Chapter 13

Question 1

What did Dimitri Iwanowsky and Martinus Beijerinck do

Answer 1

Found a mosaic disease of tobacco plants that was caused by an unusual agent.

Question 2

What was unusual about the agent found by Iwanowsky and Beijerinck?

Answer 2

The agent was too small to be seen with a light microscope

It passed through filters that retained most known bacteria

It could be grown only in media that contained living cells.

Question 3

What did Beijerinck called these unusual agents

Answer 3

Filterable virus

Question 4

In simplest terms, what are viruses

Answer 4

Viruses can be viewed as genetic information either DNA or RNA contained within a protective protein coat.

Question 5

What are viruses incapable of

Answer 5

They are inert particles incapable of metabolism, replication or motility on their own.

Question 6

What happens when a viral genome enters a cell

Answer 6

When a viral genome enters a host cell, it can hijack that cell’s replication machinery, inducing the cell to produce more viral particles.

Question 7

Viruses can be grouped into two general types

Answer 7

1. Some infect eukaryotes
2. Other infect prokaryotes

Question 8

Bacteriophages

Answer 8

Viruses that infect bacteria

Question 9

What makes viruses difficult to study

Answer 9

They are obligate intracellular parasites

Question 10

What can viruses only be visualize under

Answer 10

An electron microscope

Question 11

What do viruses require

Answer 11

Viruses require live organisms as hosts.

Question 12

What is a reason bacteriophages are important

Answer 12

Because they serve as a vehicle for horizontal gene transfer in bacteria.

They kill bacteria thereby reducing bacteria populations in nature

Question 13

Phage therapy

Answer 13

The use of bacteriophages to treat bacterial infections is explored as being an alternative to antibiotics

Question 14

At a minimum, what do virions consist of

Answer 14

A virion consists of nucleic acid surrounded by a protein coat.

Viruses only contain a single type of nucleic acid either RNA or DNA, never both.

Question 15

What are the genomes of a virus like

Answer 15

The genome may be linear or circular and either single stranded or double stranded.

Question 16

Capsid

Answer 16

Protein coat of virus

Protects the viral nucleic acid from enzymes and toxic chemicals in the environment

Question 17

What is the capsid composed of

Answer 17

It is composed of precisely arranged identical protein subunits called capsomeres.

Question 18

Nucleocapsid

Answer 18

Viral nucleic acid and its protein coat.

Question 19

What specific protein components do phages have that allow the virion to attach to specific receptor sites on host cells

Answer 19

Phages have tail fibers for attachment

Question 20

What specific protein components do animal viruses have that allow the virion to attach to specific receptor sites on host cells

Answer 20

Many animal viruses have protein structures called spikes on their surface.

Question 21

Spike

Answer 21

Or attachment proteins

Structures on the outside of the virion that bind to host cell receptors.

Question 22

Envelope of virus

Answer 22

Some viruses have an outer lipid bilayer called an envelope

Question 23

What is another name for non-enveloped viruses

Answer 23

Naked viruses

Question 24

Non-enveloped viruses

Answer 24

Type of viruses that does not have a lipid envelope.

Question 25

Enveloped viruses

Answer 25

Viruses that have a lipid bilayer surrounding their nucleocapsid They also have a viral matrix protein that links the envelope to the nucleocapsid.

Question 26

Where do enveloped viruses obtain their outer layer from

Answer 26

The host cell

Question 27

Why are enveloped viruses more susceptible to soaps, detergents and disinfectants

Answer 27

Because these chemicals damage the envelope, thereby removing the spikes and making the viruses no infectious.

Question 28

Phages are typically what

Answer 28

Typically non enveloped

Question 29

What are the three shapes viruses can be

Answer 29

1. Icosahedral 2. Helical 3. Complex

Question 30

Icosahedral viruses

Answer 30

Appear spherical when viewed with an electron microscope, but there surface is actually 20 flat triangles arranged in a manner somewhat similar to that of a soccer ball.

Question 31

Helical viruses

Answer 31

Appear cylindrical when viewed wi5 an electron microscope; their capsomeres are arranged in a helix, somewhat similar to a spiral staircase. Some are short and rigid while others are long and filamentous

Question 32

Names of virus families are derived from a variety of sources, but they all end in suffix

Answer 32

Viridae

Question 33

Arboviruses

Answer 33

Arthropod borne RNA virus, carried by vectors such as mosquitos.

Question 34

The general strategies for phage replication result in two possible outcomes

Answer 34

1. Productive infection 2. Latent state

Question 35

Productive infection

Answer 35

Viral infection in which more viral particles are produced

Question 36

Latent state

Answer 36

The state of a phage when it’s DNA is integrated into the genome of the host. In which the viral genome remains silent within the cell but is replicated along with the host cell genome.

Question 37

Another name for lytic phages

Answer 37

Virulent phages

Question 38

Lytic phage

Answer 38

Bacteriophages that lyst their host

Question 39

How do lyric phages exit the host

Answer 39

Lytic phages exit the host at the end of the infection by lysing the cell, an action that kills the cell.

Question 40

Example of lytic phage

Answer 40

T4

Question 41

T4

Answer 41

A double stranded DNA phage that infects E.coli

Question 42

Steps of T4 replication cycle.

Answer 42

1. Attachment 2. Genome entry 3. Synthesis of phage proteins and genome 4. Assembly (maturation) 5. Release

Question 43

Attachment step

Answer 43

Phage collides with host cell by chance On contact, the phage attaches to a receptor on the host cell surface or an appendage like pilus.

Question 44

Genome entry

Answer 44

The phage injects it’s genome into the cell. T4 does this by degrading a small part of the bacterial cell wall, using an enzyme located in the tip of its tail. The tail then contracts so that the phage particle appears to squat on the cell surface. This action injects the phage dna through the hosts cell wall and cytoplasmic membrane, and into the cell.

Question 45

3. Synthesis of phage proteins and genome

Answer 45

Within minutes after the phage dna is injected into a host cell, that cell’s machinery transcribes and translates some of the phage genes. The genes are expressed in a specific sequence to control the course of infection. First the proteins needed to start phage replication are made; one of these is a nuclease that degrades the hosts cells DNA. The phage genome is then synthesized. Towards the end of the replication cycle, phage structural proteins are made, including those that make up the capsid and the tail.

Question 46

Assembly (maturation)

Answer 46

Assembles the various phage components to form new phage particles. After phage head is formed, dna is packed into it; the tail is then attached, followed by the addition of the tail spikes.

Question 47

Release

Answer 47

Late in infection, the phage encoded lysozyme is produced. This enzyme digests the host cell host cell wall from within, causing the cell to lyse thereby releasing phage. These particles then infect any susceptible cells in the environment.

Question 48

Burst size

Answer 48

Number of newly formed virus particles released when the infected host cell lyses

Question 49

Temperate phage

Answer 49

A bacteriophage that can either direct a productive infection that leads to host cell lysis or remain silent within the host as a prophage (phage dna) that replicates along with the host cell genome.

Question 50

Most common example of temperate phages

Answer 50

Lambda a double stranded dna phage that infects E.coli.

Question 51

What does lambda look like outside the cell

Answer 51

It has a linear chromosome, but once that genome

Question 52

What does lambda look like inside the cell

Answer 52

Once that genome is injected into a cell, the ends join together to form a circular dna molecule.

Question 53

What are the two options that can occur to temperate phages once they are injected into the cell

Answer 53

1. Lytic infection 2. Lysogenic infection

Question 54

Lytic infection

Answer 54

Viral infection of a host cell with a subsequent production of more viral particles and lysis of the cell.

Question 55

Lysogenic infection

Answer 55

A phage encoded enzyme called an integrase inserts the phage dna into the host cell chromosome at a specific site. The prophage replicates along with the host chromosome prior to cell division, so it will be passed on to that cells progeny.

Question 56

Prophage

Answer 56

Latent form of a temperate phage; the phage DNA has generally been inserted into the host chromosome.

Question 57

Lysogen

Answer 57

A bacterium that carries phage DNA ( a prophage) integrated into its genome.

Question 58

Why are most prophage genes silent

Answer 58

Most prophage genes are silent because a phage encoded repressor protein prevents their expression.

Question 59

A prophage can remain integrated in this latent state indefinitely, except when

Answer 59

A prophage can remain integrated in this latent state indefinitely, but it can also be excised from the host chromosome by a phage encoded enzyme. When this happens, a lytic infection begins.

Question 60

What determine which type of infection occurs initially

Answer 60

Which type of infection occurs initially lytic or lysogenic is mostly random, but the metabolic state of the host cell has an effect.

Question 61

What is essential for maintaining the lysogenic (latent) state

Answer 61

The repair prevents expression of the gene required for excision and is therefore essential for maintaining the lysogenic (latent) state.