Viruses And Prions Exam Questions And
Complete Answers
Describe the general structure of a virus. - ANSWER Nucleic acid (DNA or RNA)
surrounded by a protein coat called a capsid. Virus may be naked or enveloped.
What is a complete viral particle (nucleic acid + capsid + envelope if present) called? -
ANSWER Virion
Why are viruses considered obligate intracellular parasites? - ANSWER Can only
reproduce/replicate if inside a host cell.
Why are viruses considered living? - ANSWER ➢ they evolve by natural selection
➢ they direct their own reproduction (they have the plan for replication in their nucleic
acid) and they spontaneously assemble
Why aren't viruses considered living? - ANSWER ➢ they have no cellular structure: no
cytoplasm or plasma membrane
➢ they have DNA or RNA, unlike prokaryotic and eukaryotic cells, which have both.
➢ they lack a metabolism of their own; for example, they cannot produce their own
energy (ATP) from glucose
➢ they do not contain all 4 groups of organic molecules (they lack carbs and lipids -
these molecules are present in the viral envelope, but envelopes really belong to the
host cells)
➢ they can only replicate with the help of a host cell; the host cell actually replicates the
viral nucleic acid and makes the capsid proteins
Describe the different variations on the viral nucleic acid. - ANSWER double stranded or
single stranded RNA or DNA, single or multiple segments, linear or circular
Describe the 3 basic shapes of capsids - ANSWER 1) polyhedral (capsomeres organized
into equilateral triangles, which are assembled into dome-shaped structure)
2) helical (capsomeres fit together as a helix that forms a rod-shaped structure)
3) complex (polyhedral head + helical tail)
, Explain how a virus acquires an envelope. - ANSWER The virus pushes out of the cell
membrane, forming a bud that encloses the virus - then the bud pinches off behind,
resealing the cell - as a result, the host cell is not lysed.
Where are glycoprotein spikes found? - ANSWER Glycoprotein spikes are found in the
viral envelope. Viral glycoprotein "spikes" are coded for on the viral nucleic acid and
are inserted into the host cell's plasma membrane prior to budding.
Viruses that lack envelopes are called? - ANSWER Naked
How does the presence of an envelope increase a virus's disease causing capacity?
(2-ways) - ANSWER 1.) Makes it difficult for the host's immune system to recognize the
foreign virus.
2.) Makes it easy for the enveloped virus to enter another cell (membrane fusion: cell
membranes fuse, dumping the virus into the new host cell)
What determines host range/viral specificity? - ANSWER Viral specificity is determined
by whether or not a virus can attach to a cell. Attachment depends on the presence
specific receptor sites on the surface of host cell and on specific attachment structures
on the viral capsid or envelope. Examples of receptor sites are proteins, LPS's,
glycolipids, or glycoproteins.
What are viruses that infect bacteria? - ANSWER Bacteriophages
List the 6 major steps of the lytic cycle. - ANSWER 1. Attachment - the virion attaches
itself to a specific receptor site on the surface of the host cell
2. Penetration - viral nucleic acid enters host cell (3 different ways depending on
whether virus is naked or enveloped)
3. Uncoating - removal of the capsid & envelope; only occurs if virus is enveloped.
4. Biosynthesis - viral components (nucleic acids & proteins for capsids) are
synthesized by the host cell.
5. Assembly/Maturation - components are assembled into new viruses (nucleic acids are
inserted into capsids)
6. Release - hundreds of intact virions exit host cell (2 different ways lysis or budding)
Penetration: How do enveloped virions enter the host cell? (2 ways) - ANSWER 1.
Membrane Fusion: the viral envelope fuses with host cell membrane, emptying the rest
of the virion inside the cell
Complete Answers
Describe the general structure of a virus. - ANSWER Nucleic acid (DNA or RNA)
surrounded by a protein coat called a capsid. Virus may be naked or enveloped.
What is a complete viral particle (nucleic acid + capsid + envelope if present) called? -
ANSWER Virion
Why are viruses considered obligate intracellular parasites? - ANSWER Can only
reproduce/replicate if inside a host cell.
Why are viruses considered living? - ANSWER ➢ they evolve by natural selection
➢ they direct their own reproduction (they have the plan for replication in their nucleic
acid) and they spontaneously assemble
Why aren't viruses considered living? - ANSWER ➢ they have no cellular structure: no
cytoplasm or plasma membrane
➢ they have DNA or RNA, unlike prokaryotic and eukaryotic cells, which have both.
➢ they lack a metabolism of their own; for example, they cannot produce their own
energy (ATP) from glucose
➢ they do not contain all 4 groups of organic molecules (they lack carbs and lipids -
these molecules are present in the viral envelope, but envelopes really belong to the
host cells)
➢ they can only replicate with the help of a host cell; the host cell actually replicates the
viral nucleic acid and makes the capsid proteins
Describe the different variations on the viral nucleic acid. - ANSWER double stranded or
single stranded RNA or DNA, single or multiple segments, linear or circular
Describe the 3 basic shapes of capsids - ANSWER 1) polyhedral (capsomeres organized
into equilateral triangles, which are assembled into dome-shaped structure)
2) helical (capsomeres fit together as a helix that forms a rod-shaped structure)
3) complex (polyhedral head + helical tail)
, Explain how a virus acquires an envelope. - ANSWER The virus pushes out of the cell
membrane, forming a bud that encloses the virus - then the bud pinches off behind,
resealing the cell - as a result, the host cell is not lysed.
Where are glycoprotein spikes found? - ANSWER Glycoprotein spikes are found in the
viral envelope. Viral glycoprotein "spikes" are coded for on the viral nucleic acid and
are inserted into the host cell's plasma membrane prior to budding.
Viruses that lack envelopes are called? - ANSWER Naked
How does the presence of an envelope increase a virus's disease causing capacity?
(2-ways) - ANSWER 1.) Makes it difficult for the host's immune system to recognize the
foreign virus.
2.) Makes it easy for the enveloped virus to enter another cell (membrane fusion: cell
membranes fuse, dumping the virus into the new host cell)
What determines host range/viral specificity? - ANSWER Viral specificity is determined
by whether or not a virus can attach to a cell. Attachment depends on the presence
specific receptor sites on the surface of host cell and on specific attachment structures
on the viral capsid or envelope. Examples of receptor sites are proteins, LPS's,
glycolipids, or glycoproteins.
What are viruses that infect bacteria? - ANSWER Bacteriophages
List the 6 major steps of the lytic cycle. - ANSWER 1. Attachment - the virion attaches
itself to a specific receptor site on the surface of the host cell
2. Penetration - viral nucleic acid enters host cell (3 different ways depending on
whether virus is naked or enveloped)
3. Uncoating - removal of the capsid & envelope; only occurs if virus is enveloped.
4. Biosynthesis - viral components (nucleic acids & proteins for capsids) are
synthesized by the host cell.
5. Assembly/Maturation - components are assembled into new viruses (nucleic acids are
inserted into capsids)
6. Release - hundreds of intact virions exit host cell (2 different ways lysis or budding)
Penetration: How do enveloped virions enter the host cell? (2 ways) - ANSWER 1.
Membrane Fusion: the viral envelope fuses with host cell membrane, emptying the rest
of the virion inside the cell
