* 19

Question 1

capsid

Answer 1

• protein shell enclosing the viral genome
• may be rod-shaped, polyhedral, or more complex
• built from a large number of protein subunits called CAPSOMERES, but the number of diff kinds of proteins in a capsid is small

Question 2

rod-shaped viruses

Answer 2

• ex: tobacco mosaic virus
• has a rigid, rod-shaped capsid made from over a thousand molecules or a single type of capsomere arranged in a helix: “helical virus”
• RNA inside

Question 3

adenoviruses

Answer 3

• infect animals’ respiratory tracts
• have 252 identical capsomere molecules arranged in a polyhedral capsid w/ 20 triangular facets – an icosahedron “icosahedral viruses”
• surface studded w/ protruding glycoproteins
• DNA inside

Question 4

influenza virus

Answer 4

• has membranous envelope that surrounds the capsid
• these viral envelopes, derived from the membranes of the host cell, contain host cell phoshpolipids and membrane proteins
• glycoproteins (protruding) and proteins of viral origin
• spherical
• helical virus structure inside (RNA surrounded by capsid)

Question 5

bacteriophages

Answer 5

• most complex capsids
• first 7 studied infefcted E coli: named T1 … T7
• T-evens are very similar in structure: capsids have elongated icosahedral heads enclosing DNA. attached to the head is a protein tail piece (sheath + fiber) that attaches to bacterium and injects viral DNA

Question 6

host range

Answer 6

The limited number of species whose cells can be infected by a particular virus.

Question 7

viral entry

Answer 7

• DNA injection by tail fiber
• endocytosis
• fusion of viral envelope w/ plasma membrane

Question 8

2 phage types

Answer 8

• virulent: A phage that replicates only by a lytic cycle. (ex: T4)
• temperate: A phage that is capable of replicating by either a lytic or lysogenic cycle. (ex: lambda, which resembles T4, but its tail has only 1 short tail fiber)

Question 9

lytic cycle

Answer 9

1. the T4 phage uses its tail fibers to bind to specific receptor sites on the outer surface of E coli cell
2. the tail sheath contracts, injecting the phage DNA into the cell and leaving an empty capsid outside. the cell’s DNA is hydrolyzed.
3. phage DNA directs production of phage proteins and copies of the phage genome by host and viral enzymes, using cellular components
4. 3 separate sets of proteins self-assemble to form phage heads, tails, and tail fibers. the phage genome is packaged inside the capsid as the head forms.
5. the phage directs production of an enzyme that damages the bacterial cell wall, allowing fluid to enter. the cell swells and lyses, releasing 100 - 200 phage particles.

Question 10

why haven’t phages exterminated all bacteria?

Answer 10

• bacterial mutants w/ receptors that are no longer recognized by a particular type of phage
• when DNA successfully enters a bacterium, the DNA often is idenfitied as foreign and cut up by cellular enzymes called RESTRICTION ENZYMES, which are so named b/c their activity restricts the ability of the phage to infect the bacterium (the bacterium’s own DNA is METHYLATED in a way that prevents attack by its own restriction enzymes)
• many phages coexist w/ bacteria in a state called lysogeny

Question 11

prophage

Answer 11

• during a lysogenic cycle, the lambda DNA molecule is incorporated into a specific site on the E coli chromosome by viral proteins that break both circular DNA molecules and join them to e/o.
• when integrated this way, the viral DNA is known as a prophage.
• one prophage gene codes for a protein that prevents transcription of most of the other prophage genes; thus, the phage genome is mostly silent w/in the bacterium.
• other prophage genes cause host cell to make toxins

Question 12

lysogenic cycle

Answer 12

1. phage attaches to host cell, injects its DNA
2. phage DNA circularizes
3. certain factors determine whether lytic/lysogenic cycle is entered
4. phage DNA integrates into bacterial chromosome, becoming a prophage
5. bacterium reproduces normally, copying the prophage and transmitting it to daughter cells

Question 13

replicative cycle of enveloped RNA virus

Answer 13

(single stranded RNA)

1. glycoproteins on the viral envelope bind to specific receptor molecules on host cell, promoting viral entry
2. capsid + viral genome enter cell. digestion of capsid by cellular enzymes releases the viral genome.
3. viral genome functions as template for synthesis of complementary RNA strands by a viral RNA polymerase
4. these new copies are used as templates for more replication
5. the complementary RNA strands also function as mRNAj, which is translated into both capsid proteins (in cytosol) and glycoproteins for the viral envelope (in ER and Golgi)
6. vesicles transport envelope glycoproteins to plasma membrane
7. a capsid assembles around each viral genome molecule
8. new virus buds from cell

Question 14

herpesvirus

Answer 14

• temporarily cloaked in membrane derived fro the host’s nuclear envelope; they then shed this membrane in the cytoplasm and acquire a new envelope made from Golgi membrane
• dsDNA
• replicate w/in host cell nucleus, using both viral and cellular enzymes
• copies of viral DNA can remain behind as mini-chromosomes in the nuclei of certain nerve cells; there they remain latent until some sort of physical/emotional stress triggers a new round of active virus production
• acyclovir, which resembles nucleosides, impedes herpesvirus replication by inhibitng the viral polymerase that synthesizes viral DNA

Question 15

class IV viruses

Answer 15

• single stranded RNA; animal viruses

- the genome can directly serve as mRA and thus can be translated into viral protein immediately after infection

Question 16

class V

Answer 16

• single stranded RNA
• RNA genome serves as a template for mRNA synthesis
• the RNA genome is transcribed into complementary RNA strands, which function both as mRNA and as templates for the synthesis of additional copies
• all viruses that require RNA –> RNA synthesis to make mRNA use their own viral enzyme; no such enzymes exist in most cells

Question 17

class VI

Answer 17

• retroviruses
• single stranded RNA
• RNA animal viruses w/ most complicated replicative cycles
• equipped w/ an enzyme called REVERSE TRANSCRIPTASE, which transcribes an RNA template into DNA
• ex: HIV

Question 18

HIV replicative cycle

Answer 18

1. envelope GPs enable the virus to bind to specific receptors on certain WBCs
2. virus fuses w/ cell’s plasma membrane. capsid proteins are removed, releasing the viral proteins and RNA
3. reverse transcriptase catalyzes DNA synthesis
4. reverse transcriptase catalyzes synthesis of a 2nd DNA strand compelemtnary to the first
5. the double-stranded DNA is incorporated as a PROVIRUS into the cell’s DNA
6. proviral genes are transcribed into RNA molecules, which serve as genomes for the next viral generation and as mRNAs for translation into viral protein
7. vesicles transport GPs to cell’s plasma membrane
8. capsids are assembled around viral genomes and reverse trnascriptase molecules
9. new ciruses bud off from host cell

Question 19

provirus

Answer 19

A viral genome that is permanently inserted into a host genome. (vs prophage, which leaves the host’s genome at the start of a lytic cycle)

Question 20

original sources of viral genomes

Answer 20

• plasmids: small, circular DNA molecules found in bacteria and in the unicellular eukaryotes called yeasts. exist APART from the cell’s genome, can replicate independently of the genome, and are ocasionally transferred btwn cells
• transposons: DNA segments that can move from one location to another w/in a cell’s genome
• these two and viruses share impt feature: they’re mobile genetic elements

Question 21

mimivirus

Answer 21

• dsDNA
• “mimicking microbe” b/c it’s the size of a small bacterium
• icosahedral capsid w/ 400 nm diameter
• 1.2 million bases and 1,000 genes
• some of the genes code for products previously thought to be hallmarks of cellular genomes – proteins involved in translation, DNA repair, protein folding, polysaccharide synthesis

Question 22

symptoms of viral infection

Answer 22

• viruses may damage/kill cells by causing the release of hydrolytic enzymes from lysosomes
• cause infected cells to produce toxins
• some have toxic molecular components, such as envelope proteins

Question 23

HIV antivirals

Answer 23

• azidothymidine (AZT), which resemble nucleosides, curbs HIV replication by interfering w/ DNA synthesis by reverse transcriptase
• “cocktails”: combo of 2 nucleoside mimics and a protease inhibitor, which interferes w/ an enzyme required for virus assembly

Question 24

how do viruses burst on the human disease?

Answer 24

• RNA viruses tend to have an unusually high rate of mutation b/c errors in replication their RNA genomes aren’t corrected by proofreading. some mutations change existing viruses into new strains that can cause disease, even in individuals who are immune to the ancestral virus
• dissemination by plane, etc
• spread of existing viruses from animals. (75 percent of new human diseases originate in this way)

Question 25

types of virus

Answer 25

types of flu virus: - B and C, which infect only humans and have never caused an epidemic - A, infects wide range of animals

Question 26

plant viruses: structure

Answer 26

- same basic structure and mode of replication as animal viruses - RNA genome - helical / icosahedral capsid

Question 27

plant viruses: transmittion

Answer 27

- horizontal: plant is infected from external source. plant becomes more susceptible if it's been damaged b/c the invading virus must get past the plant's epidermis. - vertical - passage of viral macromolecules from cell to cell is facilitated by virally encoded proteins that cause enlargement of plasmodesmata

Question 28

viroids

Answer 28

- circular RNA molecules, only a few hundred nucleotides long, that infect plants - don't encode proteins but can replicate in host plant cells, using host cell enzymes - cause errors in the regulatory systems that control plant growth - typical signs are abnormal development and stunted growth

Question 29

prions

Answer 29

- infectious proteins which appear to cause a number of degnerative brain diseases in various animal species - include mad cow - most likely transmitted in food - is a misfolded form of a protein normally present in brain cells. when the prion gets into a cell containing the normal form of the protein, the prion somehow converts normal protein molecules to the misfolded prion version upon contact. several prions then aggregate into a complex that can convert other normal proteins to prions, which join the chain. interferes w/ cellular functions and causes disease symptoms

Question 30

H1N1 name

Answer 30

- identifies which forms of 2 viral surface proteins are present: hemagglutinin (H) and neuraminidase (N) - H: 16 types; helps flu virus attach to host cells - N: 9 types; helps release new virus particles from infected cells

Question 31

animal virus --> human virus

Answer 31

- virus mutated as it passed from one host species to another - when an animal is infected w/ more than one strain of flu virus, the diff strains can undergo genetic recombination if the RNA molecules making up their genomes mix and match during viral assembly - coupled w/ mutation, these reassortments can lead to the emergence of a viral strain that's capable of infecting human cells