BIOC20 - LECTURE 2 EXAM QUESTIONS AND ANSWERS
WITH COMPLETE SOLUTIONS VERIFIED LATEST UPDATE
What are capsids?
They package viral genomes and transmit them to a new host cell
What is a nucleocapsid?
It is a capsid with an enclosed genome
What is a virion?
A complete, infectious virus particle, possibly including an envelope
What is an envelope?
A lipid bilayer that encapsulates a viral capsid
What microscopy methods are used to study virions?
Virions are studied by electron microscopy and X-ray diffraction
What is negative staining?
It is when viruses do not absorb the stain and appear lighter against a dark stained
background
What is positive staining?
It is when viruses absorb the stain and the target areas you are looking for are stained
(proteins, nucleic acids, lipids)
Why are positive and negative staining sub-optimal?
They both can lead to damaged tissues leading to unwanted staining of artifacts
What is cryoelectron microscopy?
,It uses liquid nitrogen to flash freeze samples allowing for extremely well preserved
structures (tissues and viruses) in samples
How are images made from CryoEM?
Data from many images are merged using a computer program giving the true image of
the structure
What is crystallization and X-ray diffraction?
This can only be used if the virus can be crystallized, if this is possible then the
crystalline structure causes a beam of incident X-rays to diffract into many specific
directions.
What are the pros and cons to crystallization and X-ray diffraction?
Pros: extremely high res images
Cons: Cannot be focused so a computer program is required to analyze diffraction
patterns to create an image
Why can viruses spontaneously self-assemble into simple, symmetric packages?
Viruses are composed of many copies of identical subunits increasing genetic economy
What is genetic economy?
It allows viruses with a very small genome to be extremely efficient while building solid
capsids; there is 2 ways to do so..
i) they make subunits that are versatile, and can be repeated and used to create one
dome
ii) express numerous proteins that interact together and form one dome
What gives capsids their symmetry?
Identical subunits
, What does capsid "shape" refer to?
The geometry of the capsid outline - spherical, cubic, tubular, etc.
What does capsid "symmetry" refer to?
The rotational and translational operations that describe it - twofold axis, threefold axis,
etc.
What are the 3 kinds of capsid symmetry that are possible?
Geometric principles have demonstrated that closed shells of identical subunits can
have..
- tetrahedral; polygon w/ 4 identical triangular faces
- cubic; 6 identical square faces
- icosahedral; 20 triangular faces
Where does the name icosahedral come from?
Greek language - eikosi = 20
What kinds of capsids HAVE NOT been described for viruses?
Tetrahedral and cubic
What are the possible icosahedral capsid rotational axis'?
Fivefold, threefold and twofold
What are the characteristics of a twofold rotational axis?
It contains 30 oval midpoints of the triangular edges. It takes two 180 degree rotations
to bring it back to its original shape
What are the characteristics of a threefold rotational axis?
It contains 20 triangular faces. It takes three 120 degree turns to bring it back to its
original shape
WITH COMPLETE SOLUTIONS VERIFIED LATEST UPDATE
What are capsids?
They package viral genomes and transmit them to a new host cell
What is a nucleocapsid?
It is a capsid with an enclosed genome
What is a virion?
A complete, infectious virus particle, possibly including an envelope
What is an envelope?
A lipid bilayer that encapsulates a viral capsid
What microscopy methods are used to study virions?
Virions are studied by electron microscopy and X-ray diffraction
What is negative staining?
It is when viruses do not absorb the stain and appear lighter against a dark stained
background
What is positive staining?
It is when viruses absorb the stain and the target areas you are looking for are stained
(proteins, nucleic acids, lipids)
Why are positive and negative staining sub-optimal?
They both can lead to damaged tissues leading to unwanted staining of artifacts
What is cryoelectron microscopy?
,It uses liquid nitrogen to flash freeze samples allowing for extremely well preserved
structures (tissues and viruses) in samples
How are images made from CryoEM?
Data from many images are merged using a computer program giving the true image of
the structure
What is crystallization and X-ray diffraction?
This can only be used if the virus can be crystallized, if this is possible then the
crystalline structure causes a beam of incident X-rays to diffract into many specific
directions.
What are the pros and cons to crystallization and X-ray diffraction?
Pros: extremely high res images
Cons: Cannot be focused so a computer program is required to analyze diffraction
patterns to create an image
Why can viruses spontaneously self-assemble into simple, symmetric packages?
Viruses are composed of many copies of identical subunits increasing genetic economy
What is genetic economy?
It allows viruses with a very small genome to be extremely efficient while building solid
capsids; there is 2 ways to do so..
i) they make subunits that are versatile, and can be repeated and used to create one
dome
ii) express numerous proteins that interact together and form one dome
What gives capsids their symmetry?
Identical subunits
, What does capsid "shape" refer to?
The geometry of the capsid outline - spherical, cubic, tubular, etc.
What does capsid "symmetry" refer to?
The rotational and translational operations that describe it - twofold axis, threefold axis,
etc.
What are the 3 kinds of capsid symmetry that are possible?
Geometric principles have demonstrated that closed shells of identical subunits can
have..
- tetrahedral; polygon w/ 4 identical triangular faces
- cubic; 6 identical square faces
- icosahedral; 20 triangular faces
Where does the name icosahedral come from?
Greek language - eikosi = 20
What kinds of capsids HAVE NOT been described for viruses?
Tetrahedral and cubic
What are the possible icosahedral capsid rotational axis'?
Fivefold, threefold and twofold
What are the characteristics of a twofold rotational axis?
It contains 30 oval midpoints of the triangular edges. It takes two 180 degree rotations
to bring it back to its original shape
What are the characteristics of a threefold rotational axis?
It contains 20 triangular faces. It takes three 120 degree turns to bring it back to its
original shape
