L11: Virus
Components Prokaryotic cell Eukaryotic cell Virion
Genetic material Present (DNA) Present (DNA) Present (DNA or RNA)
Nucleus Absent Present Absent
Endoplasmic reticulum Present Present Absent
Ribosomes Present Present Absent
Membrane present Present Present in enveloped
virus and absent in non-
(cell membrane)
enveloped virus
Cell Wall Present (in most) Present for plant Absent but have capsid to
cells and absent contain the genetic materials
for animal cells
Size in general 0.1-10 µm 10-100 µm 0.02-0.4 µm (<0.2)
Virion = Small virus particles
Virus Virion
Structure Non-cellular structure that Complete virus particle w a
contains genetic material protein capsid to protect
genetic material
Function Mobile genetic element that Inert particle that invades a
depends on host cells to cell & delivers genetic material
replicate so cell can replicate virus
Shape Can be many shapes E.g. Can be many shapes E.g.
filamentous, helical, isometric filamentous, helical, isometric,
& enveloped) enveloped & spherical)
Genetic Material Contains DNA/RNA genetic Contains DNA/RNA genetic
material that can be material that is protected by
single/double -stranded protein capsid
Classified by:
1. Type & structure of genetic material: RNA/DNA, single/double-stranded genetic
material, linear/circular, segmented/non-segmented
2. Structure of capsids: icosahedral/helical
3. Presence of envelope: Enveloped/Non-enveloped virus
4. Baltimore virus classification scheme (grps to how Mrna is produced during
replicative cycle of virus): double-stranded RNA/DNA (dsRNA/DNA), single-stranded
RNA/DNA (ssRNA/DNA), single stranded RNA with a positive polarity (ssRNA),
ssRNA with a negative polarity, diploid (two copies) ssDNA and partial dsDNA
genomes
- Pos polarity = genome RNA can serve directly as mRNA
- Neg polarity = Seq is complimentary to the mRNA
Light Microscope Electron Microscope
Illuminating source Light Electron beams
Minimum size of >0.2µm (200 nm) <0.2nm
microorganism viewable
Ability to view bacteria Can view bacteria (most) Yes (Can view bacteria and its
organelles)
Ability to view viruses Cannot view viruses Can view viruses
Cannot be viewed under light microscope as they’re small enough to be at the limit of resolution of
light microscopes
Made of proteins: Attachment proteins & capsid
Lack protein synthesis machinery: X produce attachment machinery and capsids
Requires living host to manufacture components
Replication cycle of RNA virus
Name of step What happens during the step
Step 1 Attachment The virion attaches to the host cell by specific
binding of its viral attachment proteins to the host
cell receptors.
Step 2 Penetration The virion enters the host cell via endocytosis (or
engulfment into a vesicle). Some enveloped viruses
enter via fusion of the viral envelope with the host
cell membrane.
Step 3 Uncoating The envelope (if any) and capsid of the virus is
removed, and the genetic material of the virus is
freed into the cytoplasm or nucleus.
Step 4 Replication Under the control of viral genes, the host cell
synthesizes the viral genetic material and viral
proteins (i.e. capsomeres and spike/viral
attachment protein/glycoproteins).
Step 5 Assembly Viral components are assembled to into virions.
Viral genetic material will be packaged within the
capsid, which is formed by many units of
capsomeres
Spike Proteins/viral attachment proteins are
inserted into the host cell membrane for viral
envelope.
(Note that not all viruses have envelope, hence this
step might not be present for non-enveloped
viruses).
Step 6 Release Enveloped virus bud off from the host membrane,
carrying away an envelope with protein spikes/viral
attachment protein. This complete virion is ready to
infect another cell.
Main purpose of virus infecting a cell: Make use of host cell machinery to replicate its genetic
material & viral proteins to produce many copies of itself
DNA viruses RNA viruses
Location of viral genome Nucleus cytoplasm
replication
Location of viral protein cytoplasm cytoplasm
synthesis
Types of virus that infects bacteria + basic components of said virus:
- Bacteriophage virus infects bacteria
- Structure: head, DNA, internal proteins, neck, tail sheath, tail fibre, end plate & pins
Neck
Base Plate
Replication cycle of bacteriophage
Virus that infects Virus that infects
eukaryotic cell prokaryotic cell
First step of replication Attachment Attachment
Structural component of virus Spike/ attachment protein Tail fibers
responsible for first step of
replication
Location of host cell receptor Proteins on eukaryotic Proteins on outer
cells surface of the bacterium
,LPS, pili and lipoprotein
- Virus that affect eukaryotic cells X infect prokaryotic cells.
- Unlikely for spike/surface attachment protein on a virus that infects eukaryotic cells can
recognise & bind to receptors on bacterial cell.
- Unlikely that tail fibre of bacteriophage able to recognise & bind to eukaryotic cell receptors
Range of Hosts
- Depends on presence of receptors on host cells
- If host lacks receptor for virus/component necessary for virus replication, host will
inherently be resistant to that virus
