Viruses 1

Question 1

Virus discovery

Answer 1

Chamberland filters removed bacteria from extracts
- yet extracts still caused disease
.:. microbe distinct from + smaller than bacteria caused disease

Virus nature + structure defined by electron microscope

Question 2

What are viruses?

Answer 2

Non-cellular parasitic microbes
- w/ no intrinsic metabolic capabilities

Obligate intracellular parasites
- can only replicate inside living host cells

Question 3

Viral replication

Answer 3

1. Virus attaches host cell
2. Capsid penetrates cell
3. Uncoating of capsid
   = DNA released
4. Viral DNA + proteins synthesised via host machinery
5. Assembly of new virions
6. Release + maturation of virions

Question 4

General characteristics of viruses

Answer 4

Simple structures

• few/no enzymes of their own
• consist of NAs + protein

Size

• smaller than almost all microbes
• 20nm-400nm
• size of virus unrelated to size of target host cell

Question 5

Virus structure

Answer 5

Capsid encloses genome
- symmetrical

Some have an outer lipid envelope

Question 6

capsid function

Answer 6

protect viral genome

facilitate entry into cell

Question 7

genome

• size
• made of

Answer 7

small compared to other orgs

DNA or RNA
(most bacteriophage =DNA
all plant + most animal = RNA)

Question 8

capsomeres

Answer 8

subunit of capsid

capsid mad euphorias of many individual capsomeres

Question 9

nucleocapsid/core

Answer 9

capsid w/ enclosed NAs

Question 10

tegument

Answer 10

space between nucleocapsid + envelope

may contain accessory viral proteins

Question 11

virion

Answer 11

complete mature virus particle

metabolically inert

Question 12

envelope

Answer 12

lipid membrane surrounding complex viruses

may contain glycoproteins

derived from host cell membrane during budding of viruses

Question 13

capsid

- why are capsomeres a fundamental property?

Answer 13

economy of genetic info

ease of capsid assembly

ease of intracellular release of viral genome

Question 14

economy of genetic info

Answer 14

allows virus to generate capsids from min level of genetic info

Question 15

ease of capsid assembly

Answer 15

virus particles can self-assemble
+ held together by intra-molecular forces NOT covalent bonds

.:. no additional enzyme needed to join subunit s

Question 16

ease of intracellular release of viral genome

Answer 16

dissociation of non-covalently bonded capsid

-> easy release of genome inside host

Question 17

capsid symmetry

- types + e.g.s

Answer 17

helical
e.g. Tobacco mosaic virus

icosahedral (20 faces)
e.g. Adenovirus

BUT asymmetrical
e.g. Ebola

Question 18

icosahedral capsids

- made up of…

Answer 18

20 sided polygon

each of the 12 corners = a penton = a capsomere

each face composed of a heron
= repeating units of same capsomere

hexons + pentons can be identical proteins or distinct

Question 19

helical capsids

• e.g.
• arrangement

Answer 19

e.g. tobacco mosaic virus (TMV)

capsomeres arranged in helical array to form a helical cylinder

inside surface of each capsomere contains a ridge
- viral genome in embedded

Question 20

helical capsids

- microscopes

Answer 20

shape not always evident in microscopes

influenza’s helical capsid appears roughly spherical

Question 21

genome

• encodes
• fragmented?

Answer 21

all necessary info to direct host cells to make viral proteins + replicate viral genome

some encode genomes on several distinct strands of DNA/RNA

e.g. influenza has a fragmented genome
w/ 8 separate strands of RNA

Question 22

genome

• size e.g.s
• overlapping

Answer 22

Cauliflower mosaic virus encodes 7 genes
vs E.coli encodes 4000

often encode overlapping genes
- can encode more genes on a smaller strand

Question 23

Classification of viruses

- originally

Answer 23

based on diseases caused
- BAD

dissimilar viruses may cause similar symptoms
OR
same virus may cause different symptoms depending on age, health etc of host

symptoms may also be associated with secondary bacterial infections
NOT primary viral infection

Question 24

Hepatitis

- what is it?

Answer 24

= inflammation of liver

Question 25

Virus classification | - ICTV base it on

Answer 25

genome composition + structure capsid symmetry envelope size host range

Question 26

Virus classification | - most important component

Answer 26

genome those w/ similar genome sequences likely to share a common ancestry

Question 27

genome | - composition

Answer 27

DNA or RNA ss or ds ssRNA may either be +ve sense RNA OR -ve sense RNA (copying a +ve sense strand --> complementary -ve sense strand being synthesised)

Question 28

genome | - +ve snese RNA

Answer 28

mRNA needs to be +ve sense to be translated BUT +ve sense template transcribed into a -ve sense product SO additional steps required

Question 29

genome classifications

Answer 29

Class: 1 = dsDNA 2 = ssDNA + dsDNA 3 = dsRNA 4 = +ssRNA + - ssRNA 5 = -ssRNA 6 = ssRNA-RT + DNA/RNA + dsDNA 7 = dsDNA-RT

Question 30

Baltimore classification of viral genomes - group 1 - group 2 - group 3

Answer 30

dsDNA - uses own or host DNA pol for replication ssDNA - requires DNA pol dsRNA - requires RNA-dependent RNA pol to make mRNA + genomic RNA

Question 31

Baltimore classification of viral genomes - group 4 - group 5

Answer 31

+ ssRNA - requires RNA-dependent RNA pol to make mRNA template + replication - ssRNA - requires RNA-dependent RNA pol to make mRNA + replication

Question 32

Baltimore classification of viral genomes - group 6 - group 7

Answer 32

retrovirus - uses own RTase to make dsDNA dsDNA pararetrovirus - requires plant host RTase to make dsDNA

Question 33

Hepatitis - envelope Y/N? - group

Answer 33

Hep A Virus = N +ve sense ssRNA (gp 4) Hep B = Y dsDNA (gp 7) Hep C = Y +ve sense ssRNA (gp4) Hep D = Y -ve sense ssRNA (gp5) Hep E = N +ve sense ssRNA (gp4)