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VIRAL GENOME

  VIRAL GENOME Virus is a is a small infectious agent that replicates Inside the living cells of other organism  The viral particles or virions consists of  Genetic material -DNA or RNA Protein coat -capsid  Lipid  envelope GENOME Virus as either DNA or RNA as a genetic material DNA VIRUS Virus with DNA as genetic material are called DNA viruses  The DNA of these viruses may be circular or linear . They may be double stranded or single stranded RNA VIRUS viruses with RNA as a genetic material are called as RNA viruses. The RNA in the virus may be double stranded or single stranded  It can be circular or linear  The genome size varies greatly . RNA viruses has a smaller genome size than DNA viruses BACTERIOPHAGE Viruses that infects bacteria are called as bacteriophage  It replicates within the bacteria  It is composed of protein that encapsulate the genome DNA /RNA The genome encodes few as 4 to 100S of genes  Example M13 M13 It is ...
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VIRAL GENOME

 

VIRAL GENOME

Virus is a is a small infectious agent that replicates Inside the living cells of other organism 
The viral particles or virions consists of 
  • Genetic material -DNA or RNA
  • Protein coat -capsid 
  • Lipid  envelope
GENOME
Virus as either DNA or RNA as a genetic material
DNA VIRUS

  • Virus with DNA as genetic material are called DNA viruses 
  • The DNA of these viruses may be circular or linear .
  • They may be double stranded or single stranded

RNA VIRUS

  • viruses with RNA as a genetic material are called as RNA viruses.

  • The RNA in the virus may be double stranded or single stranded 
  • It can be circular or linear 
  • The genome size varies greatly .
  • RNA viruses has a smaller genome size than DNA viruses

BACTERIOPHAGE

  • Viruses that infects bacteria are called as bacteriophage 
  • It replicates within the bacteria 
  • It is composed of protein that encapsulate the genome DNA /RNA
  • The genome encodes few as 4 to 100S of genes 
  • Example M13

M13

  • It is a virus that infect E coli. 
  • It is composed of single stranded circular DNA the genome encased in a flexible tube made of protein 
  • The genome has 6407 nucleotides Which encodes 10 genes and one intergenomic sequences


Replication

  • M13 replicates by rolling circle model of replication

  • ROLLING CIRCLE MODEL OF REPLICATION or σ replication
  • Rolling circle model of replication explains replication in single  single stranded DNA(+ve strand)viruses such as M13 and T4.
  • The chromosome became double stranded by synthesising negative strand.
  • The outer strand duplex is cut /nick at specific point by endonuclease

  • As the result of cut/nick, a linear strand with 3’ and 5’ ends are produced.
  • After the nick formation 3’ end is free to rotate on unbroken strand and 5’ end attached to plasma membrane of host bacterium.
  • The broken strand attached to the plasma membrane is tail region serves as template for synthesis of new DNA strands. 
  • The unbroken strand is used as template and complementary strand is synthesised.

  • As the synthesis of new DNA strand proceeds, the unbroken parental strand rolls back and unwinds further.

  • In tail region the DNA synthesised as discontinuous segment. 
  • Each segment proceed by RNA primer.

  • After complete synthesis of circle the tail is cut off by action of specific endonuclease into unit length called progeny rod.
  • The progeny rod undergo circularisation to form new circular molecule.
  • During this process the gap is closed by ligase
  • The newly formed circular molecule became a template for  new replication.




Phage 174

  • It is single stranded DNA virus
  • It has 5386 nucleotides encoding 11 proteins of these 11 genes only 8 genes are necessary for viral morphogenesis.
  • It has GC content of 44% and 95%of the nucleotide are coding genes





REPLICATION
It involves looped rolling circle model of replication
LOOPED ROLLING CIRCLE MODEL OF REPLICATION

  • Looped rolling circle model of replication is used to generate a progeny of single stranded circular DNA from double stranded circular template eg:Phage 174
  • A phage protein called gene A protein makes a nick at origin and in the process becomes covalently linked to the newly formed 5’-P terminus.

  • Using the Replication protein and SSBP protein and polymerase III holoenzyme, chain growth occurs from 3’OH group , displacing broken parental strand called +strand.
  • This strand becomes coated with SSBP protein and not replicated 
  • Leading strand synthesis continues until the origin reached.
  • The gene A protein binds to the 3’OH group at +strand and it joins the 3’OH and 5’ of +ve strand using the energy obtained from original nicking event.

  • It the  dissociate and attached to the newly synthesised +ve strand.
  • This process can continue indefinitely generating number of circular +ve strands.

ANIMAL VIRUSES

  • Viruses that infect animals are called animal viruses.
  • They have protein shell capsid and genetic material made of DNA or RNA that is packed inside the capsid

INFLUENZA VIRUS

  • They are negative sense single stranded segment virus
  • The genome is 13588 bases long contains 8RNA segment
  • It codes for 10-14 proteins




PLANT VIRUS

  • Virus that infects plants are called plant viruses.
  • 75% of plant viruses has 5SRNA as genome.
  • 65% has 15SRNA and 10% has 5sRNA
  • 5% has ds RNA.Very few plant virus has 5SDNA.

TOBACCO MOSAIC VIRUS(TMV)

  • It has +sense 5SRNA virus
  • It infect wide variety of plants especially tobacco plant.
  • The genome contains 6.3-6.5kbp.
  • The 3’ terminus has tRNA- like structure and 5’ terminus has methylated guanine
  • It encodes 4 genes 
  • 1.Replicase proteins(2)
  • 2.RNA dependent RNA polymerase
  • 3.Capsid protein

1. Replicase Proteins (126 kDa & 183 kDa)

  • 📍 Located at the 5′ end of the genome
126 kDa protein
  • Has methyltransferase and helicase activities
  • Involved in:
  • RNA capping
  • Unwinding RNA during replication
183 kDa protein
  • Produced by read-through of the 126 kDa ORF
  • Contains RNA-dependent RNA polymerase (RdRp) domain
  • Essential for:
  • Viral RNA replication
  • Synthesis of negative-strand RNA
  • 👉 Together, these form the replication complex.
3kbp:

  • It produce movement Protein (MP) – ~30 kDa
  • Enables virus movement from cell to cell
  • Modifies plasmodesmata
  • Allows viral RNA to pass into adjacent plant cells
  •  Key role: Local spread within plant tissues

4kbp:

  • It encodes coat Protein (CP) – ~17.5 kDa
  • Encapsidates viral RNA
  • Forms the rigid rod-shaped virion




References
1.Lewin's Genes twelve
2.Molecular Biology of the Gene by Watson,Baker,Bell,Gann,Levine&Losick













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