BIOL. 1406 Chapter 19 Viruses

Question 1

How does a virus make more viruses?

Answer 1

A generic virus containing a viral genome enters the host cell and releases the genome;
The viral genome is replicated by enzymes;
Copies of genome are transcribed by host enzymes;
Viral mRNA is translated into viral proteins;
New viruses are released form the cell.

With some viruses, the host cell bursts, releasing new viruses.
With other viruses, the host cell remains alive, releasing new viruses.

Question 2

Viruses

Answer 2

Cellular parasites that exist by manipulating cellular processes.
Scientists use molecular tools to manipulate cells.

Question 3

Virus

Answer 3

An infectious particle, consisting of viral genome that is packaged into a protein coat and, in some cases, a membranous envelope
Cannot reproduce or carry out processes outside of host cell

Question 4

Viral Genomens

Answer 4

Single/double-stranded DNA or RNA;
Either a single linear or circular molecule of the nucleic acid

Question 5

Capsid

Answer 5

The protein shell that encloses the viral genome;
Made up of small subunits - capsomeres:
Can have a variety of structures; may be helical or icosahedral

Question 6

Viral envelopes

Answer 6

Derived from membranes of host cells; surround the capsid of influenza and many other viruses found in animals

Question 7

Bacteriophages

Answer 7

Phages;
Viruses that infect bacteria;
They have an elongated capsid head that encloses their DNA;
A protein tail piece attaches the phage to the host and injects the phage DNA inside

Question 8

Host Range

Answer 8

A number of species it can infect;
some viruses have a broad one while others can only infect one species

Question 9

Two alternative reproductive mechanisms of phages

Answer 9

Lytic and lysogenic cycles

Question 10

Lytic Cycle

Answer 10

A phage replica give cycle that culminated during host cell’s death;
Produces new phages and breaks apart the host cell wall, releasing the progeny viruses

Question 11

Virulent phage

Answer 11

A phage that reproduces by the lytic cycle

Question 12

Lysogenic cycle

Answer 12

phage reproductive cycle that replicates viral DNA without destroying the host cell;
viral genome is incorporated in host cell’s DNA;
every time a cell divides, cell’s DNA replicates with the phage’s DNA shared by the two daughter cells;
an environmental signal can trigger bacteria to enter the lytic cycle;
some prophages are expressed during lysogeny, while others secrete bacterial toxins harmful to humans

Question 13

Temperate Phages

Answer 13

Bacteriophages that use both the lytic and lysogenic cycles

Question 14

Prophage

Answer 14

The integrate viral DNA inside the host’s genome

Question 15

Bacterial Defenses Against Phages

Answer 15

natural selection favors mutants with surface proteins that are not recognized as receptors by a particular type of phage;
foreign DNA can be identified as such and cut up by restriction enzymes (inside cell);
bacteria and archaea can protect themselves with the use CRISPR-Cas9

Question 16

CRISPR-Cas9

Answer 16

a DNA sequence inside bacteria that contains clustered, regularly, interspaced, short, palindromic repeats;
each repeat codes for a viral genome that has entered host’s cell;
once transcribed, it attaches to the Cas9 protein and finds the complementary viral genome that entered the cell;
the protein cuts up the invaded agent, disabling it;
the technique may be used to alter, delete, or insert nucleotides

Question 17

Two key variables that are used to identify animal viruses

Answer 17

an RNA or DNA genome that can be single- or double-stranded;
presence or absence of a membranous envelope;
Whereas few bacteriophages have an envelope, many animal viruses do.
Whereas few bacteriophages have an RNA genome, many animal viruses do.

Question 18

Viral Envelopes

Answer 18

many viruses that infect animals have a membranous envelope;
after synthesis, capsids acquire an envelope from host’s cell membrane;
others exit out of the nuclear envelope with an envelope that is replaced by the Golgi Apparatus

Question 19

Viral RNA Genome

Answer 19

broadest variety found in viruses that infect animals;
Genome is retroviruses that use reverse transcriptase to synthesize DNA from RNA

Question 20

Retrivirus

Answer 20

contains a viral RNA;
uses reverse transcriptase to copy DNA from an RNA ;
HIV (Human Immunodeficiency Virus) that causes AIDS (Acquired Immunodeficiency Syndrome)

Question 21

Provirus

Answer 21

the viral DNA that is integrated into the animal host’s 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 genomes for viral particles released from cell

Question 22

Evolution of viruses

Answer 22

Since viruses can replicate only within cells, they probably evolved as bits of cellular nucleic acid;
candidates for the source of viral genome include plasmids and transposons;
(plasmids, transposons, and viruses are all mobile genetic elements)

Question 23

Formidable pathogens in animals and plants

Answer 23

viruses and prions

Question 24

Prion

Answer 24

a misfolded protein that can transmit its misfoldedness to normal variants of the same protein and trigger cellular death

Question 25

Vaccine

Answer 25

A harmless derivative of a pathogen that stimulates the immune system to mount defenses against the harmful pathogen;
can prevent certain viral illnesses;
viral infections cannot be treated with antibiotics;
antiviral drugs can help to treat, not cure, viral infections by inhibiting synthesis of viral DNA and by interfering with viral assembly

Question 26

Causes of rapidly emerging viral disease in humans

Answer 26

1) mutations of existing viruses into new ones that can spread more easily;
2) spread of viral disease from a small, isolated human population
3) spread of existing viruses in animals (3/4 of human disease originate this way)

Question 27

Flu Epidemics

Answer 27

cause by type A influenza viruses;
infect a wide variety of animals: birds, pigs, horses, and humans;
given standardized names based on the viral surface proteins hemagglutin (HA) and neuraminidase (NA);
18 types of HA and 11 types of NA have been identified

Question 28

H5N1 strain

Answer 28

an influenza strain, circulating among people that is very difficult;
very difficult to mount an effective immune response to it;
not transmitted from person to person

Question 29

H1N1

Answer 29

a deadly strain;
swine flu;
combination of swine, avian, and human influenza genes

Question 30

Pandemic

Answer 30

A global epidemic

Question 31

Influenza viruses

Answer 31

flu viruses;
have nine RNA segments in their genome, leading to many new genetic combinations;
have a high rate of mutation;
normal seasonal flu viruses are not considered emerging viruses because variants of these viruses have been circulating among humans for a long time;
these viruses undergo mutation and reassortment;
variations that are most likely to occur each year are selected to generate vaccines

Question 32

Plant Viruses

Answer 32

most have an RNA genome;
many have a helical capsid, while other have an icosahedral capsid;
spread by two major routes:
horizontal transmission and vertical transmission

Question 33

Horizontal Transmission

Answer 33

virus, entering through damaged cell’s walls

Question 34

Vertical Transmission

Answer 34

virus, inherited from a parent

Question 35

Prions

Answer 35

infectious, incorrectly-folded proteins that appear to cause generative brain diseases in animals;
can be transmitted in food, act slowly, and are virtually indestructible;
able to convert a normal form of protein into misfolded version;
when several prions aggregate into a complex that can covert proteins into prions, which join the chain;
involved in diseases such as Alzeihmer’s and Parkinson’s