Question 1

What does it mean to be acellular?

Accepted Answer

Lacking cells.

Question 2

What is a capsid?

Accepted Answer

Protein coating of the viral core. Some viral capsids are simple polyhedral "spheres", whereas others are quite complex in structure.

Question 3

What is a capsomere?

Accepted Answer

Protein subunit that makes up the capsid.

Question 4

What is an envelope?

Accepted Answer

Lipid bilayer that envelopes some viruses.

Question 5

What is a group I virus?

Accepted Answer

Virus with a dsDNA genome. Their mRNA is produced by transcription directly from the DNA template in much the same way as with cellular DNA.

Question 6

What is a group II virus?

Accepted Answer

Virus with a ssDNA genome. They convert their single-stranded genomes into a dsDNA intermediate before transcription to mRNA can occur.

Question 7

What is a group III virus?

Accepted Answer

Virus with a dsRNA genome. The strands separate, and one of them is used as a template for the generation of mRNA using the RNA-dependent RNA polymerase encoded by the virus.

Question 8

What is a group IV virus?

Accepted Answer

Virus with a ssRNA genome with positive polarity. Intermediates of dsRNA, called replicative intermediates, are made in the process of copying the genomic RNA. Multiple, full-length RNA strands of negative polarity are formed from these intermediates, which may then serve as templates for the production of RNA with positive polarity, including both full-length genomic RNA and shorter viral mRNAs.

Question 9

What is a group V virus?

Accepted Answer

Virus with a ssRNA genome with negative polarity. dsRNA intermediates are used to make copies of the genome and produce mRNA. In this case, the negative-stranded genome can be converted directly to mRNA. Additionally, full-length positive RNA strands are made to serve as templates for the production of the negative-stranded genome.

Question 10

What is a group VI virus?

Accepted Answer

Virus with a ssRNA genome converted into dsDNA by reverse transcriptase. Group VI viruses have diploid (two copies) ssRNA genomes that must be converted to dsDNA, which is then transported to the nucleus of the host cell and inserted into the host genome. Then, mRNA can be produced by transcription of the viral DNA that was integrated into the host genome.

Question 11

What is a group VII virus?

Accepted Answer

Viruses with partial dsDNA genomes that make ssRNA intermediates that act as mRNA, but are also converted back into dsDNA genomes by reverse transcriptase, necessary for genome replication.

Question 12

What is a matrix protein?

Accepted Answer

Envelope protein that stabilizes the envelope and often plays a role in the assembly of progeny virions.

Question 13

What is negative polarity?

Accepted Answer

ssRNA viruses with genomes complimentary to their mRNA.

Question 14

What is positive polarity?

Accepted Answer

ssRNA viruses with genomes that contain the same base sequences and codons found in their mRNA. It means that the genomic RNA can serve directly as mRNA.

Question 15

What is a replicative intermediate?

Accepted Answer

dsRNA intermediate made in the process of copying genomic RNA.

Question 16

What is reverse transcriptase?

Accepted Answer

Enzyme found in Baltimore groups VI and VII that converts single-stranded RNA into double-stranded DNA.

Question 17

What is a viral receptor?

Accepted Answer

Glycoprotein used to attach a virus to host cells via molecules on the cell.

Question 18

What is a virion?

Accepted Answer

Individual virus particle outside a host cell.

Question 19

What is the virus core?

Accepted Answer

Contains the virus genome. It may be either DNA or RNA. Viral genomes tend to be small, containing only those genes that encode proteins that the virus cannot get from the host cell. The genetic material may be single- or double-stranded. It may also be linear or circular. While most viruses contain a single nucleic acid, others have genomes that have several, which are called segments.

Question 20

How were viruses first discovered?

Accepted Answer

Viruses were first discovered after the development of a porcelain filter, called the Chamberland-Pasteur filter, which could remove all bacteria visible in the microscope from any liquid sample. In 1886, Adolph Meyer demonstrated that a disease of tobacco plants, tobacco mosaic disease, could be transferred from a diseased plant to a healthy one via liquid plant extracts. In 1892, Dmitri Ivanowski showed that this disease could be transmitted in this way even after the Chamberland-Pasteur filter had removed all viable bacteria from the extract. Still, it was many years before it was proven that these "filterable" infectious agents were not simply very small bacteria but were a new type of very small, disease-causing particle.

Question 21

How large are viruses?

Accepted Answer

Virions are very small, about 20-250 nanometers in diameter, and are thus, unlike bacteria (which are about 100 times larger), not visible with a light microscope, with the exception of some large virions of the poxvirus family. It was not until the development of the electron microscope in the late 1930s that scientists got their first good view of the structure of the tobacco mosaic virus (TMV) and other viruses. The surface structure of virions can be observed by both scanning and transmission electron microscopy, whereas the internal structures of the virus can only be observed in images from a transmission electron microscope.

Question 22

How might viruses have evolved?

Accepted Answer

Little is known about how viruses originated, because viruses do not fossilize. Researchers must conjecture by investigating how today's viruses evolve and by using biochemical and genetic information to create speculative virus histories. While most findings agree that viruses don't have a single common ancestor, scholars have yet to find a single hypothesis about virus origins that is fully accepted in the field. One such hypothesis, called devolution or the regressive hypothesis, proposes to explain the origin of viruses by suggesting that viruses evolved from free-living cells. However, many components of how this process might have occurred are a mystery. A second hypothesis (called escapist or the progressive hypothesis) accounts for viruses having either an RNA or a DNA genome and suggests that viruses originated from RNA and DNA molecules that escaped from a host cell. A third hypothesis posits a system of self-replication similar to that of other self-replicating molecules, likely evolving alongside the cells they rely on as hosts; studies of some plant pathogens support this hypothesis. The emerging field called virus molecular systematics attempts to explain the origin of viruses through comparisons of sequenced genetic material.

Question 23

What are some characteristics of viral morphology?

Accepted Answer

They are acellular. A virion consists of a nucleic acid core, an outer protein coating or capsid, and sometimes an outer envelope made of protein and phospholipid membranes derived from the host cell. Viruses may also contain additional proteins, such as enzymes. Viral morphology is quite diverse. Virion complexity does not correlate with complexity of the host, with some of the most complex virion structures observed in bacteriophages.

Question 24

What are filamentous viruses?

Accepted Answer

Viruses that are long and cylindrical.

Question 25

What are isometric or icosahedral viruses?

Accepted Answer

Viruses that have shapes that are roughly spherical.

Question 26

What are enveloped viruses?

Accepted Answer

Viruses that have membranes surrounding capsids. Animal viruses, such as HIV, are frequently enveloped. Glycoproteins embedded in the viral envelope are used to attach to host cells. Other envelope proteins are the matrix proteins that stabilize the envelope and often play a role in the assembly of progeny virions. Because of the fragility of the envelope, non-enveloped viruses are more resistant to changes in temperature, pH, and some disinfectants than enveloped viruses.

Question 27

What are head and tail viruses?

Accepted Answer

Viruses that infect bacteria and have a head that is similar to icosahedral viruses and a tail shape like filamentous viruses.

Question 28

What is viral reception?

Accepted Answer

Many viruses use some sort of glycoprotein to attach to their host cells via molecules on the cell called viral receptors. For these viruses, attachment is a requirement for later penetration of the cell membrane, so they can complete their replication inside the cell. The receptors that viruses use are molecules that are normally found on cell surfaces and have their own physiological functions. Viruses have simply evolved to make use of these molecules for their own replication.

Question 29

How does HIV infect a cell?

Accepted Answer

HIV uses the CD4 molecule on T lymphocytes as one of its receptors. CD4 is a cell adhesion molecule which functions to keep different types of immune cells in close proximity to each other during the generation of a T lymphocyte immune response. After attaching to the CD4 receptor, HIV fuses with the cell membrane, releasing viral contents into the cell, such as HIV RNA, reverse transcriptase, integrase, and other viral proteins. A preinitiation complex is formed between viral ssRNA and reverse transcriptase, and produces viral DNA. The viral DNA is transported across the nucleus and integrated into the host DNA by integrase. New viral RNA produced by transcription of viral DNA is used as genomic RNA and to make viral proteins. New viral RNA and proteins move to the cell surface and a new, immature HIV forms. Finally, the virus matures by protease, releasing individual HIV proteins.

Question 30

How does KSHV (Kaposi's sarcoma-associated herpesvirus) infect a cell?

Accepted Answer

The KSHV virus bind the xCT receptor on the surface of human cells. xCT receptors protect cells against stress. Stressed cells express more xCT receptors than non-stressed cells. The KSHV virion causes cells to become stressed, thereby increasing expression of the receptor to which it binds.

Question 31

What is the T4 bacteriophage's morphology?

Accepted Answer

The T4 bacteriophage is among the most complex virions known, which infects E. coli , has a tail structure that the viruses uses to attach to host cells and a head structure that houses its DNA.

Question 32

What is an adenovirus?

Accepted Answer

A non-enveloped animal virus that causes respiratory illnesses in humans. It uses glycoprotein spikes protruding from its capsomeres to attach to host cells. It has a double-stranded DNA genome enclosed in an icosahedral capsid that is 90-100 nm across. It is transmitted orally and causes a variety of illnesses in vertebrates, including human eye and respiratory infections.

Question 33

What is HIV?

Accepted Answer

Human immunodeficiency viruses. They are enveloped, icosahedral retroviruses that infect humans, and are the causative agent in AIDS. They have ssRNA genomes, and are in group VI of the Baltimore classification scheme.

Question 34

In general, how do DNA viruses replicate?

Accepted Answer

In DNA viruses, the viral DNA directs the host cell's replication proteins to synthesize new copies of the viral genome and to transcribe and translate that genome into viral proteins.

Question 35

How do RNA viruses replicate and why are mutations more common in RNA viruses than in DNA viruses?

Accepted Answer

To replicate their genomes in the host cell, RNA viruses encode enzymes that can replicate RNA from the RNA genome (RNA-dependent RNA polymerase, RdRp), which cannot be done by the host cell. These RNA polymerase enzymes are more likely to make copying errors than DNA polymerases, and therefore often make mistakes during transcription. For this reason, mutations in RNA viruses occur more frequently than in DNA viruses. This causes them to change and adapt more rapidly to their host.

Question 36

What is the Baltimore classification scheme?

Accepted Answer

The most commonly used classification method today. It groups viruses according to how the mRNA is produced during the replicative cycle of the virus.

Question 37

What is a non-segmented genome?

Accepted Answer

Viral genome that consists of a single segment of genetic material.

Question 38

What is a segmented genome?

Accepted Answer

Viral genome that is divided into multiple segments.

Question 39

What is the rabies virus?

Accepted Answer

A virus with a single-stranded RNA (ssRNA) core and an enveloped helical capsid. Rabies transmission occurs when saliva from an infected mammal enters a wound. The virus travels through neurons in the peripheral nervous system to the central nervous system where it impairs brain function, and then travels to other tissues. The virus can infect any mammal, and most die within weeks of infection.

Question 40

What is the variola virus?

Accepted Answer

A virus with a double-stranded DNA (dsDNA) core and a complex capsid. It is the causative agent of smallpox.

Question 41

What is smallpox?

Accepted Answer

A human virus transmitted by inhalation of the variola virus, localized in the skin, mouth, and throat, which causes a characteristic rash. Before its eradication in 1979, infection resulted in a 30-35% mortality rate.

Question 42

What are the classifications of capsids?

Accepted Answer

Naked icosahedral, enveloped icosahedral, enveloped helical, naked helical, and complex.

Question 43

What is a complex capsid?

Accepted Answer

Complex with many proteins; some have combinations of icosahedral and helical capsid structures.

Question 44

Who developed the Baltimore classification scheme?

Accepted Answer

Nobel Prize-winning biologist David Baltimore in the early 1970s.

Question 45

What is an acute disease?

Accepted Answer

Disease where the symptoms rise and fall within a short period of time.

Question 46

What is an asymptomatic disease?

Accepted Answer

Disease where there are no symptoms and the individual is unaware of being infected unless lab tests are performed.

Question 47

What is AZT?

Accepted Answer

Anti-HIV drug that inhibits the viral enzyme reverse transcriptase.

Question 48

What is a bacteriophage?

Accepted Answer

Virus that infects bacteria.

Question 49

What is budding?

Accepted Answer

Method of exit from the cell used in certain animal viruses, where virions leave the cell individually by capturing a piece of the host plasma membrane. During the budding process, the cell does not undergo lysis and is not immediately killed, however, the damage to the cells that the virus infects may make it impossible for the cells to function normally, even though they remain alive for a period of time. Budding is the process whereby many enveloped viruses acquire their lipid bilayer.

Question 50

What is cell necrosis?

Accepted Answer

Cell death.

Question 51

What is a chronic infection?

Accepted Answer

Describes when the virus persists in the body for a long period of time.

Question 52

What does it mean to be cytopathic?

Accepted Answer

Causing cell damage.

Question 53

What is fusion?

Accepted Answer

Method of entry by some enveloped viruses, where the viral envelope fuses with the plasma membrane of the host cell.

Question 54

What is a gall?

Accepted Answer

Appearance of a plant tumor.

Question 55

What is horizontal transmission?

Accepted Answer

Transmission of disease between unrelated individuals.

Question 56

What is hyperplasia?

Accepted Answer

Abnormally high cell growth and division.

Question 57

What is hypoplasia?

Accepted Answer

Abnormally low cell growth and division.

Question 58

What is an intermittent symptom?

Accepted Answer

Symptom that occurs periodically.

Question 59

What is latency?

Accepted Answer

Virus that remains in the body for a long period of time but only causes intermittent symptoms.

Question 60

What is lysis?

Accepted Answer

Bursting of a cell.

Question 61

What is the lytic cycle?

Accepted Answer

Type of virus replication in which virions are released through lysis, or bursting, of the cell. In the lytic cycle, the phage first infects a cell. The phage DNA then circularizes, remaining separate from the host DNA. The phage DNA replicates and phage proteins are made. New phage particles are assembled. The cell eventually lyses, releasing the phage.

21: Viruses Flashcards

Viral Evolution, Morphology, and Classification, Virus Infections and Hosts, Prevention and Treatment of Viral Infections, Other Acellular Entities - Prions and Viroids

Brainscape's Knowledge GenomeTM

21: Viruses Flashcards

Viral Evolution, Morphology, and Classification, Virus Infections and Hosts, Prevention and Treatment of Viral Infections, Other Acellular Entities - Prions and Viroids

Brainscape's Knowledge Genome^TM