viruses in the lab?
It requires live cells (viruses are obligate intracellular parasites) and viruses are very specific for the cells they infect.
What cytopathic effect does Epstein Barrr virus have on lymphocytes?
Makes them large and misshapen
Penetration: How do most naked animal virions penetrate the host cell?
Most naked animal viruses enter the host cell when the viral capsid adsorbs (attaches) to the host cell surface & only the viral nucleic acid enters cell; the capsid is left on the outside
Give 3 examples of viral species.
Mumps, Measles, Rubella, Human Immunodeficiency Virus, Human Papillomavirus
Describe the general structure of a virus.
Nucleic acid (DNA or RNA) surrounded by a protein coat called a capsid. Virus may be naked or enveloped.
What can trigger lysogenic viruses to go into the lytic cycle?
Temperature change, UV, stress, drugs
Penetration: How do bacteriophages penetrate the host cell?
The bacteriophage is a naked virus. The tail pins penetrate the thick bacterial cell envelope, injecting only the viral nucleic acid; the capsid stays on the outside of the cell and disintegrates.
How do viruses cause their host cell to lyse?
The host cell produces the enzyme lysozyme, encoded on the viral nucleic acid.
What is the disadvantage of growing animal viruses in chicken embryos?
The problems of viral specificity - viruses are very specific for the cells they infect. Not all viruses would infect chicken embryo cells.
How does the bacteria Corynebacterium diphtheriae cause disease?
The toxin produced by the bacteria is coded for on its prophage (integrated bacteriophage). So, it the bacteria can only cause disease if it is infected by a specific virus.
How does viral infection lead to syncytia formation?
The viral fusion protein that allows enveloped viruses to enter a host cell by membrane fusion causes the plasma membranes of adjacent cells to fuse.
Explain how a virus acquires an envelope.
The virus pushes out of the cell membrane, forming a bud that encloses the virus - then the bud pinches off behind, resealing the cell - as a result, the host cell is not lysed.
How is syncytia formation an advantage for viruses?
The viruses can spread without having to diffuse through an extracellular environment - this facilitates rapid viral dissemination and makes it difficult to attack them with antibodies.
Inclusion bodies are collections of viral components, such as capsids and nucleic acids, inside a host cell.
These structures can be seen under the microscope. Rabies virus produces these structures specifically called negribodies in infected nerve cells.
How do physicians diagnose most viral illnesses? Signs/Symptoms Why?
Too costly and takes too much time to culture (by the time a definite diagnosis comes back the patient has usually recovered).
Where are glycoprotein spikes found? Glycoprotein spikes are found in the viral envelope.
Viral glycoprotein "spikes" are coded for on the viral nucleic acid and are inserted into the host cell's plasma membrane prior to budding.
What determines host range/viral specificity?
Viral specificity is determined by whether or not a virus can attach to a cell. Attachment depends on the presence specific receptor sites on the surface of host cell and on specific attachment structures on the viral capsid or envelope. Examples of receptor sites are proteins, LPS's, glycolipids, or glycoproteins.
What is a complete viral particle (nucleic acid + capsid + envelope if present) called?
Virion
Why are they important? Glycoprotein spikes will only bind to certain receptors on the host cell surface and are essential for host specificity and viral infectivity; they can be thought of as "docking" proteins.
Why are they important? Glycoprotein spikes will only bind to certain receptors on the host cell surface and are essential for host specificity and viral infectivity; they can be thought of as "docking" proteins.
Is the viral classification system natural or artificial? Artificial
artificial
Give an example of a Retrovirus. HIV (human immunodeficiency virus) -
causes AIDS
Viruses that kill the host cell by lysis are called (quick death)
cytocidal.
Viruses that damage the host cell by budding are called . (slow death)
cytopathic
Describe the different variations on the viral nucleic acid.
double stranded or single stranded RNA or DNA, single or multiple segments, linear or circular
Only enveloped (naked or enveloped) viruses can form syncytia.
enveloped
Viral family names end in -
iridae.
Why aren't viruses considered living?
lack carbs and lipids - these molecules are present in the viral envelope, but envelopes really belong to the host cells) they can only replicate with the help of a host cell; the host cell actually replicates the viral nucleic acid and makes the capsid proteins
Viruses that lack envelopes are called
naked.
Describe the 3 basic shapes of capsids.
polyhedral (capsomeres organized into equilateral triangles, which are assembled into dome-shaped structure) helical (capsomeres fit together as a helix that forms a rod-shaped structure) complex (polyhedral head + helical tail)
Why are viruses considered living?
they evolve by natural selection they direct their own reproduction (they have the plan for replication in their nucleic acid) and they spontaneously assemble
Why aren't viruses considered living? they have no cellular structure: no cytoplasm or plasma membrane they have DNA or RNA, unlike prokaryotic and eukaryotic cells, which have both.
they lack a metabolism of their own; for example, they cannot produce their own energy (ATP) from glucose they do not contain all 4 groups of organic molecules (they
Enveloped viruses are released from the host cell by budding.
true
Naked viruses are released from the host cell by lysis.
true
One of the most famous continuous cell line is the HeLa cell line (after Henrietta Lacks).
true
Viral DNA that has become integrated into the host bacterial cell's chromosome is called a prophage
true
Bacteriophages are
viruses that infect bacteria.
Viral DNA that has become integrated in an animal cell's chromosome is called a provirus .
yes