Chapter 19 - Viruses

Question 1

What is a virus?

Answer 1

A very small infectious particle consisting of little more than genes (a nucleic acid) packaged in a protein coat (in some cases, a membraneous envelope)

• Cannot reproduce or carry out metabolism outside of a host cell
• Not alive, but exist in a shady area between life-forms and chemicals

Question 2

What do viral genomes consist of?

Answer 2

• Double or single stranded DNA (DNA virus)
• Double or single stranded RNA (RNA virus)
• The genome is usually organized as a single linear or circular molecule of nucleic acid, although the genomes of some viruses consist of multiple molecules of nucleic acid
• Have between 3 and several thousand genes in their genomes

Question 3

What is a capsid?

Answer 3

The protein shell enclosing the viral genome; capsids are built from protein subunits called capsomeres

Depending on the type of virus, the capsid may be rod-shaped, polyhedral, or more complex in shape

*A capsid can have a variety of structures

Question 4

What are viral envelopes?

Answer 4

Derived from the membranes of the host cell; contain host cell phospholipids and membrane viral origin (they also contain proteins and glycoproteins of viral origin)

Question 5

What are bacteriaphages (phages)?

Answer 5

Viruses that infect bacteria; have the most complex capsids among viruses

Phages have an elogated capsid head that encloses their DNA; a protein tail piece attaches the phage to the host and injects the phage DNA inside

Question 6

What is the host range?

Answer 6

Each virus has a host range, a limited number of host cells in can infect

Question 7

What are the general features of a viral replicative cycle?

Answer 7

• Once a viral genome has entered a cell, the cell begins to manufacture viral proteins
• The virus makes use of host enzymes, ribosomes, tRNAs, amino acids, ATP, and other molecules
• Viral nucleic acid molecules and capsomeres spontaneously self-assemble into new viruses

Question 8

What is the lytic cycle?

Answer 8

A phage replicative cycle that culminates in death of the host cell; refers to the last stage of infection, during which the bacterium lyses (breaks open) and releases the phages that were produced within the cell

Question 9

What is a virulent phage?

Answer 9

A phage that replicates only by a lytic cycle

Question 10

What are restriction enzymes?

Answer 10

Cellular enzymes that cut up foreign DNA; bacteria defense against phages, restriction enzymes recognize and cut up certain phage DNA

Restriction enzymes restrict the ability of the phage to replicate within the bacterium

Question 11

What is the lysogenic cycle?

Answer 11

Allows replication of the phage genome without destroying the host; the viral DNA molecule is incorporated into the host cell’s chromosome

Question 12

What are temperate phages?

Answer 12

Phages capable of using both modes of replicating within a bacterium

Question 13

What is a prophage?

Answer 13

When integrated into the bacterial chromosome, the integrated viral DNA is known as a prophage

Every time the host divides, it copies the phage DNA and passes the copies to daughter cells

Question 14

What are the steps of the lytic cycle?

Answer 14

1. Attachment - phage binds to specific surface proteins that act as receptors
2. Entry of phage DNA and degradation of host DNA - the phage DNA is injected into the cell
3. Synthesis of viral genomes and proteins - the phage DNA directs production of phage proteins and copies of the phage genome by host and viral enzymes, using components within the cell
4. Self assembly - proteins self assemble to form phage heads, tails, and tail fibers
5. Release - the phage directs production of an enzyme that damages the bacterial cell wall, allowing fluid to enter; the cell swells and finally bursts, releasing 100 to 200 phage particles

Question 15

What are the steps of the lysogenic cycle?

Answer 15

1. Phage DNA integrates into the bacterial chromosome, becoming a prophage
2. The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells
3. Daughter cell with prophage - many cell division produce a large population of bacteria infected wit the prophage

*Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle (An environmental signal - a certain chemical or high energy radiation - can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode)

Question 16

What are the two key variables used to classify viruses that infect animals?

Answer 16

1. An RNA or DNA genome

2. A single stranded or double stranded genome

Question 17

Few bacteriophages have an envelope or an RNA genome, what type of viruses have both?

Answer 17

Many animal viruses have both

Question 18

What are viral envelopes?

Answer 18

• Many viruses that infect animals have a membranous envelope
• Viral glycoproteins on the envelope bind to specific receptor molecules on the surface of a host cell
• Some viral envelopes are derived from the host cell’s plasma membrane as the viral capsids exit

Question 19

What are the steps of the replicative cycle of an enveloped RNA virus?

Answer 19

1. Glycoproteins on the viral envelope bind to specific receptor molecules on the host cell, promoting viral uptake by the cell
2. The capsid and viral genome enter the cell; digestion of the capsid by cellular enzymes releases the viral genome
3. The viral genome functions as a template for synthesis of complementary RNA strands by a viral RNA polymerase
4. New copies of viral genome RNA are made using the complementary RNA strands as templates
5. cRNA strands also function as mRNA, which is translated into both capsid proteins (in the cytosol) and glycoproteins for the viral envelope (in the ER and Golgi apparatus)
6. Vesicles transport envelope glycoproteins to the plasma membrane
7. A capsid assembles around each viral genome molecule
8. Each new virus buds from the cell, its envelope studded with viral glycoproteins embedded in membrane derived from the host cell

Question 20

What do all viruses that use an RNA genome as a template for mRNA transcription require?

Answer 20

RNA —> RNA synthesis;

These viruses use a viral enzyme capable of carrying out this process; there are no such enzymes in most cells

The broadest variety of RNA genomes is found in viruses that infect animals

Question 21

What are retroviruses?

Answer 21

The RNA animal viruses with the most complicated replicative cycle (class VI)

Retroviruses use reverse transcriptase to copy their RNA genome into DNA

*Retro means backwards

Question 22

What is reverse transcriptase?

Answer 22

An enzyme which transcribes an RNA template into DNA, providing an RNA —> DNA information flow, the opposite of the usual direction

Question 23

What is the retrovirus that causes AIDS (acquired immunodeficiency syndrome)?

Answer 23

HIV (human immunodeficiency virus);

HIV and other retroviruses are enveloped viruses that contain two identical molecules of single stranded RNA and two molecules of reverse transcriptase

After HIV enters a host cell, its reverse transcriptase molecules are released into the cytoplasm, where they catalyze synthesis of viral DNA; the newly made viral DNA then enters the cell’ nucleus and integrates into the DNA of a chromosome; the integrated viral DNA (provirus) never leaves the host’s genome, remaining a permanent resident of the cell

Question 24

What is a provirus?

Answer 24

The integrated viral DNA which never leaves the host’s genome, remaining a permanent resident of the cell

• Unlike a prophage, a provirus remains a permanent resident of the host cell
• RNA polymerase transcribes the proviral DNA into RNA molecules
• The RNA molecules function both as mRNA for synthesis of viral proteins and as genomes for new virus particles released from the cell

Question 25

Discuss the evolution of viruses.

Answer 25

• Viruses do not fit our definition of living organisms; since viruses can replicate only within cells, they probably evolved as bits of cellular nucleic acid
• Candidates for the source of viral genomes include plasmids and transposons

Question 26

What are the three types of mobile genetic elements?

Answer 26

1. Plasmids - small circular DNA molecules found in bacteria and in the unicellular eukaryotes called yeasts; plasmids exists apart from and can replicate independently of the bacterial chromosome and are occasionally transferred between cells
2. Transposons - DNA segments that can move from onel ocation to another within a cell’s genome
3. Viruses

Question 27

What are examples of formidable pathogens in plants and animals?

Answer 27

Viruses, viroids, and prions;

• Diseases caused by viral infections affect humans, agricultural crops, and livestock worldwide
• Smaller, less complex entities called viroids and prions also cause disease in plants and animals, respectively

Question 28

Discus viral diseases in animals.

Answer 28

• Viruses may damage or kill cells by causing the release of hydrolytic enzymes from lysosomes
• Some viruses cause infected cells to produce toxins that lead to disease symptoms
• Others have molecular components such as envelope proteins that are toxic

Question 29

What is a complex and critical part of the body’s natural defenses?

Answer 29

The immune system

Question 30

What are vaccines?

Answer 30

A harmless variant or derivative of a pathogen that stimulates the immune system to mount defenses against the harmful pathogen

• Vaccines can prevent certain viral illnesses
• Viral infections cannot be treated by antibiotics
• Antiviral drugs can help to treat, though not cure, viral infections

Question 31

How do vaccines work?

Answer 31

By teaching the body to recognize and fight invaders;

• Subunit - small amount of virus or copy of virus in the form of a vaccine
• Body reacts by creating antibodies or killer cells
• Upon viral infection, antibodies and killer cells are there waiting to attack

Question 32

What is a difference between an epidemic and a pandemic?

Answer 32

An epidemic is restricted to a local region; a pandemic is global

Question 33

Discuss emerging viruses.

Answer 33

• Emerging viruses are those that suddenly become apparent
• In 2009, a general outbreak (epidemic) of a flu-like illness appeared in Mexico and the United States, caused by an influenza virus named H1N1
• Flu epidemics are caused by new strains of influenza virus to which people have little immunity
• Viral diseases in a small isolated population can emerge and become global (pandemic)
• New viral diseases can emerge when viruses spread from animals to humans (zoonosis)
• Viral strains that jump species can exchange genetic information with other viruses to which humans have no immunity

Question 34

What three processes contribute to the emergence of viral diseases?

Answer 34

1. The mutation of existing viruses - RNA viruses tend to have an unusually high rate of mutation because viral RNA polymerases do not proofread and correct errors in replicating their RNA genomes (some mutations change existing viruses into new genetic varieties that can cause disease)
2. The dissemination of a viral disease from a small, isolated human population
3. The spread of existing viruses from other animals

Question 35

What is an antigenic drift?

Answer 35

A change in virus protein sequence due to a mutation over time;

• Natural mutation over time of known strains of influenza (or other things, in a more general sense) which may lead to a loss of immunity, or in vaccine mismatch
• Most common
• Occurs in all types of influenza

*A mechanism for variation in viruses that involves the accumulation of mutations within the genes that code for antibody-binding sites

Question 36

What is an antigenic shift?

Answer 36

A specific case of reassortment or viral shift that confers a phenotypic change

• Important for the emergence of new viral pathogens as it is a pathway that viruses may follow to enter a new niche
• Avian influenza and human influenza

*The process by which two or more different strains of a virus, or strains of two or more different viruses, combine to form a new subtype having a mixture of the surface antigens of the two or more original strains

Question 37

Discuss viral diseases in plants.

Answer 37

• More than 2,000 types of viral diseases of plants are known and cause spots on leaves and fruits, stunted growth, and damaged flowers or roots
• Most plant viruses have an RNA genome
• Many have a helical capsid, while others have an icosahedral capsid

Question 38

What are two major routes of spreading viral diseases in plants?

Answer 38

1. Horizontal transmission - entering through damaged cell walls
2. Vertical transmission - inheriting the virus from a parent

Question 39

What are viroids?

Answer 39

Circular RNA molecules, only a few hundred nucleotides long, that infect plants

• Viroids do not encode proteins but can replicate in host plant cells, apparently using host cell enzymes
• Cause errors in the regulatory systems that control plant growth
• Typical signs of viroid diseases are abnormal development and stunted growth

Question 40

What are prions?

Answer 40

Infectious proteins which appear to cause a number of degenerative brain diseases in various animal species; slow-acting (an incubation period of at least ten years before symptoms develop), virtually indestructible infectious proteins (not destroyed or deactivated by heating to normal cooking temperatures)

• Prions propagate by converting normal proteins into the prion version
• Scrapie in sheep, mad cow disease, and Creutzfeldt-Jakob disease in humans are all caused by prions