VIRAL GENETICS

1
??? ???? ?????? ??????
2

• VIRAL GENETICS

3

• Structure
• A virus particle, also known as a virion, is
  essentially a nucleic acid (DNA or RNA) enclosed
  in a protein shell or coat. Viruses are extremely
  small, approximately 15 - 25 nanometers in
  diameter.

4
GENETIC MATERIAL

• The "genetic material," depends on the nature and
  function of the specific virus.
• Types
• Double-stranded DNA
• Double-stranded RNA
• Single-stranded DNA or
• Single-stranded RNA .

5
GENETIC MATERIAL

• The viral genome can consist of
• Very small number of genes
• or Up to hundreds of genes depending on the type
  of virus. The genome is typically organized as
  a long molecule that is usually straight or
  circular.

6
(No Transcript)
7
PROTEIN COAT

• The protein coat that envelopes the genetic
  material is known as a capsid. It can have
  several shapes
• polyhedral
• rod or
• "complex.
• The protein subunits of the capsid are called
  capsomeres.

8
Influenza virus

• In addition to the protein coat, some viruses
  have specialized structures.
• e.g. the flu virus has a membrane-like envelope
  around its capsid.
• The envelope has both host cell and viral
  components and assists the virus in infecting its
  host..

9
GENETICS OF VIRUSES

• Viruses can store their genetic information in
  six different types of nucleic acid which are
  named based on how that nucleic acid eventually
  becomes transcribed to the viral mRNA capable of
  binding to host cell ribosomes and being
  translated into viral proteins..

10
Production of Viral mRNA From1- DNA

• .

11
Production of Viral mRNA from2-RNA

12
Production of Viral mRNA from2-RNA
13

•   
•  Viruses grow rapidly, there are usually a large
  number of progeny virions per cell., therefore,
  more chance of mutation. The nature of the
  viral genome (RNA or DNA segmented or
  non-segmented) plays an important. role in
  Viruses change.  

14
GENETIC CHANGE IN VIRUSES

• Viruses are continuously changing as a result of
  genetic selection. They undergo subtle genetic
  changes through mutation and major genetic
  changes through recombination.
• Mutation occurs when an error is incorporated in
  the viral genome.
• Recombination occurs when coinfecting viruses
  exchange genetic information, creating a novel
  virus.

15
MUTATION

• The mutation rates of DNA viruses approximate
  those of eukaryotic cells, yielding in theory one
  mutant virus in several hundred to many thousand
  genome copies.
• RNA viruses have much higher mutation rates,
  perhaps one mutation per virus genome copy.
  Mutations can be deleterious, neutral, or
  occasionally favorable.
• Only mutations that do not interfere with
  essential virus functions can persist in a virus
  population.

16
MUTATION

• Mutations can produce viruses with new antigenic
  determinants.
• The appearance of an antigenically novel virus
  through mutation is called antigenic drift.
• Antigenically altered viruses may be able to
  cause disease in previously resistant or immune
  hosts.

17
RECOMBINATION

• Recombination involves the exchange of genetic
  material between two related viruses during
  coinfection of a host cell.
• Recombination by Independent Assortment
  independent assortment can occur among viruses
  with segmented genomes. Genes that reside on
  different pieces of nucleic acid are randomly
  assorted. This can result in the generation of
  viruses with new antigenic determinants and new
  host ranges. Development of viruses with new
  antigenic determinants through independent
  assortment is called antigenic shift.

18

• Recombination of Incompletely Linked Genes
  Genes that reside on the same piece of nucleic
  acid may undergo recombination. The closer two
  genes are together, the rarer is recombination
  between them (partial linkage).

19
MUTANTS

• Spontaneous mutations
• These arise naturally during viral replication
  e.g. due to errors by the genome-replicating
  polymerase
• DNA viruses tend to more genetically stable than
  RNA viruses. There are error correction
  mechanisms in the host cell for DNA repair, but
  probably not for RNA..
• .

20
MUTANTS

• Mutations that are induced by physical or
  chemical means
• Chemical 
• Agents acting directly on bases, e.g. nitrous
  acidAgents acting indirectly, e.g. base analogs
  which mispair more frequently than normal bases
  thus generating mutations.
• Physical
• Agents such as UV light or X-rays.

21
TYPES OF MUTATION

• Mutants can be point mutants (one base replaced
  by another) or insertion/deletion mutants..

22

• Attenuated mutants  Many viral mutants cause
  much milder symptoms (or no symptoms) compared to
  the parental virus .

23
(No Transcript)
24
APPLIED GENETICS

• There is a new vaccine (approved June 2003) for
  influenza virus.
• The vaccine is trivalent it contains 3 strains
  of influenza virus 
• The viruses are cold adapted strains which can
  grow well at 25 degrees C and so grow in the
  upper respiratory tract where it is cooler. The
  viruses are temperature-sensitive and grow poorly
  in the warmer lower respiratory tract..

25
REFRENCES

• http//biology.about.com/library/weekly/aa110900a.
  htm
• http//www-micro.msb.le.ac.uk/3035/HBV.html
• http//cancerweb.ncl.ac.uk/cgi-bin/omd?tautomerism
  
• http//pathmicro.med.sc.edu/lecture/RETRO.HTM

26
REFRENCES

• Cox NJ, Brammer TL, Regnery HL Influenza global
  surveillance for epidemic and pandemic variants.
  Eur J Epidemiol 10467, 1994
• Gao L, Chain B, Sinclair C et al Immune response
  to human papillomavirus type 16 E6 gene in a live
  vaccinia vector. J gen Virol 75157, 1994
• Holland J, Spindler K, Horodyski F et al Rapid
  evolution of RNA genomes. Science 2151577, 1982
• Honess RW, Buchan A, Halliburton IW, Watson DH
  Recombination and linkage between structural and
  regulatory genes of herpes simplex virus type I
  study of the functional organization of the
  genome. J Virol 34716, 1980
• Palese P, Young JF Variation of influenza A, B,
  and C viruses. Science 2151468, 1982
• Paoletti E, Perkus ME, Piccini A Live
  recombinant vaccines using genetically engineered
  vaccinia virus. Antiviral Res, suppl 1301, 1985
• Radding CM Homologous pairing and strand
  exchange in genetic recombination. Annu Rev Genet
  16405, 1982
• W. Robert Fleischmann, Jr
• Bbc.co.uk
• Influenza virus Image courtesy of Linda M.
  Stannard, University of Cape Town.

27

• Romanova LI, Blinov VM, Tolskaya EA et al The
  primary structure of crossover regions of
  intertypic poliovirus recombinants a model of
  recombination between RNA genomes. Virology
  155202, 1986
• Schaffer PA, Tevethia MJ, Benyesh-Melnick M
  Recombination between temperature sensitive
  mutants of herpes simplex virus type 1. Virology
  58219, 1974
• Scholtissek C Source for influenza pandemics.
  Eur J Epidemiol 10455, 1994
• Siegfried W. Perspectives in gene therapy with
  recombinant adenoviruses. Exp Clin Endocrinol
  1017, 1993
• Smith FI, Palese P Variation in influenza virus
  genes epidemiological, pathogenic, and
  evolutionary consequences. p. 319. In Krug RM
  (ed) The Influenza Viruses. Plenum, New York,
  1989
• Webster RG, Bean WJ, Gorman OT et al Evolution
  and ecology of influenza A viruses. Microbiol Rev
  56152, 1992
• Viral Genetics