BIOL 191 Introductory MicrobiologyChap. 13 Viruses, Viroids, and Prions Study Guide

KEY

I. General Characteristics of Viruses

a. Introi. Table 13.1 Comparing Viruses and Bacteria

Are viruses… Are typical bacteria…Intracellular parasites? Yes, NoDo they have a plasma membrane? No, YesDo they contain ribosomes? No, YesDo they reproduce by binary fission? No, YesDo they possess both RNA and DNA in the same structure? No, YesAre they sensitive to antibiotics? No, YesAre they sensitive to interferons? Yes, No

ii. *Obligatory*intracellular*parasites*: What does this mean?In order to reproduce, they have to be inside of a living cell

iii. Define ‘virus’ Entities that contain a single type of nucleic acid in the virion, have a protein coat (capsid), multiply inside living cells by using the machinery of the cell, and cause the cell to synthesize structures that will allow the virus to infect another cell

b. Host Range. What does the host *range* depend on? What is a bacteriophage? The host range includes the species that a particular virus can infect. It is usually very specific. It is determined by attachment capabilities and the availability of cellular components that are required for viral multiplication. A bacteriophage (phage for short) is a virus that infects only bacteria.

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c. Viral Sizes The typical viral size is 20-1000 nm (nanometer 10-9 m) in length

II. Viral Structure - Define ‘Virion’ The complete viral particle at the moment of attachment (or the moment it leaves the cell). It is the only structure capable of transmission from one host to another.

Will a virus survive outside of a host cell and be able to transmit to another host if it is not in this form? No

a. Nucleic Acid –

i. GeneticsChap. 8 Fig. 8.2 The Flow of Genetic Information Know the basics of DNA expression and replication. These processes are necessary for viral multiplication.

ii. What types of nucleic acids may viruses have? Depending on the viral species, a virus may have- in the virion- one of the following: ssRNA, dsRNA, ssDNA, dsDNA

b. Capsid – Outer protein shell (coat)

c. Envelope – A structure that surrounds viruses that bud out of the host cell. It consists of a portion of the host cell membrane that ‘sticks’ to the virus as it pushes through the host cell membrane to leave the cell

d. Spikes - Structures that project out of the viral envelopeExamples: influenza virus (H and N stand for subtypes of these spikes). See discussion on p. 374-375.

1. H (Hemagglutinin) proteins

Hemagglutinin is one of two virally-coded envelope protein spikes of the influenza virus. Hemagglutinin is responsible for host cell binding and subsequent fusion of viral envelope and host membrane.

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ss = single stranded

ds = double stranded

2. N (Neuraminidase) proteinsProjections from surfaces of influenza viruses containing neuraminidase are involved in the release of viruses from infected cells. Mosby's Medical Dictionary, 8th edition. © 2009, Elsevier.

e. General Morphology- Be able to identify and label the morphological structures on the last page of the Study Guide

1. Helical (Capsomeres arranged in a helix, but they resemble a rod)

2. Polyhedral (Many sided)

3. Enveloped (Contain an envelope)

4. Complex viruses: A virus with additional structures and/or a combination of the basic morphological shapes

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Examples:

Herpesvirus- Polyhedral enveloped

Influenza-Helical enveloped

III. Taxonomy of Viruses

a. What is ‘taxonomy’?The study of categorizing (grouping) organisms

b. How does the International Committee on Taxonomy of Viruses group (classify) viruses? Nucleic acids and Structure

c. What is a viral ‘species’? A group of viruses sharing the same genetic material and host range

IV. Isolation, Cultivation and Identification of Viruses

a. How are bacterial and animal viruses grown in the lab?

-Bacteriophages: Are grown in bacterial cells (for example in a petri plate).

Plaques are clear areas where a phage has infected and killed the bacteria. Viruses that have been released from these now dead cells are most likely now in nearby bacteria cells

Animal viruses: Live animals (some, such as mice, may have been altered to produce human cells in which human viruses can grow), chick embryos, animal cell cultures (similar to bacteria in a petri plate and plaques)

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Be able to identify the type of virus grown in a bacterial culture such as the one in the illustration to the left. The ‘dots’ are plaques. Fig. 13.6

“T” stands for ‘type’. T-even (T-2, T-4, etc.) bacteriophages are dsDNA bacteriophages that have the complex morphology shown in Fig. 13.11.

b. What are some ways viruses are identified?

i. Characteristics of spikes (p. 371)ii. Reaction to antibodies

iii. Cytopathic effects (characteristics of a cell after infection)

iv. Characterization of nucleic acids

Viral Multiplication: First bacteriophage, then animal virusesIn Chap. 8 Microbial Genetics- be sure you know what DNA replication, protein synthesis (transcription/translation), mRNA , tRNA , and rRNA refer to.

What genes do viruses have? Viruses are very small and cannot contain all of the genes

needed for viral multiplication. The host cell must have the other necessary genes and the molecules and structures coded for by those genes that are required for that specific virus to infect that specific cell.

The genes viruses do contain include those that code for capsid proteins and other surface molecules (such as those for spikes) and possibly a few of the enzymes required to continue the viral life cycle. The virus generally does not contain genes for proteins the host cell already has.

What enzymes do virions contain? Some do not contain any enzymes in their virions. Other may

contain one or a few which function to aid the virus in entering the cell or replicating its genetic material once inside.

V. Multiplication of Bacteriophages: Lytic cycle or Lysogenic cycles

a. T-Even Bacteriophage Lytic Cycle

Know general info about the T-even bacteriophages- Large, with a complex morphology, infect E. coli

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Fig. 13.11 The lytic cycle of a T-even bacteriphage The following illustration is not on the exam, but students should know the order of the steps, what they mean, and the basic concepts of the lytic cycle- The phage infects the cell, immediately takes over the cell and forces the cell to make more phage- the cycle ends with the cells lysing (thus, dying), and the new phage virions are released to infect another nearby bacterial cell

- Attachment

-Penetration

-Biosynthesis

-Maturation

-Release

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b. Bacteriphage Lambda Lysogenic Cycle These viruses may go through a lytic cycle, but also may enter a latent (inactive) stage where the viral dsDNA is incorporated into the host cell DNA. If the bacterial cell is not harmed by this, the bacterial cell will function normally and divide, thus replicating not only its own DNA but also that of the PROPHAGE (the viral DNA while it is incorporated in the bacterial DNA). Both bacterial daughter cells will therefore be infected and contain prophage in their DNA.

BE ABLE TO LABEL THE PROPHAGE AND RECOGNIZE WHICH SIDE OF THE ILLUSTRATION IS THE LYTIC OR LYSOGENIC STAGE

Fig. 13. 12 The lysogenic cycle of bacteriophage lambda in E. coli

-Attachment-Phage DNA circularization-If the phage enters the lysogenic cycle: See above for answers

Define prophage. What happens when the bacterium reproduces? Can lysogenic viruses be lytic?

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What are important possible results of lysogeny?-The bacterial cell cannot be infected by another of the same type of phage (however, other types of phage may infect this cell)-The bacterial cell may exhibit new genetic properties from the DNA brought into the bacterial cell by the phage (see discussion of phage conversion causing diphtheria, streptococcal toxic shock, and botulism on p. 384)-Specialized transduction, in which only DNA on either side of the prophage DNA can be transferred. Fig. 13.13.--This is different from generalized transduction, in which any bacterial DNA can be transferred from one bacterial cell to another. Chap. 8 Fig. 8.29.

VI. Multiplication of Animal Viruses

a. How do animal viruses differ from phages? Table 13.3 Know this example: Rather than injecting the genetic material into the bacterial cell (leaving the phage capsid behind), the capsid of an animal virus enters the host cell, and, therefore, the virus must uncoat (digest the viral capsid) to release the viral genetic material into the cell.

b. Why might some people be resistant to a specific virus but not others? Receptor sites on the surfaces of human cells are inherited and different from one person to another. Therefore, a virus may be able to attach to one person’s receptors but not another’s.

c. How is ‘attachment’ related to drug development against viruses? Drugs may be developed that block the receptor of the animal or the viruses’ attachment sites.

d. Animal DNA virus multiplication Fig. 13.15 Fig. Replication of a DNA Animal Virus

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i. Attachmentii. Entry

iii. Uncoatingiv. (Bio)synthesis of early viral proteins (needed for viral

DNA replication), synthesis of viral DNA, and then synthesis of late proteins (for example, the capsid)

v. Maturationvi. Release – through budding or rupture

e. Animal RNA viral multiplicationFig. 13.17 Pathways of multiplication used by various RNA-containing viruses

i. Attachmentii. Entry

iii. Uncoatingiv. Biosynthesis of viral RNA in

1. ssRNA + (sense) stranded viruses (+ strand is equivalent to mRNA)…Know what mRNA is.

2. ssRNA – (antisense) stranded viruses (– strand is complementary to the + strand-it does not contain the correct genetic code)

3. dsRNA viruses4. Retroviruses (see discussion on next page)

v. Synthesis of viral proteinsvi. Maturation

vii. Release- through budding or rupture

Know what is needed in order to replicate any strand of nucleic acid (DNA or RNA)- The complementary strand is required, along with enzymes capable of creating the new strand from the complementary strand

Understand what is required to synthesize proteins and the name of the processes: transcription, translation, ribosomes, free RNA nucleotides

RETROVIRUSES: includes HIV

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Know what a complementary strand means and why it is necessary for synthesis of any strand of DNA or RNA.

Define retrovirus: ss+RNA viruses that also contain the enzyme reverse transcriptase in the virion

Be able to label Fig. 13.19 below and describe what is happening at (and in-between some) the cells. See arrows for important parts.

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Be able to recognize what is happening in this figure.

VII. Viruses and Cancer

A. Define a. Oncogenes: Cancer causing genesb. Oncogenic viruses (oncoviruses): Viruses that induce

tumors by a variety of methods – They may bring in cancer causing oncogenes with them or mutate or otherwise change ‘silent’ oncogenes previously in the individual before infection

c. Transformed cells have become cancerous

B. What % of cancers is known to be virus-induced? ~10%

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C. What is the OUTSTANDING FEATURE of all oncogenic viruses?The virus’ DNA must integrate into the host cell DNA

D. DNA Oncogenic Viruses: Examplesa. HPV (human papillomavirus – cervical and anal cancer)b. EB virus causes Burkitt’s lymphoma (this disease also

showed researchers that a virus can be transferred through tissue transplants)

c. HBV (hepatitis B - liver cancer)

E. RNA Oncogenic Viruses

-Only RNA viruses that are retroviruses can cause cancer. Why? (See the outstanding feature of oncoviruses above)

Only retroviruses can create dsDNA from their RNA that then can integrate into the host cell chromosome, a required characteristic of oncoviruses.

VIII. Latent and Persistent Viral Infections: Define and know examples from text discussion

Fig. 13.21 Latent and persistent viral infectionsLatent viral infections can remain in the host for an indefinite time without producing new viruses or causing symptoms. Some type of STRESS may ‘trigger’ the virus into becoming ‘active’ and forcing the cell to make more viruses. This is analogous to lysogeny in bacteriophage.An example is shingles (chickenpox [varicella herpes] in nerve cells).

A persistent viral infection produces viruses, but at low levels for long periods of time. These are usually fatal.Examples: measles that years later develops into a form of encephalitis, HIV, HBV liver cancer, HPV cervical cancer

IX. Prions See Fig. 13.22 How a protein can be infectious

A. Know examples and how they are different from viruses

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Prions are pure protein (no nucleic acid) that are infectious. They are able to ‘replicate’ when the abnormal prion form of this normal mammal brain protein transforms the normal brain protein into the abnormal prion form. They cause spongiform encephalitis (large vacuoles/plaques develop and destroy the brain).

B. Nervous System Diseases caused by Prions Chap. 22 p. 629-632Mad cow disease, scrapie (in sheep), kuru and Creutzfeldt-Jakob (CJD) in humans

Since this brain protein is encoded in the DNA of all mammals, the disease may also be caused by genetic mutation or inheritance.

X. Plant Viruses and Viroids

A. Why are plants somewhat protected against many diseases?

Plants have a hard cell wall made of cellulose so usually a viral infection must involve wounds to the cell wall or be brought in through other parasites (such as insects).

B. Define viroid

Short pieces of naked RNA (no DNA or proteins) that (so far) are only know to infect plants

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Enveloped

Polyhedral

Complex

Helical