Lecture 2 Flashcards
What is the one thing all viruses have in common?
Symmetry in structure
Issue DNA viruses must overcome?
DNA viruses must overcome issue with very stable double stranded DNA, its stiffness. Cells need to use tricks to condense it
Advantage RNA viruses have over DNA viruses in structure?
RNA is mostly single stranded and can fold back to stem loops, easily condensed.
Disadvantage RNA viruses have against DNA viruses?
RNA is much less stable, leading to upper limit for RNA viral genomes. Only dsDNA is stable enough for very complex viruses.
How are RNA viruses genomes condensed?
Complex secondary structures formed with e.g. catalytic activity (ribozymes), a branched polymer; important for assembly.
Why is symmetry used by viruses?
Genetic economy (minimalism). Requires less energy. Building blocks are interchangeable. Size of container vs coding length = more genes require larger container, which would require more genes etc... nucleic acid is 6 times heavier than the protein it encodes Recyclability of building blocks.
Summary: symmetry is a good way to ‘reuse’ building blocks to reduce complexity of genome so that it is small enough to fit in the container it encodes.
First virus to be shown to have structural symmetry?
Tobacco Mosaic Virus.
Shown to have 2D, helical, rod-like particles.
What other viruses shows 2D symmetry?
Bacteriophage M13 (ssDNA) shows 2D symmetry, but not as stiff as TMV. More flexibility. The virus that infects E. coli.
Ebola virus also displays helical symmetry.
What part of the virus resembles icosahedral symmetry?
The Capsid
What rotation orders correspond to what degree of rotation?
Order 2: 180 degrees
Order 3: 120 degrees
Order 4: 90 degrees
What order of rotational symmetry does an equilateral triangle have
Order 3
What is icosahedral symmetry?
6 axes of 5-fold symmetry.
10 axes of 3-fold symmetry.
15 axes of 2-fold symmetry
What is the Caspar and Klug theory of quasi-equivalence?
How larger, more complex viruses assemble their capsid.
Icosahedral symmetry only accounts for at most 60 subunits in capsid.
Viruses with larger capsids use quasi-equivalence, using four triangles to make one even larger triangles, etc…
What does quasi-equivalence mean in terms of subunits meeting?
The corners of the larger ‘triangle’ has 5 subunits meeting, whereas the corners of the smaller triangles have 6 subunits.
What does quasi-equivalence mean in terms of subunits meeting?
The corners of the larger ‘triangle’ has 5 subunits meeting, whereas the corners of the smaller triangles have 6 subunits. Visible under a microscope as pentamers and hexamers.
