Chapter 26

Question 1

Nature of Viruses

Answer 1

All viruses have same basic structure​

Nucleic acid core surrounded by protein.​

No cytoplasm – not a cell.​

Nucleic acid can be D N A or R N A​

Circular or linear.​

Single- or double-stranded.​

R N A viruses may be segmented or not.

Question 2

Virus structure

Answer 2

Nearly all viruses form a protein sheath, or capsid, around their nucleic acid core​

Composed of repeats of 1 to a few proteins.​

Some viruses store specialized enzymes with nucleic acid core​

Reverse transcriptase not found in host.​

Many animal viruses have an envelope​

Derived from host cell membrane with viral proteins.

Question 3

Viral hosts

Answer 3

Viruses are obligate intracellular parasites, found in every kind of organism

Question 4

Host range

Answer 4

types of organisms infected

Question 5

Tissue tropism

Answer 5

inside a host, the virus may only infect certain tissues.

Question 6

Viral Replication

Answer 6

Infecting virus can be thought of as a set of instructions​

Viral genome tricks host cell into making viruses​

Cell with a virus is often damaged by infection​

Viruses can only reproduce inside cells​

Outside, they are metabolically inert virions

Viruses lack their own ribosomes and enzymes for protein and nucleic acid synthesis​

Virus hijacks the cell’s transcription and translation machineries to express​

Early genes​

Intermediate genes​

Late genes​

End result is assembly and release of viruses​

Question 7

Most viral capsids come in two simple shapes

Answer 7

Helical – rodlike or threadlike. Ex: T M V ​

Icosahedral – almost spherical.

Question 8

Viral morphology is highly diverse

Answer 8

Naked viruses (no envelope). Ex: poliovirus​

Some viruses are complex.​

T-even bacteriophages – binal (twofold) symmetry.​

Enveloped viruses (influenza).

Question 9

Viral Genomes

Answer 9

Vary greatly in both type of nucleic acid (D N A or R N A) and number of strands (single- or double-stranded)​

Most R N A viruses are single-stranded​

Include influenza, measles, common cold.​

Replicate in the host cell’s cytoplasm.​

Replication is error−prone, so high rates of mutation = difficult targets for immune system and vaccines/drugs.

Question 10

Retroviruses

Answer 10

Have single-stranded R N A genome that is reverse-transcribed into double-stranded D N A.​

Employ reverse transcriptase to copy viral R N A into D N A.

Question 11

Most D N A viruses are double-stranded

Answer 11

Replicated in nucleus of eukaryotic host cell.

Question 12

Giant viruses

Answer 12

Challenge assumptions about viruses due to size and genome characteristics​

Most viruses have diameters from about 20 to 250 nm​

But Mimivirus has a virion 750nm in diameter, and a genome of 1.2 Mb​

Question 13

Taxonomy

Answer 13

International Committee on Taxonomy of Viruses (I C T V) uses order, family, subfamily, and genus.

Question 14

Classification can be based on

Answer 14

Taxonomy

The disease they cause

The host they infect

Genome expression

Question 15

Classification by Disease or Host

Answer 15

Both methods are limited​

Not all viruses cause disease​

Some viruses cause different diseases under different conditions or times during infection​

Some viruses like the common cold can be caused by several viruses​

Some viruses infect different types of organisms – ex. influenza

Question 16

Classification by Genome Expression

Answer 16

More broadly applicable than other classification types​

Baltimore classification sorts viruses based on the relationship between genome structure and genome expression​

Advances in metagenomics have revealed important roles for viruses in ecology and evolution

Question 17

Metagenomics

Answer 17

Metagenomics lets us study ALL viruses​

Isolate D N A from environmental sample, amplify, sequence and compare to the database

Question 18

Bacteriophage

Answer 18

Viruses that infect bacteria

Question 19

Reproductive cycles of bacteriophage: lytic cycle

Answer 19

Attachment or adsorption​

Target part of bacterial outer surface.​

Penetration or injection​

T4 pierces cell wall to inject viral genome.​

Synthesis​

Phage takes over the cell’s replication and protein synthesis enzymes to synthesize viral components.

Assembly​

Assembly of components.​

Release​

Mature virus particles are released through enzyme that lyses host or budding through host cell wall.

Question 20

Latent phase

Answer 20

Virus does not immediately kill infected cell​

Question 21

Vibrio cholerae phage conversion

Answer 21

Lysogenic phage introduced a gene coding for cholera toxin.​

Gene became incorporated into host genome.​

Converts harmless bacteria into disease-causing form.

Question 22

Persistent infections

Answer 22

latent or chronic

Question 23

Acute infections

Answer 23

rapid replication of virus, can lead to sudden symptom onset

Question 24

Influenza

Answer 24

One of the most lethal viruses in human history​

Influenza pandemic of 1918 and 1919 thought to have infected 1/3 of the world’s population.​

Flu viruses are enveloped, Type V animal viruses

Question 25

Hemagglutinin (H)

Answer 25

Aids in viral entry

Question 26

Neuraminidase (N)

Answer 26

Aids in viral exit.

Question 27

Antigenic drift

Answer 27

small changes in the HA and NA proteins such that previous vaccine-induced immunity is no longer protective

Question 28

Flu pandemics

Answer 28

Spanish flu of 1918, A(H1N1)​

Killed 50 to 100 million worldwide.​

Asian flu of 1957, A(H2N2)​

Killed over 100,000 Americans.​

Hong Kong flu of 1968, A(H3N2)​

Infected 50 million in U.S., killing 70,000.

Question 29

Coronaviruses

Answer 29

Enveloped viruses with (+) single-stranded R N A genomes​

3 cause severe disease: ​

M E R S-CoV​

S A R S-CoV​

S A R S-CoV-2​

4 others cause mild cold like symptoms

Question 30

Zoonotic

Answer 30

infect variety of animals, spill over from nature

Question 31

Coronaviruses 2

Answer 31

S A R S-CoV – Severe acute respiratory syndrome ​

From China in late 2002, spread to ~24 countries.​

S A R S-CoV-2 – coronavirus disease 2019 (COVID-19) ​

Spread globally from Wuhan, China, Jan 2020.​

By Nov 2020, the W H O reported 58+ million infections and 1.4 million deaths.​

By late 2020, vaccines were available, that could give 70 to 95% protection.

Question 32

SARS-CoV-2

Answer 32

Infects respiratory epithelia by binding of spike proteins to A C E2 protein on cell surface​

Wide range of morbidities:​

Some asymptomatic​

Respiratory tract symptoms​

Neurological symptoms ​

Acute respiratory distress, pneumonia, kidney failure, death​

Long-term complications

Question 33

Persistent viral infections

Answer 33

Persistent viral infections can be chronic, latent, or slow​

The virus or genome can be detected in the host for years or for life​

Examples: H I V and herpes viruses

Question 34

Human Immunodeficiency Virus

Answer 34

H I V causes acquired immune deficiency syndrome (A I D S)​

H I V infection is chronic​

H I V infects and ultimately destroys certain types of immune cells

Question 35

HIV Infection Cycle

Answer 35

Attachment

Entry through endocytosis

Replication

Assembly

Release

Question 36

Latent viral infections

Answer 36

Viruses that “hide” in the host to evade the immune system​

Can establish lifelong latent infections​

H S V-1 and H S V-2 trigger acute disease episodes and then become latent inside neurons​

Physiological stimuli can trigger their reactivation and lead to additional disease episodes

Question 37

Viruses and Cancer

Answer 37

Viruses can cause cancer by altering the growth properties of human cells​

-Blocking cell death.​

-Promoting cell division.

Question 38

Prions

Answer 38

Proteinaceous infectious particles”​

Cause transmissible spongiform encephalopathies (T S Es)​

“Mad cow” disease – B S E​

Scrapie in sheep​

Creutzfeldt–Jacob disease in humans (C J D)​

Host has normal prion proteins

causes normal PrP to misfold, cause disease.

Question 39

Viroids

Answer 39

iny naked molecules of circular R N A​

250 to 400 nts​

Cause disease in economically important crops​

Use host protein to replicate​

Small interfering R N As (siR N As) may be produced during infection​

Interfere with plant growth and development.​