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LECTURE 33: Viral Evolution

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LECTURE 33: Viral Evolution. By Randall Fisher B.Sc. MBS [email protected] Structure of this lecture:. Split into groups (ten groups of 12.794 students) Recap of last lecture 3 Theories of Viral Origin 3 minute and 47 second interval Baltimore scheme of Viral Classification Questions.

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structure of this lecture
Structure of this lecture:
  • Split into groups (ten groups of 12.794 students)
  • Recap of last lecture
  • 3 Theories of Viral Origin
  • 3 minute and 47 second interval
  • Baltimore scheme of Viral Classification
  • Questions
question 1
Question 1
  • What is the approximate size of the largest RNA Viral genome?
  • And the largest DNA Viral genome?
question 2
Question 2
  • DNA or RNA?
question 3




Question 3
  • A: Positive sense
  • B: Negative sense
  • C: Nonsense
  • D: 50 cents
question 4
Question 4
  • What attribute, exclusive to Viral RNA genomes, does this image demonstrate?



question 5



Question 5
  • What the hell is that?


question 6
Question 6
  • What is the name of the subfamily of Orthomyxoviruses that is clinically important to man (and woman… well… some of them).
question 7
Question 7
  • How are Paramyxoviruses transmitted?
question 8
Question 8
  • What is the shape of the Rhabdovirus virion?


question 9
Question 9
  • Which virus is Prof. Fieldings favorite virus?
question 10
Question 10
  • Which RNA virus synthesizes the enzyme Reverse Transcriptase?
theories of viral origin
Theories of Viral Origin
  • Regressive theory
    • Degenerate forms of parasites
    • Mitochondria and chloroplasts?
  • Progressive theory
    • Normal cellular n.a. ability to replicate autonomously,
    • DNA viruses = plasmids or transposable elements.
    • Retroviruses = retrotransposons
    • RNA virus = mRNA.
  • Coevolution theory:
    • Viruses coevolved with life
baltimore scheme of viral classification
Baltimore Scheme of Viral Classification
  • Scheme that encompass all viruses, based on the:
      • nature of genomes (type of nucleic acids)
      • Modes of replication and gene expression
  • May not be phylogenetic, no common origin
  • Used by International Committee on Taxonomy of Viruses (ICTV) with other parameters
  • Revised Baltimore scheme based on fundamental importance of mRNA in the replication cycle of viruses
  • Viruses do not contain the molecules necessary to translate mRNA, rely on host cell machinery
baltimore scheme of viral classification18
Baltimore Scheme of Viral Classification
  • They must make mRNA that can be recognized by host cell ribosomes
  • Either the genes are stored in the 5\'→3\' direction (positive or + polarity), analogous to the direction in which genes are represented in mRNA in cells,
baltimore scheme of viral classification19
Baltimore Scheme of Viral Classification
  • or the genes are stored in the opposite, 3\'→5\' direction (negative or - polarity).
  • In other words: + or - polarity of RNA:
    • "+" is able to serve as mRNA.
    • "-" is the complement of “+”, must function as template to make a complementary strand of + RNA before any translation can occur.
lets get cracking
Lets get cracking!
  • DNA viruses
    • Group I - dsDNA viruses (double stranded DNA)
      • mRNA may come from either strand and transcription similar to host’s
    • Group II - ssDNA viruses (single stranded DNA)
      • DNA + or – depending on virus studied
      • DNA converted to ds before synthesis of mRNA
rna viruses group iii
RNA Viruses: Group III
  • RNA viruses
    • Group III - dsRNA viruses (double stranded RNA)
      • Most of these viruses have segmented genomes
      • mRNA from one template of each segment
      • Mechanism similar to transcription from dsDNA genome
      • Enzymes needed not in uninfected cells
      • Enzymes encoded by virus, packaged in virion, carried into cell
group iv
Group IV
  • Group IV - (+)ssRNA viruses (positive single stranded RNA or mRNA like)
    • Same sense as mRNA (+), can be translated
    • Enzymes for RNA synthesis virus encoded, made after infection initiated
group v
Group V
  • Group V - (-)ssRNA viruses (negative single-stranded RNA)
    • Genomes complimentary to mRNA (- sense)
    • Synthesis of mRNA by transcription from genome strand
    • Requires novel virus encoded enzymes
    • Some Group V viruses are known as ‘ambisense’ viruses
    • These use newly made ‘antigenome’ RNA strand as template for mRNA production and Enzymes needed also virus encoded
reverse transcribing viruses groups vi and vii
Reverse Transcribing Viruses: Groups VI and VII
  • DNA and RNA Reverse Transcribing viruses
    • Group VI - ssRNA-RT viruses (single stranded RNA)
      • Generate dsDNA intermediate before replication
      • Carried out by reverse transcriptase enzyme; carried in virion
    • Group VII - dsDNA-RT viruses (double stranded DNA)
      • Also known as “reversiviruses”
      • Replication strategy via positive-sense ssRNA intermediate and RT enzyme
      • Inverse, but similar to VI
the end
The END!!!
  • For now…