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LECTURE 33: Viral Evolution. By Randall Fisher B.Sc. MBS 2355827@uwc.ac.za. 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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Lecture 33 viral evolution l.jpg

LECTURE 33: Viral Evolution

By Randall Fisher



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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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Question 1

  • What is the approximate size of the largest RNA Viral genome?

  • And the largest DNA Viral genome?

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Question 2

  • DNA or RNA?

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Question 3

  • A: Positive sense

  • B: Negative sense

  • C: Nonsense

  • D: 50 cents

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Question 4

  • What attribute, exclusive to Viral RNA genomes, does this image demonstrate?



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Question 5

  • What the hell is that?


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Question 6

  • What is the name of the subfamily of Orthomyxoviruses that is clinically important to man (and woman… well… some of them).

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Question 7

  • How are Paramyxoviruses transmitted?

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Question 8

  • What is the shape of the Rhabdovirus virion?


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Question 9

  • Which virus is Prof. Fieldings favorite virus?

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Question 10

  • Which RNA virus synthesizes the enzyme Reverse Transcriptase?

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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

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LECTURE 34: The Baltimore Scheme of Viral Classification

By: the same old guy

With the same old B.Sc.


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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

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    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,

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    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.

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    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

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    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

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    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

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    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

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    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

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    The END!!!

    • For now…