SM L2

Question 1

Classification of Polioviruses

Answer 1

Family - Picornaviridae (150 varites)
Genus - 5 Genera, Rhinovuris, Hepatovirus, Cardiovirus, Apthovirus, Enteroviru (includes poliviruses)
Species (Polio Virus)

Question 2

Characteristics of Polioviruses

Answer 2

Small (30nm).
3 serotypes (Type I, II, or III)
Single strand “+ sense” RNA
Nonenveloped (Naked)
Icosahedral Capsid symmetry
Capsid is resistant to acidity, & detergents
4 components to capsid: VP1 (blue), VP2 (red), VP3 (yellow), VP4 (internal) proteins
Xray crystallography reveals a canyon about 5-fold axis of symmetry
Causes - paralytic poliomyelitis
Transmission: Faecaloral

Question 3

Paralytic poliomyeltitis

Answer 3

Depicted in Egyptian wall painting
Most polio infections are unapparent/mild 1-2% reach CNS…high fever, back pain, muscle spasms (non paralytic poliomyelitis)
In 0.1% of cases…partial/complete muscle paralysis when relaxed (paralytic poliomyelitis)
If paralysis remains after months becomes permanent
Destruction of large motor neurones causes acute flaccid paralysis (AFP)

Question 4

Cure for Polio - salk vaccine

Answer 4

1955 - injectable salk vaccine.
Salk vaccine - formalin inacticated at neutral pH.
Probelm as not all vaccine as attenuated
Attenuated microorganisms can be used in vaccines, where they stimulate an immune response without causing disease, and can provide long-lasting protection against future infections by the same microorganism

Question 5

Cure for Polio - Sabin

Answer 5

1963 Sabin Vaccine
Also known as the oral polio vaccine (OPV)
The vaccine is made from 3 weakened, live polio virus strains, which are given orally in a sugar cube or liquid drops.
The strains are attenuated meaning that they replicate but do not cause disease
Developed by Albert Sabin

Question 6

Innactivated Polio Vaccine

Answer 6

PV contains polioviruses of three types which have been inactivated by formaldehyde. It is only available as a combination vaccine

Question 7

Tropism

Answer 7

The pattern of host cells which a virus can invade and replicate in (i.e. the biological niche)

Question 8

How is tropisim determined?

Answer 8

Tropism is determined by the host cells’ to viral entry (often expression of a receptor), and whether the cell is susceptibility permissive for replication (intracellular biochemistry)

Question 9

Poliovirus tropism

Answer 9

Gut M neurones, ( neurotropiccells ( enterotropic ), lymphoid cells.

Question 10

Do all primate cells express the PV receptor?

Answer 10

All primate cells express the PV receptor (PVR; CD155) (susceptible), but only gut, CNS and certain other cells are permissive
Polio virus binds to receptor (PVR) on susceptible cells

Question 11

Poliovirus: Genome organisation

Answer 11

single copy of genome (+ve sense)
single open reading frame (very simple)
untranslated regions (UTR), capsid proteins, proteases, polymerase/RNA synthesis

Question 12

Poliovirus replication cycle

Answer 12

1.Virus binds to receptor (PVR) on susceptible cell & enters
2.Virus uncoats to release +RNA (Red)
3.+RNA translated to proteins (including polymerase) in permissive cell
4.Polymerase makes -RNA template from the +RNA
5.Now, +RNA made from -RNA template
6.Progeny virus assembled (using mRNA from 5 as its nucleic acid)
7.Cell Lyses and virus released

Question 13

Viremia

Answer 13

Virus in blood

Question 14

Classification of HIV virus

Answer 14

Family - retroviridae
Genus - Lentiviruses
Includes: Includes: HIV-1, HIV-2, SIV, FIV, BIV, Visna-Maedi virus
Species - HIV

Question 15

Which species of HIV infect humans?

Answer 15

HIV-1 and HIV-2

Question 16

HIV-1

Answer 16

HIV-1 is thought to have originated in sourthern cameroon after jumping from wild chimps to humans during 20th century
-HIV-1 is more virulent. Easily transmitted and is the cause of the majority of HIV infections globally.

Question 17

HIV-2

Answer 17

*HIV-2 may have originated from an Old World monkey
of Guinea-Bissau, Gabon and Cameroon called the Sooty
Mangabey
HIV-2 is less transmittable and is largely confined to
West Africa.

Question 18

Hunter theory

Answer 18

transference of the virus
from chimp to human when a human was bitten
by a chimp or was cut while butchering one,
and the human became infected.

Question 19

Main symptomes of AIDS

Answer 19

Many people with HIV
have no symptoms for
several years. Others
may develop symptoms
similar to flu, usually two
to six weeks after
infection.

Question 20

Human immunodeficiency virus

Answer 20

• Size=110nm
• 2 copies of Single strand “+ sense”
  RNA
• Reverse transcriptase, integrase,
  protease
• Icosahedral Capsid symmetry
• Encased by a matrix protein within
  an envelope
• Envelope is lipid bilayer derived
  from host membrane but contains
  viral protein gp120
• Causes Acquired Immunodeficiency
  disease (AIDS)
• Transmission: Sexually transmitted
  (STD) or other exchange body fluids
  (i.v. drug abuse, vertical, unscreened
  blood donation)
• 2 strains (HIV I, II).

Question 21

What way is HIV transmitted

Answer 21

sexually or body fluids

Question 22

HIV; Genome organisation

Answer 22

• 2 copies of genome (9719 nucleotides)
• Genes encoded in all 3 reading frames
• alternative splicing
• Long terminal repeats (LTR)
• 3 major genes:
  gag (capsid, matrix,& protease)
  pol (RT & integrase);
  env (envelope proteins gp120)
• Accessory genes: nef, tat, vif (regulatory)

Question 23

HIV tropism

Answer 23

primate cells expressing CD4 (such as
helper CD4+ T cells) and macrophages (M-tropic)
Also, either/or the chemokine receptor CCR5 (macs R5
HIV) or CXCR4 (T-cells X4 HIV).

Question 24

What does it mean that HIV is lymphotropic?

Answer 24

HIV infection…….you see LOW levels of CD4+ cells

Question 25

What are the four mechanisms for HIV?

Answer 25

Four main mechanisms:
1. direct viral killing of infected cells
2. increased rates of apoptosis in infected cells
3. killing of infected CD4+ T cells by CD8 cytotoxic
lymphocytes that recognize infected cells.
4. Death of uninfected CD4 ‘bystander cells’ cells by cell
suicide (2014)

Question 26

HIV replication cycle

Answer 26

Virus binds to
receptor (CD4,
CCR5, CXCR4)
on helper T
cell
2. Viral core (capsid)
enters cell &
releases +RNA
3.+RNA is converted
to DNA by RT
4. +/-DNA is
transported to
nucleus and
integrates into
host genome
(Provirus)
5. mRNA for viral
proteins & viral
genomes made
from provirus +/-
DNA template 6. Progeny virus
assembled at cell
surface
7. Immature
virus buds
from cell
then
matures**

Question 27

Reasons for genetic variability…evasion of host immune system