this talks about virus structure , assembling , virus like organisms Prepared from lange microbiology

1. III-Virology
Ch28
By :Khaled Alkhodari
Sadi Nakhala
Mohammed Albelbesi

2. III Basic Virology
Chapter 28
1-Virus structure
By: Khaled HANI Alkhodari
Supervisor : Dr. Said Alghorrah

3. Contents :
The flu virus
Introduction
Khaled Alkhodari
Size & Shape of Viruses
Viral Nucleic Acids
Viral Capsid & Symmetry
Viral Proteins Sadi Alnakhala
Atypical Virus-Like Agents Mohammed Albelbesi

4. Remember

5. They range from
20 to 300 nm in
diameter
~= largest protein
 smallest cell
Size

6. Shape
-In colloquial terms (spheres, rods, bullets, or bricks).
-In reality they are structures of precise
geometric symmetry.
-Is determined by the capsid of the virus.

8. The viral genome is located
internally and can be:
- single / double-stranded DNA -
- single / double-stranded RNA
Only in Virus
Can be either linear or circular
DNA is always a single molecule
RNA = single / several pieces
VIRAL NUCLEIC ACIDS
Both influenza virus and rota
virus have a segmented RNA
genome
Almost all viruses are
haploid.The exception is the
retrovirus is diploid.

9. VIRAL CAPSID & SYMMETRY
Capsid :protein made up from
Capsomers :
•Consisting of 1/>proteins
• Seen in EM as a spherical particle, st with
a central hole
• Arrangement=geometric structure of virus
Nucleocapsid = NA + Capsid

10. VIRAL CAPSID & SYMMETRY
Nucleocapsids have two forms of
symmetry:
1.Icosahedral: capsomers=20▲
symmetric figure (icosahedron)
with approximate outline of a
sphere.
Enveloped or Naked.

11. VIRAL CAPSID & SYMMETRY
2. Helical: the capsomers are arranged in
a hollow coil that appears rod-shaped.
The helix can be either rigid or flexible.
All helical human viruses are
enclosed by an envelope
There are no naked helical viruses

13. VIRAL CAPSID & SYMMETRY
Building the virus particle from
identical protein subunits has 2
advantages:
1- Reduces the need for genetic
information.
2- Promotes self-assembly (no enzyme
or energy is required)

14. Any Questions ?

15. References

16. Thank you


17. PRESENTED BY : SADI YEHIA NKHALA
INTRODUCED TO : DR SAID AL-GHORRA
Viral structure
2- proteins

18.  the surface proteins of the virus, whether
they are the capsid proteins or the envelope
glycoproteins, are the principal antigens
against which the host mounts its immune
response to viruses.
18

19. The important functions of viral proteins
 The capsid proteins protect the genome DNA or RNA from degradation
by nucleases.
 The proteins on the surface of the virus mediate the attachment of the
virus to specific receptors on the host cell surface
 Outer viral proteins are also important antigens that induce neutralizing
antibody and activate cytotoxic T cells to kill virus-infected cells. These
outer viral proteins not only induce antibodies, but are also the target of
antibodies (i.e., antibodies bind to these viral proteins and prevent
[“neutralize”] the virus from entering the cell and replicating).
19

20. To remember ( bacterial note)
The somatic o-antigen is a polysaccharide on the exterior that
differs in each species and frequently differs between strains of
a single species .
Because the number of permutations of this array is very large,
many antigenic types exist , for example, more than 1500
antigenic types have been identified for Salmonella.
20

21. SEROTYPES !
( ANTIGENIC DETERMINANTS)
 The term “serotype” is used to describe a subcategory of a virus
based on its surface antigens. For example, measles virus has one
serotype, polioviruses have three serotypes , and rhinoviruses have
over 100 serotypes.
 This is because all measles viruses have only one antigenic
determinant on its surface protein that induces neutralizing
antibody capable of preventing infection. In contrast, polioviruses
have three different antigenic determinants on its surface proteins
,i.e., poliovirus type 1 has one kind of antigenic determinant ,
poliovirus type 2 has a different antigenic determinant, and
poliovirus type 3 has a different antigenic determinant from types
1 and 2
21

22. Medical implications related to serotypes
 person can be immune (have antibodies) to
poliovirus type 1 and still get the disease
,poliomyelitis caused by poliovirus types 2 or
3.
 The other implication is the polio vaccine must
contain all three serotypes in order to be
completely protective.
22

23. Internal viral proteins
( types and variablility)
 Some of the internal viral proteins are structural
(e.g. , the capsid proteins of the enveloped viruses), whereas
others are enzymes (e.g., the polymerases that synthesize the
viral mRNA).
 The internal viral proteins vary depending on the virus. Some
viruses have a DNA or RNA polymerase attached to the
genome; others do not.
 If a virus has an envelope, then a matrix protein that mediates
the interaction between the capsid proteins and the envelope
proteins is present.
23

24. Summary :
 Internal proteins may be :
1-capsid proteins ( in an enveloped viruses )
2-enzymes ( polymerase )
3-matrix proteins ( in an enveloped viruses )
24

25. Produced Proteins
( Superantigens )
 Some viruses produce proteins that act as “superantigens”
similar in their action to the superantigens produced by
bacteria, such as the toxic shock syndrome toxin of
Staphylococcus aureus .
 Viruses known to produce superantigens include:
1- Two members of the herpes virus family, namely, Epstein-
Barr virus and cytomegalovirus,
2- The retrovirus( mouse mammary tumor virus.)
25

26. Regulatory Proteins:
 Some viruses contain regulatory proteins in a
structure called the tegument, which is located
between the nucleocapsid and the envelope. These
regulatory proteins include transcription and
translation factors that control either viral or cellular
processes. Members of the herpesvirus family, such
as herpes simplex virus and cytomegalovirus, have a
prominent, well-characterized tegument.
26

27. Viral Envelope
 The envelope is a lipoprotein
membrane composed of lipid
derived from the host cell membrane
and protein that is virus-specific.
 Furthermore, there are frequently
glycoproteins in the form of spike-
like projections on the surface, which
attach to host cell receptors during
the entry of the virus into the cell.
27

28. Viral Envelope
 The envelope of most viruses is derived from the cell’s outer
membrane( cytoplasmic membrane ) , with the notable
exception of herpesviruses that derive their envelope from
the cell’s nuclear membrane.
 In general, the presence of an envelope confers instability on
the virus. Enveloped viruses are more sensitive to heat ,
drying , detergents, and lipid solvents such as alcohol and
ether than are nonenveloped viruses, which are composed
only of nucleic acid and capsid proteins.
28

29. Viral Envelope
 An interesting clinical correlate of this observation is
that virtually all viruses that are transmitted by the
fecal– oral route (those that have to survive in the
environment) do not have an envelope, that is, they
are naked nucleocapsid viruses. These include
viruses such as hepatitis A virus, poliovirus,
Coxsackie virus, echovirus, Norwalk virus, and
rotavirus.
29

30. Viral Envelope
 In contrast, enveloped viruses are most often
transmitted by direct contact, such as by blood or by
sexual transmission. Examples of these include human
immunodeficiency virus, herpes simplex virus type 2,
and hepatitis B and C viruses. Other enveloped viruses
are transmitted directly by insect bite (e.g., yellow fever
virus and West Nile virus) or by animal bite (e.g., rabies
virus).
30

31. Viral Envelope
 Many other enveloped viruses are transmitted
from person to person in respiratory aerosol
droplets, such as influenza virus, measles virus,
rubella virus, respiratory syncytial virus, and
varicella-zoster virus. If the droplets do not infect
directly, they can dry out in the environment, and
these enveloped viruses are rapidly inactivated.
.
31

32. Viral Envelope
 Note that rhinoviruses, which are transmitted
by respiratory droplets, are naked
nucleocapsid viruses and can survive in the
environment for significant periods. They
therefore can also be transmitted by hands
that make contact with the virus on
contaminated surfaces
32

33. 33
 Summary
1-Nonenveloped visrus :
Feco-oral route / Respiratory droplets
2-Enveloped viruses :
Direct contact / Bites / Respiratory droplets

34. 34

35. Prepared by : Mohammed Elbelbessi
supervised by : Dr- Said Elghora

36. Characteristics of typical viruses
1- viruses range in size from (20 – 300 nm)
20 ~ the size of large proteins
300 ~ the size of the smallest cells
2- viruses contain either DNA or RNA , but not both
3- All viruses have a protein coat called a capsid
4- viral proteins serve several important function

37. Atypical viruses
There are four exceptions to the typical viruses .
1- Defective viruses
2- Pseudovirions
3- Viroids
4- Prions

38. Defective viruses
Defective viruses are composed of nucleic acid and protein
But , they can't replicate without a “ Helper virus “ .
Fgf
During growth of most human viruses , more defective viruses are produced than infectious
viruses
Ratio ( 100 :1 )

39. Defective viruses
*- The growth of defective and non-defective viruses replicate simultaneously
Because defective particles formation can interfere with the
growth of the infectious particles . It has been hypothesized
that the defective viruses may aid in recovery from an
infection by limiting the ability of infectious particles to grow
e.g , ( production of protecting viruses )

40. Pseudovirions contain host cell DNA instead of viral DNA
*- when the cell DNA is fragmented during cell injury ,
pieces of DNA is incorporated within the capsid protein
*- Pseudovirions unlike defective viruses can infect cells , but they don’t replicate
Pseudovirions

41. Viroids
• Consist only of single molecule of circular RNA
Without protein coat or envelope . There is extensive homology
between bases in viroid RNA
And doesn’t code for any protein
*- viroids replicate , but the mechanism is poorly understood.
*- cause several plant disease , but not human ones

42. Prions
Prions are infectious particles that are composed
only of proteins ( No detectable nucleic acid ) .
*- they cause several diseases such as Creutzfeldt-Jakob disease in
humans and scrapie in animals.
Creutzfeldt-Jakob = mad cow disease = BSE

43. Prions

44. Prions
*- Prions are more resistant to inactivation by ultraviolet light and
heat than viruses .
*- prions composed of a single glycoprotein with a molecular weight
(27,000 – 30,000 ) .
*- prion proteins are encoded by single circular gene And this gene is
found in equal numbers in infected and uninfected animals .

45. Prions
To differentiate between proteins found in infected and uninfected
cells. Post- transitional modification of the prion proteins are
hypothesized .
These modification include change in the structure of
Proteins from alpha – helical form to beta- pleated sheet .
Change occurs ( affected with scrapie )

46. Functions of normal prion protein
1-one of the signal transduction proteins in neurons
2- copper- binding protein
3- protease-sensitive in normal cells , and protease- resistant in infected
cells
4- No immune response formed against this protein
and so, No inflammatory response .

47. Prions
Prions proteins in infected brain tissue form rod-shaped particles that
are morphologically and histochemically indistinguishable from
amyloid
Amyloid : substance formed in brain tissue after various central nervous
system diseases
amyloidosis also occur in other organs such as bone and joints

48. Prions