Micro Ch 13
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What is the structure of a virus?
1. DNA or RNA - not both - both can be single or double stranded.
2. Capsid = protective protein coat - made of capsomeres
3. Envelope - not all viruses have it
4. Spikes - some have spikes for attachment to host
Are viruses alive?
Are viruses specific?
What is Host Range?
Viruses are obligatory intercellular parasite - they cannot reproduce outside of a host cell and thus are not classified as alive.
Virsus are very specific - most often can only infect 1type of cell in 1 species -not always the case (bird flu/ swine flu)
Host Range - spectrum of host cells the virus can infect - this is determined by specific host attachement sites
What size are viruses?
What are the shapes of viruses?
Most are much smaller than bacteria. They range in size from 30 nm (nanometers) to 1 um (micrometer) - (most bacteria are 1 um in size
- Viral shapes are determined by the capsid
- 1. helical - hollow protein capsule hold DNA or RNA
- 2. polyhedral - icosahedral = 20 sided equilateral triangle faces
- 3. enveloped - only in some - some have spikes on envelope that can mutate (influenza) - roughly spherical
- 4. complex - bacteriophage: head (capsid), body (sheath), legs (fibers)
What is a bacteriophage?
What is a Virion?
- A complex virus that affects only bacteria - also called phages
- -on culture plate - they form plaques on a lawn of bacteria
Virion = fully assembled virus
List 3 ways viruses are grown for study.
Virsues must be grown in living cells:
- 1. Living animals (living cells of host)
- 2. Embryonated eggs (living cells of host)
- 3. Cell cultures (1 living cell type)
can be used to diagnose viral infections in man - Known viral antigen
are mixed with patients serum to detect antibodies
against the virus.
- Detection is not always easy becuase most viruses are inside the cell (not in the bloodstream)
- Western blot test is used to confirm HIV infection - looks at serum antibody binding to prepared HIV proteins.
5 stages of bacteriophage replication (Lytic cycle)
What is the lytic cycle?
What is the lysogenic cycle?
- 1. Attachment - phage attaches by tail fibers to host cell
- 2. Penetration - phage tail releases lysozyme to break open cell wall → sheath contracts → tail core (pin) driven into cell wall → DNA injected into cell (cuspid stays outside)
3. Biosynthesis -
of phage DNA and proteins - using host cell machinery
- assembly of phage particles into virions
(complete viral particle)
- phage lysosome lyses cell wall - virus released and cell dies
Lytic cycle - phage replication results in lysis and death of cell
Lysogenic cycle = Prophage (instead of Biosynthesis) - DNA is incorporated in host DNA and then is replicated with each cell division (host cell lives on)
a bacteriophage can do either or both at the same time
Viruses use host cell's machinery to replicate itself
Virus make thousands of itself within 1 cell when it replicates
Host cell either dies (lysis) or lives indefinately to reproduce the virus (HIV integrates itself into host cell genome)
What are the differences in animal vrial replication and bacteriophage?
- Penetration - by endocytosis - entire viron enters cell
- Uncoating - of cuspid proteins (by viral or host enzymes)
- 1. Attachement
- 2. Penetration - endocytosis
- 3. Uncoating
- 4. Biosynthesis
- 5. Maturation
- 6. Release - by rupture (lysis) or budding (enveloped virus)
One step growth
What is the eclipse period?
One step growth - virions present in culture until attachment - then no new virons are found in the culture until after biosynthesis and maturation have taken place. Most infected cells die as a result of infection- consequently, new virions will not be produced.
- The eclipse period is the time between infection of the cell and the appearance of the mature virus within the cell.
What is a provirus vs. a prophage?
Provirus - a piece of viral DNA that is integrated into a host cell chromosome - the provirus never comes out of the chromosome- thus it is protected from the host's immune system and antiviral drugs
Prophage - phage DNA inserts itself in the circular bacterial DNA - every time the cell replicates, it replicates the phophage DNA also. Some future even will cause the phage DNA to pop out and initiate the lytic cycle
Why is HIV a "retro" virus - describe its replication.
What is reverse transcriptase?
Does HIV form a "provirus"?
- HIV = Human Immunodeficiency virus
- 1. HIV virus enters a human T-cell
- 2. uses reverse transcriptase (an enzyme) to copy its RNA into double straded DNA
- 3. DNA then inserts into T-cell DNA -PROVIRUS
- 4. Provirus - always stays inserted - replicates with T-cell division - can turn on at future point and reproduce itself in numbers
RETRo virus - name comes from REverse TRansscriptase
Retrovirus is derived from the name REverse TRanscriptase - the enzyme that
What is budding?
Budding is one method of release of an virus from the host cell (animal only) - The assempled capsid (containing nucleic acid) is pushed through the plasma membrane of the host cell - The plasma membrane adheres to the virus and form the envelope which pinches off the host cell
What is an oncogenic virus?
An oncogene - where is it located? what turns it on?
Oncogenic virus = cancer causing virus
Oncogenes - mutant host cell regulatory genes that can cuase cancer when turned on by a oncogenic virus
all oncogenic viruses integrate themselves into host DNA
a transformed cell is a host cell that is infected by an ongogenic virus and turns into a tumor cell
Latent viral infection
persistent viral infection
Latent - virus remians asymptematic for long periods of time - chickenpox/ shingles, herpes 1 (cold sores)
- DNA virus
- veneral warts - cervical cancer
- 100 different HPVs
- - replicate on body surface cells (skin/ mucosa)
- - vaccine
Hepatitis B virus
- DNA virus
- Infects the hepatocyte - cell lysis
- Acute Hepititis B viral - usually self-limited
- Chronic active hepititis → cirrhosis of the liver
- can cause hepatoma (cancer)
- Rx - interferon (chronic acive hep)
- Drug induced hepititis
Cause the common cold
Adenovirus, rhinovirus, coronavirus
- smallpox - DNA
- 30% mortality - to Rx exists -
- can vaccinate up to 3 days after exposure
- pheumonia, encephalitis, corneal ulceration (blindness)
Cowpox = original vaccine (milk maids)
- 1. Herpes simplex I - oral
- 2. Herpes simplex II - genital - both inhibit DNA polymerase
- 3. Varicella zoster - chickenpox - lives in sensory neurons
- 4. Kaposi's sarcoma - viral cancer (AIDS dd)
- 5. Epstein-Barr virus - mononucleosis, Burkitt's lymphoma
- infects motor neurons - flaccid paralysis
- Rubella - german measels
- -fetal abnormalities if mother infected during 1st trimester
- MMR vaccine
- rabies -
- death from respiratory failure or acute enchehilitis (travels to brain along peripheral nerves)
- ANY mammel can get it
- 100% mortality once symtomatic
- post exposure vaccination is effective
- RNA virus
- RSV - respiratory syncytial virus - infant bronchoilitis
- Parainfluenza - croup (laryngotracheobronchitis)
- - bronchoilitis
Mumps - infects parotid gland & testes - vaccine
measels - vaccine
- RNA enveloped virus - A, B, C
- antigenic shirt - major change
- antigenic drift - minor change
- spikes mutate often - new vaccine every year
- RNA retrovirus - HIV (Human Immunodeficiency Virus)
- uses its own reverse transcriptase (singel stranded RNA → double stranded DNA)
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