Understanding Viruses: From Structure and Infection to Immunity
Viruses and their Life Cycles
What are Viruses?
Viruses that infect bacteria are referred to as bacteriophages.
A virion is a complete, extracellular virus particle.
The protein projections on the surface of a virus that are involved in attachment to the host cell are called spikes.
Stages of a Phage Infection
- Attachment
- Penetration
- Transcription
- Replication of nucleic acid and protein
- Assembly
- Release
The bacterial viruses that are released by a process termed extrusion are called filamentous phages.
Animal Virus Infection
The receptors to which animal virus attachment proteins usually bind are glycoproteins.
Resistance of some animals to certain viral diseases is based on lack of specific receptors on the host cell.
The enveloped viruses typically obtain their envelope from the host plasma membrane AND as they exit the host.
Viral Infections and their Consequences
Carriers may have a persistent infection AND may be a source of infection.
The best-known examples of viruses that cause latent infections are herpes AND chickenpox.
The site at which a virus has infected and subsequently lysed the infected cell, releasing its progeny to infect and lyse surrounding cells, thereby forming a “clear zone”, is referred to as a plaque.
Prions and Viroids
Prions affect the nervous system.
Viroids characteristically are composed of ssRNA.
True or False
Bacteriophage, unlike animal viruses, often have special viral-specific enzymes carried in the capsid which enter the host cell at the same time as the nucleic acid. False
Naked and enveloped viruses both may enter the host via endocytosis. True
Short Answer
Question 1
Would you expect the number of virions to be the same if you measured them by the plaque assay or by counting using the electron microscope? Why?
Answer
No-the plaque assay only measures viable virus particles, while the electron microscope cannot distinguish between defective and viable virus.
Question 2
An antibiotic is added to a culture of E. coli, resulting in death of the cells. Bacteriophage are then added. Would the phage replicate in the E. coli cells? Why or why not?
Answer
No, because the virus would depend too much on having the active machinery of a living cell for its replication. Without the ability of the cell to try to replace what is lost/damaged/used as the virus goes through its life cycle, the virus couldn’t reproduce effectively.
Question 3
Is an antigenic shift alone likely to cause pandemics?
Answer
No, you need to have a change in both antigenic shift and drift
Question 4
Clustered regularly interspaced short palindromic repeats (CRISPR)
Answer
They integrate fragments from phage DNA and their own chromosomes and target for destruction of any DNA that contains the same fragments in the future
Comparing Viruses and Immunity
Bacteriophages vs. Animal Viruses
Bacteriophages and animal viruses differ because bacteriophages leave the capsid outside the cell, while animal virus entry involves the entry of the whole nucleocapsid.
Innate Immunity
Skin and mucous membranes are the first line of innate immunity, act as physical barriers to infection AND contain antimicrobial secretions.
The four cardinal signs of inflammation are: redness, heat, swelling, pain.
Pyrogens are fever-inducing substances.
Adaptive Immunity
Antibodies are made by Plasma cells or memory B cells.
Which of the following do not induce a strong immune response? lipids AND nucleic acids or (simple sugars)
Which is the first antibody class made during the primary response to an antigen? IgM
The first kind of leukocyte lured to the site of inflammation is the neutrophil.
The key molecule upon which all complement pathways converge is C3.
The Immune System
Normal microflora are the organisms that typically reside on your body AND protect against infection by pathogens.
Secondary lymphoid organs are strategically located in the body AND facilitate interactions between cells.
Ag-Ab binding may result in All of the choices are correct.
- neutralization.
- immobilization.
- agglutination.
- opsonization.
The cellular organelle responsible for the digestion of ingested infectious agents is the phagolysosome.
Interferons, complement, lysozyme, and lactoferrin are all examples of nonspecific antimicrobial factors.
CD8 cells are often T cytotoxic cells.
How long after initiation of a primary response do significant amounts of antibody appear in the blood? 10-14 days
Which of the following statements about interferon is incorrect? It only works on a few specific types of virus. (it protects the cell against viral infection)
Short Answer
Question 1
Smoking impairs the ciliated cells of the middle portion of the respiratory tract. Many analgesic drugs (painkillers) impair peristalsis (the churning motion of the digestive tract). The result of either of these activities leads to an increased risk of infection in their respective areas. Why?
Answer
The churning of the areas helps propel the pathogen out of these areas. Allows them to stay and cause an infection
Question 2
How is a t-cell receptor different from a b-cell receptor
Answer
T-cell receptors need to have an antigen broken down inside the cell and presented to them in the MHC1 molecule, b-cells don’t they directly produce antibodies
Question 3
Toll-like receptors
Answer
Each recognizes a specific “danger” molecule AND are embedded in the cell membrane
Question 4
A term synonymous with antibody is
Answer
immunoglobulin
Question 5
What is a virulent phage? (lysis phage)
Answer
Virulent phage A bacteriophage that causes the destruction of the host bacterium by lysis.
Question 6
What is a temperate Phage? (lysogenic phage)
Answer
Temperate refers to the ability of some bacteriophages (notably coliphage λ) to display a lysogenic life cycle. Many (but not all) temperate phages can integrate their genomes into their host bacterium’s chromosome, together becoming a lysogen as the phage genome becomes a prophage.
Question 7
What is the difference between specialized and generalized transduction?
Answer
generalized- any part of the bacterial gene is being transduced by the virus into another bacteria
specialized- It is only specific genes that are flanking either side of the prophage; are the one that actually gets transduced
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Question 8
Difference between acute and persistent infection
Answer
Acute- symptoms are rapid; 5-7 days; resolves all by itself
Persistent- remains for as long as the person is alive; the host is a carrier; upon infection, they don’t lys out the cell
Question 9
What are the different steps involved in phagocytosis
Answer
- Chemotaxis
- Recognition and attachment
- Engulfment
- Phagosome Maturation and Phagolysosome Formation
- Destruction and Digestion
- Exocytosis
Question 10
What is the difference between humoral and cell-mediated immunity?
Answer
Humoral- done by B-cells; b_cells would recognize the antigen then be selective; then will do cloning; and then convert into plasma cells; plasma cells will secrete antibodies; the antibodies will bind to the antigen and destroy it
Cell- when the t-helper cell recognizes the antigens; the cytotoxic t-cell will get the signal; go and bind to the infected cell; will secrete perforin; cause pores; then death
Question 11
What is the difference between t dependent and t independent antigen?
Answer
dependent- depend on helper cells
independent- (lps flagella will go bind directly to the cell) don’t need to be recognized by the t helper cell