Structure of Viruses

Question 1

What are viruses

Answer 1

Must have a host to replicate
Exploit host cell metabolic processes
Contain nucleic acid genetic material
V. small - some have 5 genes

Question 2

General structure

Answer 2

Genome + protein subunits

Virion= extracellular form

Question 3

Symmetry

Answer 3

Nucleocapsids are always symmetrical
Either:
Spherical (Icosahedral symmetry) - eg human papillomavirus
Rod (Helical symmetry) - eg tobacco mosaic

Question 4

Complex viruses

Answer 4

Example - bacteriophage T4
Icosahedral head
Helical tail
Number of tail fibres

Question 5

Genomes

Answer 5

Most viruses have RNA

Question 6

Baltimore Classification

Answer 6

Viruses are classified according to the nature of their genetic material

Question 7

Virus Envelope

Answer 7

Some viruses acquire a lipid bilayer “envelope” as they bus from the surface of cells
Often the envelope contains virus proteins important for infecting other cells

Question 8

Virus enzymes

Answer 8

E.g some bacteriophage contain lysosome needed for entry of their DNA
E.g Influenza viruses carry neuraminidase to allow release from infected cell

Question 9

Other virus enzymes

Answer 9

RNA viruses always have their own replicase

Retroviruses carry their own replicase, “reverse transcriptase” that copies RNA into DNA

Question 10

Virus replication

Answer 10

One cellular process that ALL viruses require = protein translation on the ribosomes

Question 11

Virus replication

Answer 11

1. Attachment (adsorption)
2. Penetration (injection)
3. Synthesis (of nucleic acid and protein)
4. Assembly and packaging
5. Release (lysis)

Question 12

Attachment and entry - Animal cells

Answer 12

Viruses bind to specific receptor
Viruses enter animal cells by Endocytosis or Membrane fusion
Host cell specificity

Question 13

Virus entry cont.

Answer 13

The virus envelope fuses with the plasma membrane of the cell
The virus RNA is released into the cytoplasm of the cell

Question 14

Production of viral nucleic acid

Answer 14

• Production of some virus proteins
• requiring viral mRNA
• then genome is replicated

Question 15

Forming the virus

Answer 15

Some virus proteins form the virus coat
The virus coat forms around the nucleic acid
Virus capsids self assemble

Question 16

Release of the virus

Answer 16

Viruses get out of cells by

• budding through cell membrane (virus then acquires envelope)
• Cell lysis
• Cell death

Question 17

Subviral entities

Answer 17

Helper or defective viruses
-e.g. bacteriophage P4 of E.coli
P4 does not encode a capsid protein and relies on phage P2 to provide its capsid

Question 18

Subviral entities

Answer 18

Helper or defective viruses (II)

-e.g. adeno-associated virus (satellite virus) AAV need adenovirus (unrelated virus) to replicate

Question 19

Subviral entities

Answer 19

Viroids are found in plants and cause disease
They are small circular RNA molecules that do not have a protein coat
The special structure of viroid RNA makes it stable inside and outside of cells
Viroids contain NO protein-coding genes

Question 20

Subviral entities

Answer 20

Prions consist entirely of protein
No RNA or DNA but they are infectious!
-cause BSE (Mad cow disease)
-cause Creutzfeldt-Jakob disease (CJD)